Jasmin discovered that her extreme fatigue, muscle pain, vision problems, and vomiting were due to problems in her mitochondria, like the damaged mitochondria shown in red in Figure 4.14.1. Mitochondria are small, membrane-bound organelles found in eukaryotic cells that provide energy for the cells of the body. They do this by carrying out the final two steps of aerobic cellular respiration: the Krebs cycle and electron transport. This is the major way that the human body breaks down the sugar glucose from food into a form of energy cells can use, namely the molecule ATP.

Because mitochondria provide energy for cells, you can understand why Jasmin was experiencing extreme fatigue, particularly after running. Her damaged mitochondria could not keep up with her need for energy, particularly after intense exercise, which requires a lot of additional energy. What is perhaps not so obvious are the reasons for her other symptoms, such as blurry vision, muscle spasms, and vomiting. All of the cells in the body require energy in order to function properly. Mitochondrial diseases can cause problems in mitochondria in any cell of the body, including muscle cells and cells of the nervous system, which includes the brain and nerves. The nervous system and muscles work together to control vision and digestive system functions, such as vomiting, so when they are not functioning properly, a variety of symptoms can emerge. This also explains why Jasmin’s niece, who has a similar mitochondrial disease, has symptoms related to brain function, such as seizures and learning disabilities. Our cells are microscopic, and mitochondria are even tinier — but they are essential for the proper functioning of our bodies. When they are damaged, serious health effects can occur.

 

One seemingly confusing aspect of mitochondrial diseases is that the type of symptoms, severity of symptoms, and age of onset can vary wildly between people — even within the same family! In Jasmin’s case, she did not notice symptoms until adulthood, while her niece had more severe symptoms starting at a much younger age. This makes sense when you know more about how mitochondrial diseases work.

Inherited mitochondrial diseases can be due to damage in either the DNA in the nucleus of cells or in the DNA in the mitochondria themselves. Recall that mitochondria are thought to have evolved from prokaryotic organisms that were once free-living, but were then infected or engulfed by larger cells. One of the pieces of evidence that supports this endosymbiotic theory is that mitochondria have their own, separate DNA. When the mitochondrial DNA is damaged (or mutated) it can result in some types of mitochondrial diseases. However, these mutations do not typically affect all of the mitochondria in a cell. During cell division, organelles such as mitochondria are replicated and passed down to the new daughter cells. If some of the mitochondria are damaged, and others are not, the daughter cells can have different amounts of damaged mitochondria. This helps explain the wide range of symptoms in people with mitochondrial diseases — even ones in the same family — because different cells in their bodies are affected in varying degrees. Jasmin’s niece was affected strongly and her symptoms were noticed early, while Jasmin’s symptoms were more mild and did not become apparent until adulthood.

There is still much more that needs to be discovered about the different types of mitochondrial diseases. But by learning about cells, their organelles, how they obtain energy, and how they divide, you should now have a better understanding of the biology behind these diseases.

Apply your understanding of cells to your own life. Can you think of other diseases that affect cellular structures or functions. Do they affect people you know? Since your entire body is made of cells, when cells are damaged or not functioning properly, it can cause a wide variety of health problems.

Attributions

Figure 4.14.1

Mitochondrial Disease muscle sample by Nephron is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.

Figure 4.14.2

Aunt and Niece by Tatiana Rodriguez on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Reference

Wikipedia contributors. (2020, June 6). Mitochondrial disease. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Mitochondrial_disease&oldid=961126371

Refers to the body system consisting of the heart, blood vessels and the blood. Blood contains oxygen and other nutrients which your body needs to survive. The body takes these essential nutrients from the blood.

We stay alive because millions of different chemical reactions are taking place inside our bodies all the time. Each of our cells is like the busy auto assembly line pictured in Figure 3.10.1. Raw materials, half-finished products, and waste materials are constantly being used, produced, transported, and excreted. The "workers" on the cellular assembly line are mainly enzymes. These are the proteins that make biochemical reactions happen.

Chemical reactions that take place inside living things are called biochemical reactions. The sum of all the biochemical reactions in an organism is called metabolism. Metabolism includes both exothermic (energy-releasing) chemical reactions and endothermic (energy-absorbing) chemical reactions.

Catabolic Reactions

Exothermic reactions in organisms are called catabolic reactions. These reactions break down molecules into smaller units and release energy. An example of a catabolic reaction is the breakdown of glucose during cellular respiration, which releases energy that cells need to carry out life processes.

Anabolic Reactions

Endothermic reactions in organisms are called anabolic reactions. These reactions build up bigger molecules from smaller ones and absorb energy. An example of an anabolic reaction is the joining of amino acids to form a protein. Which type of reactions — catabolic or anabolic — do you think occur when your body digests food?

Most of the biochemical reactions that happen inside of living organisms require help. Why is this the case? For one thing, temperatures inside living things are usually too low for biochemical reactions to occur quickly enough to maintain life. The concentrations of reactants may also be too low for them to come together and react. Where do the biochemical reactions get the help they need to proceed? From the enzymes.

An enzyme is a protein that speeds up a biochemical reaction. It is a biological catalyst. An enzyme generally works by reducing the amount of activation energy needed to start the reaction. The graph in Figure 3.10.2 shows the activation energy needed for glucose to combine with oxygen. Less activation energy is needed when the correct enzyme is present than when it is not present.

How Well Enzymes Work

Enzymes are involved in most biochemical reactions, and they do their jobs extremely well. A typical biochemical reaction that would take several days or even several centuries to happen without an enzyme is likely to occur in just a split second with the proper enzyme! Without enzymes to speed up biochemical reactions, most organisms could not survive.

Enzymes are substrate-specific. The substrate of an enzyme is the specific substance it affects. Each enzyme works only with a particular substrate, which explains why there are so many different enzymes. In addition, for an enzyme to work, it requires specific conditions, such as the right temperature and pH. Some enzymes work best under acidic conditions, for example, while others work best in neutral environments.

Enzyme-Deficiency Disorders

There are hundreds of known inherited metabolic disorders in humans. In most of them, a single enzyme is either not produced by the body at all, or is otherwise produced in a form that doesn't work. The missing or defective enzyme is like an absentee worker on the cell's assembly line. Imagine the auto assembly line from the image at the start of this section.  What if the worker who installed the steering wheel was absent?  How would this impact the overall functioning of the vehicle?  When an enzyme is missing, toxic chemicals build up, or an essential product isn't made. Generally, the normal enzyme is missing because the individual with the disorder inherited two copies of a gene mutation, which may have originated many generations previously.

Any given inherited metabolic disorder is generally quite rare in the general population. However, there are so many different metabolic disorders that a total of one in 1,000 to 2,500 newborns can be expected to have one.

Figure 3.10.1

Auto Assembly line by Brian Snelson on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.

Figure 3.10.2

Enzyme_activation_energy by G. Andruk [IMeowbot at the English language Wikipedia], is used under a CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/) license.

References

Amoeba Sisters. (2016, August 28). Enzymes (updated). YouTube. https://www.youtube.com/watch?v=qgVFkRn8f10&feature=youtu.be

TED-Ed. (2015, January 15). What triggers a chemical reaction? - Kareem Jarrah. YouTube. https://www.youtube.com/watch?v=8m6RtOpqvtU&feature=youtu.be

Created by CK-12 Foundation/Adapted by Christine Miller

Fashion Statement

This colourful hairstyle makes quite a fashion statement. Many people spend a lot of time and money on their hair, even if they don’t have an exceptional hairstyle like this one. Besides its display value, hair actually has important physiological functions.

What is Hair?

Hair is a filament that grows from a hair follicle in the dermis of the skin. It consists mainly of tightly packed, keratin-filled cells called keratinocytes. The human body is covered with hair follicles, with the exception of a few areas, including the mucous membranes, lips, palms of the hands, and soles of the feet.

Structure of Hair

The part of the hair located within the follicle is called the hair root. The root is the only living part of the hair. The part of the hair that is visible above the surface of the skin is the hair shaft. The shaft of the hair has no biochemical activity and is considered dead.

Follicle and Root

Hair growth begins inside a follicle (see Figure 10.5.2 below). Each hair follicle contains stem cells that can keep dividing, which allows hair to grow. The stem cells can also regrow a new hair after one falls out. Another structure associated with a hair follicle is a sebaceous gland that produces oily sebum. The sebum lubricates and helps to waterproof the hair. A tiny arrector pili muscle is also attached to the follicle. When it contracts, the follicle moves, and the hair in the follicle stands up.

 

Shaft

The hair shaft is a hard filament that may grow very long. Hair normally grows in length by about half an inch a month. In cross-section, a hair shaft can be divided into three zones, called the cuticle, cortex, and medulla.

• The cuticle (or outer coat) is the outermost zone of the hair shaft. It consists of several layers of flat, thin keratinocytes that overlap one another like shingles on a roof. This arrangement helps the cuticle repel water. The cuticle is also covered with a layer of lipids, just one molecule thick, which increases its ability to repel water. This is the zone of the hair shaft that is visible to the eye.
• The cortex is the middle zone of the hair shaft, and it is also the widest part. The cortex is highly structured and organized, consisting of keratin bundles in rod-like structures. These structures give hair its mechanical strength. The cortex also contains melanin, which gives hair its colour.
• The medulla is the innermost zone of the hair shaft. This is a small, disorganized, and more open area at the center of the hair shaft. The medulla is not always present. When it is present, it contains highly pigmented cells full of keratin.

Characteristics of Hair

Two visible characteristics of hair are its colour and texture. In adult males, the extent of balding is another visible characteristic. All three characteristics are genetically controlled.

Hair Colour

All natural hair colours are the result of melanin, which is produced in hair follicles and packed into granules in the hair. Two forms of melanin are found in human hair: eumelanin and pheomelanin. Eumelanin is the dominant pigment in brown hair and black hair, and pheomelanin is the dominant pigment in red hair. Blond hair results when you have only a small amount of melanin in the hair. Gray and white hair occur when melanin production slows down, and eventually stops.

Figure 10.5.3 Variation in hair colouration. Which types of melanin are present for each hair colour shown?

Hair Texture

Hair exists in a variety of textures. The main aspects of hair texture are the curl pattern, thickness, and consistency.

• The shape of the hair follicle determines the shape of the hair shaft. The shape of the hair shaft, in turn, determines the curl pattern of the hair. Round hair shafts produce straight hair. Hair shafts that are oval or have other shapes produce wavy or curly hair .
• The size of the hair follicle determines the thickness of hair. Thicker hair has greater volume than thinner hair.
• The consistency of hair is determined by the hair follicle volume and the condition of the hair shaft. The consistency of hair is generally classified as fine, medium, or coarse. Fine hair has the smallest circumference, and coarse hair has the largest circumference. Medium hair falls in between these two extremes. Coarse hair also has a more open cuticle than thin or medium hair does, which causes it to be more porous.
  

  

Functions of Hair

In humans, one function of head hair is to provide insulation and help the head retain heat. Head hair also protects the skin on the head from damage by UV light.

The function of hair in other locations on the body is debated. One idea is that body hair helps keep us warm in cold weather. When the body is too cold, arrector pili muscles contract and cause hairs to stand up (shown in Figure 10.5.5), trapping a layer of warm air above the epidermis. However, this is more effective in mammals that have thick hair or fur than it is in relatively hairless human beings.

Human hair has an important sensory function, as well. Sensory receptors in the hair follicles can sense when the hair moves, whether it moves because of a breeze, or because of the touch of a physical object. The receptors may also provide sensory awareness of the presence of parasites on the skin.

 

Some hairs, such as the eyelashes, are especially sensitive to the presence of potentially harmful matter. The eyelashes grow at the edge of the eyelid and can sense when dirt, dust, or another potentially harmful object is too close to the eye. The eye reflexively closes as a result of this sensation. The eyebrows also provide some protection to the eyes. They protect the eyes from dirt, sweat, and rain. In addition, the eyebrows play a key role in nonverbal communication (see Figure 10.5.6). They help express emotions such as sadness, anger, surprise, and excitement.

Hair in Human Evolution

Among mammals, humans are nearly unique in having undergone significant loss of body hair during their evolution. Humans are also unlike most other mammals in having curly hair as one variation in hair texture. Even non-human primates (see Figure 10.5.7) all have straight hair. This suggests that curly hair evolved at some point during human evolution.

Loss of Body Hair

One hypothesis for the loss of body hair in the human lineage is that it would have facilitated cooling of the body by the evaporation of sweat. Humans also evolved far more sweat glands than other mammals, which is consistent with this hypothesis, because sweat evaporates more quickly from less hairy skin. Another hypothesis for human hair loss is that it would have led to fewer parasites on the skin. This might have been especially important when humans started living together in larger, more crowded social groups.

These hypotheses may explain why we lost body hair, but they can’t explain why we didn’t also lose head hair and hair in the pubic region and armpits. It is possible that head hair was retained because it protected the scalp from UV light. As our bipedal ancestors walked on the open savannas of equatorial Africa, the skin on the head would have been an area exposed to the most direct rays of sunlight in an upright hominid. Pubic and armpit hair may have been retained because they served as signs of sexual maturity, which would have been important for successful mating and reproduction.

Evolution of Curly Hair

Greater protection from UV light has also been posited as a possible selective agent favoring the evolution of curly hair. Researchers have found that straight hair allows more light to pass into the body through the hair shaft via the follicle than does curly hair. In this way, human hair is like a fibre optic cable. It allows light to pass through easily when it is straight, but it impedes the passage of light when it is kinked or coiled. This is indirect evidence that UV light may have been a selective agent leading to the evolution of curly hair.

Social and Cultural Significance of Hair

Hair has great social significance for human beings. Body hair is an indicator of biological sex, because hair distribution is sexually dimorphic. Adult males are generally hairier than adult females, and facial hair in particular is a notable secondary male sex characteristic. Hair may also be an indicator of age. White hair is a sign of older age in both males and females, and male pattern baldness is a sign of older age in males. In addition, hair colour and texture can be a sign of ethnic ancestry.

Hair also has great cultural significance. Hairstyle and colour may be an indicator of social group membership and for better or worse can be associated with specific stereotypes. Head shaving has been used in many times and places as a punishment, especially for women. On the other hand, in some cultures, cutting off one’s hair symbolizes liberation from one’s past. In other cultures, it is a sign of mourning. There are also many religious-based practices involving hair. For example, the majority of Muslim women hide their hair with a headscarf. Sikh men grow their hair long and cover it with a turban. Amish men (like the one pictured in Figure 10.5.8) grow facial hair only after they marry — but just a beard, and not a mustache.

Unfortunately, sometimes hairstyle, colour and characteristics are used to apply stereotypes, particularly with respect to women.  "Blonde jokes" are a good example of how negative stereotypes are maintained despite having no actual truth behind them.  Many stereotypes related to hair are hidden, even from persons perpetrating the stereotype.  Often a hairstyle is judged by another as having ties to gender, sexuality, worldview and/or socioeconomic status; even when these inferences are woefully inaccurate.  It is important to be aware of our own biases and determine if these biases are appropriate - take a look at the collage in Figure 10.5.9.  What are your initial reactions?  Are these reactions founded in fact?  Do you harbor an unfair bias?

Figure 10.5.9 What are your biases?  Are they fair?

 

Attributions

Figure 10.5.1

Hair by jessica-dabrowski-TETR8YLSqt4 [photo] by Jessica Dabrowski on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 10.5.2

Blausen_0438_HairFollicleAnatomy_02 by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 10.5.3

• Standing tall by Ilaya Raja on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Blond-haired woman smiling by Carlos Lindner on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Smith Mountain Lake redhead by Chris Ross Harris on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Through the look of experience by Laura Margarita Cedeño Peralta on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 10.5.4

Curly hair by chris-benson-clvEami9RN4 [photo] by Chris Benson on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 10.5.5

1024px-PilioerectionAnimation by AnthonyCaccese on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en) license.

Figure 10.5.6

Pout by alexander-dummer-Em8I8Z_DwA4 [photo] by Alexander Dummer on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 10.5.7

Cotton_top_tamarin_monkey._(12046035746) by Bernard Spragg. NZ, from Christchurch, New Zealand on Wikimedia Commons is used under a CC0 1.0 Universal
Public Domain Dedication license (https://creativecommons.org/publicdomain/zero/1.0/deed.en).

Figure 10.5.8

Amish hairstyle by CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.
©CK-12 Foundation Licensed under  • Terms of Use • Attribution
Figure 10.5.9

• Rainbow Hair Bubble Man by Behrouz Jafarnezhad on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Pink hair in Atlanta, United States by Tammie Allen on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Magdalena 2 by Valerie Elash on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Perfect Style by Daria Volkova on Unsplash is used under the Unsplash License (https://unsplash.com/license)
• Stay Classy by Fayiz Musthafa on Unsplash is used under the Unsplash License (https://unsplash.com/license)
• Take your time by Jan Tinneberg on Unsplash is used under the Unsplash License (https://unsplash.com/license)

References

Blausen.com staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Brainard, J/ CK-12 Foundation. (2016). Figure 7 This style of facial hair is adopted by most Amish men after they marry [digital image]. In CK-12 College Human Biology (Section 12.5) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-college-human-biology/section/12.5/

Sony Pictures Animation. (2019, December 5). Hair love | Oscar®-winning short film (Full) | Sony Pictures Animation. YouTube. https://www.youtube.com/watch?v=kNw8V_Fkw28

TED-Ed. (2015, August 25). Why do some people go bald? – Sarthak Sinha. YouTube. https://www.youtube.com/watch?v=8diYLhl8bWU

TEDx Talks. (2015, February 4). Why do we care about hair | Naomi Abigail | TEDxBaDinh. YouTube. https://www.youtube.com/watch?v=hDW5e3NR1Cw

Created by CK-12 Foundation/Adapted by Christine Miller

A tiny sperm from dad breaks through the surface of a huge egg from mom. Voilà! In nine months, a new son or daughter will be born. Like most other multicellular organisms, human beings reproduce sexually. In human sexual reproduction, males produce sperm and females produce eggs, and a new offspring forms when a sperm unites with an egg. How do sperm and eggs form? And how do they arrive together at the right place and time so they can unite to form a new offspring? These are functions of the reproductive system.

The reproductive system is the human organ system responsible for the production and fertilization of gametes (sperm or eggs) and, in females, the carrying of a fetus. Both male and female reproductive systems have organs called gonads that produce gametes. A gamete is a haploid cell that combines with another haploid gamete during fertilization, forming a single diploid cell called a zygote. Besides producing gametes, the gonads also produce sex hormones. Sex hormones are endocrine hormones that control the development of sex organs before birth, sexual maturation at puberty, and reproduction once sexual maturation has occurred. Other reproductive system organs have various functions, such as maturing gametes, delivering gametes to the site of fertilization, and providing an environment for the development and growth of an offspring.

The reproductive system is the only human organ system that is significantly different between males and females. Embryonic structures that will develop into the reproductive system start out the same in males and females, but by birth, the reproductive systems have differentiated. How does this happen?

Sex Differentiation

Starting around the seventh week after conception in genetically male (XY) embryos, a gene called SRY on the Y chromosome (shown in Figure 18.2.2) initiates the production of multiple proteins. These proteins cause undifferentiated gonadal tissue to develop into male gonads (testes). The male gonads then secrete hormones — including the male sex hormone testosterone — that trigger other changes in the developing offspring (now called a fetus), causing it to develop a complete male reproductive system. Without a Y chromosome, an embryo will develop female gonads (ovaries) that will produce the female sex hormone estrogen. Estrogen, in turn, will lead to the formation of the other organs of a normal female reproductive system.

Homologous Structures

Undifferentiated embryonic tissues develop into different structures in male and female fetuses. Structures that arise from the same tissues in males and females are called homologous structures. The male testes and female ovaries, for example, are homologous structures that develop from the undifferentiated gonads of the embryo. Likewise, the male penis and female clitoris are homologous structures that develop from the same embryonic tissues.

Sex Hormones and Maturation

Male and female reproductive systems are different at birth, but they are immature and incapable of producing gametes or sex hormones. Maturation of the reproductive system occurs during puberty, when hormones from the hypothalamus and pituitary gland stimulate the testes or ovaries to start producing sex hormones again. The main sex hormones are testosterone in males and estrogen in females. Sex hormones, in turn, lead to the growth and maturation of the reproductive organs, rapid body growth, and the development of secondary sex characteristics. Secondary sex characteristics are traits that are different in mature males and females, but are not directly involved in reproduction. They include facial hair in males and breasts in females.

The main structures of the male reproductive system are external to the body and illustrated in Figure 18.2.3. The two testes (singular, testis) hang between the thighs in a sac of skin called the scrotum. The testes produce both sperm and testosterone. Resting atop each testis is a coiled structure called the epididymis (plural, epididymes). The function of the epididymes is to mature and store sperm. The penis is a tubular organ that contains the urethra and has the ability to stiffen during sexual arousal. Sperm passes out of the body through the urethra during a sexual climax (orgasm). This release of sperm is called ejaculation.

In addition to these organs, the male reproductive system consists of several ducts and glands that are internal to the body. The ducts, which include the vas deferens (also called the ductus deferens), transport sperm from the epididymis to the urethra. The glands, which include the prostate gland and seminal vesicles, produce fluids that become part of semen. Semen is the fluid that carries sperm through the urethra and out of the body. It contains substances that control pH and provide sperm with nutrients for energy.

The main structures of the female reproductive system are internal to the body and shown in the following figure. They include the paired ovaries, which are small, ovoid structures that produce ova and secrete estrogen. The two oviducts (sometimes called Fallopian tubes or uterine tubes) start near the ovaries and end at the uterus. Their function is to transport ova from the ovaries to the uterus. If an egg is fertilized, it usually occurs while it is traveling through an oviduct. The uterus is a pear-shaped muscular organ that functions to carry a fetus until birth. It can expand greatly to accommodate a growing fetus, and its muscular walls can contract forcefully during labour to push the baby out of the uterus and into the vagina. The vagina is a tubular tract connecting the uterus to the outside of the body. The vagina is where sperm are usually deposited during sexual intercourse and ejaculation. The vagina is also called the birth canal because a baby travels through the vagina to leave the body during birth.

The external structures of the female reproductive system are referred to collectively as the vulva. They include the clitoris, which is homologous to the male penis. They also include two pairs of labia (singular, labium), which surround and protect the openings of the urethra and vagina.

Attributions

Figure 18.2.1

Sperm-egg by Unknown author on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/public_domain).  

Figure 18.2.2

Y Chromosome by Christinelmiller on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license. 

Figure 18.2.3

3D_Medical_Animation_Vas_Deferens by https://www.scientificanimations.com on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 18.2.4

Blausen_0399_FemaleReproSystem_01 by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

References 

Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Reactions. (2015, June 8). Hormones and gender transition. YouTube. https://www.youtube.com/watch?v=l5knvmy1Z3s&feature=youtu.be

TED-Ed. (2012, April 23). Sex determination: More complicated than you thought. YouTube. https://www.youtube.com/watch?v=kMWxuF9YW38&feature=youtu.be

TED-Ed. (2017, April 24). The evolution of animal genitalia - Menno Schilthuizen. YouTube. https://www.youtube.com/watch?v=vcPJkz-D5II&feature=youtu.be

 

Image shows a pictomicrograph of a protozoan parasite of the Giardia lamblia species. It is roughly cone-shaped, with several flagella trailing from the narrow end of it.

Woman looks thoughtfully out at the countryside.

 

Who's Who?

Figure 3.7.1 Identical twins show clearly the importance of genes in making us who we are. Genes would not be possible without nucleic acids.

Nucleic acids are the class of biochemical compounds that includes DNA and RNA. These molecules are built of small monomers called nucleotides. Many nucleotides bind together to form a chain called a polynucleotide. The nucleic acid DNA (deoxyribonucleic acid) consists of two polynucleotide chains or strands. Thus, DNA is sometimes called double-stranded. The nucleic acid RNA (ribonucleic acid) consists of just one polynucleotide chain or strand, so RNA is sometimes called single-stranded.

Each nucleotide consists of three smaller molecules:

1. A sugar molecule (the sugar deoxyribose in DNA and the sugar ribose in RNA)
2. A phosphate group
3. A nitrogen base

The nitrogen bases in a nucleic acid stick out from the backbone. There are four different nitrogen bases: cytosine, adenine, guanine, and either thymine (in DNA) or uracil (in RNA). In DNA, bonds form between bases on the two nucleotide chains and hold the chains together. Each type of base binds with just one other type of base: cytosine always binds with guanine, and adenine always binds with thymine. These pairs of bases are called complementary base  pairs.

The binding of complementary bases causes DNA molecules automatically to take their well-known double helix shape, which is shown in the animation in Figure 3.7.3. A double helix is like a spiral staircase. It forms naturally and is very strong, making the two polynucleotide chains difficult to break apart.

DNA Molecule. Hydrogen bonds between complementary bases help form the double helix of a DNA molecule. The letters A, T, G, and C stand for the bases adenine, thymine, guanine, and cytosine. The sequence of these four bases in DNA is a code that carries instructions for making proteins. Shown is a representation of how the double helix folds into a chromosome.

 

DNA makes up genes, and the sequence of bases in DNA makes up the genetic code. Between “starts” and “stops,” the code carries instructions for the correct sequence of amino acids in a protein. RNA uses the information in DNA to assemble the correct amino acids and help make the protein. The information in DNA is passed from parent cells to daughter cells whenever cells divide, and it is also passed from parents to offspring when organisms reproduce. This is how inherited characteristics are passed from one generation to the next.

ATP is Energy

There is one type of specialized nucleic acid that exists only as a monomer.  It stands apart from the other nucleic acids because it does not code for, or help create, proteins.   This molecule is ATP, which stands for adenosine triphosphate.  It consists of a sugar, adenosine, and three phosphate groups.  It's primary role is as the basic energy currency in the cell.  The way ATP works is all based on the phosphates.  As shown in Figure 3.7.4, a large amount of energy is stored in the bond between the second and third phosphate group.  When this bond is broken, it functions as an exothermic reaction and this energy can be used to power other processes taking place in the cell.

 

Reference

TED-Ed. (2012, November 26). DNA: The book of you - Joe Hanson. YouTube, 2012. https://www.youtube.com/watch?v=aeAL6xThfL8&feature=youtu.be

 

 

People tend to carry similar traits to their biological parents, as illustrated by the family tree. Beyond just appearance, you can also inherit traits from your parents that you can’t see.

Rebecca becomes very aware of this fact when she visits her new doctor for a physical exam. Her doctor asks several questions about her family medical history, including whether Rebecca has or had relatives with cancer. Rebecca tells her that her grandmother, aunt, and uncle — who have all passed away — had cancer. They all had breast cancer, including her uncle, and her aunt also had ovarian cancer. Her doctor asks how old they were when they were diagnosed with cancer. Rebecca is not sure exactly, but she knows that her grandmother was fairly young at the time, probably in her forties.

Rebecca’s doctor explains that while the vast majority of cancers are not due to inherited factors, a cluster of cancers within a family may indicate that there are mutations in certain genes that increase the risk of getting certain types of cancer, particularly breast and ovarian cancer. Some signs that cancers may be due to these genetic factors are present in Rebecca’s family, such as cancer with an early age of onset (e.g., breast cancer before age 50), breast cancer in men, and breast cancer and ovarian cancer within the same person or family.

Based on her family medical history, Rebecca’s doctor recommends that she see a genetic counselor, because these professionals can help determine whether the high incidence of cancers in her family could be due to inherited mutations in their genes. If so, they can test Rebecca to find out whether she has the particular variations of these genes that would increase her risk of getting cancer.

When Rebecca sees the genetic counselor, he asks how her grandmother, aunt, and uncle with cancer are related to her. She says that these relatives are all on her mother’s side — they are her mother’s mother and siblings. The genetic counselor records this information in the form of a specific type of family tree, called a pedigree, indicating which relatives had which type of cancer, and how they are related to each other and to Rebecca.

He also asks her ethnicity. Rebecca says that her family on both sides are Ashkenazi Jews (Jews whose ancestors came from central and eastern Europe). “But what does that have to do with anything?” she asks. The counselor tells Rebecca that mutations in two tumor-suppressor genes called BRCA1 and BRCA2, located on chromosome 17 and 13, respectively, are particularly prevalent in people of Ashkenazi Jewish descent and greatly increase the risk of getting cancer. About one in 40 Ashkenazi Jewish people have one of these mutations, compared to about one in 800 in the general population. Her ethnicity, along with the types of cancer, age of onset, and the specific relationships between her family members who had cancer, indicate to the counselor that she is a good candidate for genetic testing for the presence of these mutations.

Rebecca says that her 72-year-old mother never had cancer, nor had many other relatives on that side of the family. How could the cancers be genetic? The genetic counselor explains that the mutations in the BRCA1 and BRCA2 genes, while dominant, are not inherited by everyone in a family. Also, even people with mutations in these genes do not necessarily get cancer — the mutations simply increase their risk of getting cancer. For instance, 55 to 65 per cent of women with a harmful mutation in the BRCA1 gene will get breast cancer before age 70, compared to 12 per cent of women in the general population who will get breast cancer sometime over the course of their lives.

Rebecca is not sure she wants to know whether she has a higher risk of cancer. The genetic counselor understands her apprehension, but explains that if she knows that she has harmful mutations in either of these genes, her doctor will screen her for cancer more often and at earlier ages. Therefore, any cancers she may develop are likely to be caught earlier when they are often much more treatable. Rebecca decides to go through with the testing, which involves taking a blood sample, and nervously waits for her results.

Attributions

Figure 5.1.1

Family Tree [all individual face images] from Clker.com used and adapted by Christine Miller under a CC0 1.0 public domain dedication license (https://creativecommons.org/publicdomain/zero/1.0/).

Figure 5.1.2

Rebecca by Kyle Broad on Unsplash is used under the Unsplash License (https://unsplash.com/license).

References

Wikipedia contributors. (2020, June 27). Ashkenazi Jews. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Ashkenazi_Jews&oldid=964691647

Wikipedia contributors. (2020, June 22). BRCA1. In Wikipedia. https://en.wikipedia.org/w/index.php?title=BRCA1&oldid=963868423

Wikipedia contributors. (2020, May 25). BRCA2. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=BRCA2&oldid=958722957

Created by: CK-12/Adapted by Christine Miller

People tend to carry similar traits to their biological parents, as illustrated by the family tree. Beyond just appearance, you can also inherit traits from your parents that you can’t see.

Rebecca becomes very aware of this fact when she visits her new doctor for a physical exam. Her doctor asks several questions about her family medical history, including whether Rebecca has or had relatives with cancer. Rebecca tells her that her grandmother, aunt, and uncle — who have all passed away — had cancer. They all had breast cancer, including her uncle, and her aunt also had ovarian cancer. Her doctor asks how old they were when they were diagnosed with cancer. Rebecca is not sure exactly, but she knows that her grandmother was fairly young at the time, probably in her forties.

Rebecca’s doctor explains that while the vast majority of cancers are not due to inherited factors, a cluster of cancers within a family may indicate that there are mutations in certain genes that increase the risk of getting certain types of cancer, particularly breast and ovarian cancer. Some signs that cancers may be due to these genetic factors are present in Rebecca’s family, such as cancer with an early age of onset (e.g., breast cancer before age 50), breast cancer in men, and breast cancer and ovarian cancer within the same person or family.

Based on her family medical history, Rebecca’s doctor recommends that she see a genetic counselor, because these professionals can help determine whether the high incidence of cancers in her family could be due to inherited mutations in their genes. If so, they can test Rebecca to find out whether she has the particular variations of these genes that would increase her risk of getting cancer.

When Rebecca sees the genetic counselor, he asks how her grandmother, aunt, and uncle with cancer are related to her. She says that these relatives are all on her mother’s side — they are her mother’s mother and siblings. The genetic counselor records this information in the form of a specific type of family tree, called a pedigree, indicating which relatives had which type of cancer, and how they are related to each other and to Rebecca.

He also asks her ethnicity. Rebecca says that her family on both sides are Ashkenazi Jews (Jews whose ancestors came from central and eastern Europe). “But what does that have to do with anything?” she asks. The counselor tells Rebecca that mutations in two tumor-suppressor genes called BRCA1 and BRCA2, located on chromosome 17 and 13, respectively, are particularly prevalent in people of Ashkenazi Jewish descent and greatly increase the risk of getting cancer. About one in 40 Ashkenazi Jewish people have one of these mutations, compared to about one in 800 in the general population. Her ethnicity, along with the types of cancer, age of onset, and the specific relationships between her family members who had cancer, indicate to the counselor that she is a good candidate for genetic testing for the presence of these mutations.

Rebecca says that her 72-year-old mother never had cancer, nor had many other relatives on that side of the family. How could the cancers be genetic? The genetic counselor explains that the mutations in the BRCA1 and BRCA2 genes, while dominant, are not inherited by everyone in a family. Also, even people with mutations in these genes do not necessarily get cancer — the mutations simply increase their risk of getting cancer. For instance, 55 to 65 per cent of women with a harmful mutation in the BRCA1 gene will get breast cancer before age 70, compared to 12 per cent of women in the general population who will get breast cancer sometime over the course of their lives.

Rebecca is not sure she wants to know whether she has a higher risk of cancer. The genetic counselor understands her apprehension, but explains that if she knows that she has harmful mutations in either of these genes, her doctor will screen her for cancer more often and at earlier ages. Therefore, any cancers she may develop are likely to be caught earlier when they are often much more treatable. Rebecca decides to go through with the testing, which involves taking a blood sample, and nervously waits for her results.

Attributions

Figure 5.1.1

Family Tree [all individual face images] from Clker.com used and adapted by Christine Miller under a CC0 1.0 public domain dedication license (https://creativecommons.org/publicdomain/zero/1.0/).

Figure 5.1.2

Rebecca by Kyle Broad on Unsplash is used under the Unsplash License (https://unsplash.com/license).

References

Wikipedia contributors. (2020, June 27). Ashkenazi Jews. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Ashkenazi_Jews&oldid=964691647

Wikipedia contributors. (2020, June 22). BRCA1. In Wikipedia. https://en.wikipedia.org/w/index.php?title=BRCA1&oldid=963868423

Wikipedia contributors. (2020, May 25). BRCA2. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=BRCA2&oldid=958722957

Image shows young adult twin females.

Created by: CK-12/Adapted by Christine Miller

Identical Twins, Identical Genes

You probably can tell by their close resemblance that these two young ladies are identical twins (Figure 5.2.1). Identical twins develop from the same fertilized egg, so they inherit copies of the same chromosomes and have all the same genes. Unless you have an identical twin, no one else in the world has exactly the same genes as you. What are genes? How are they related to chromosomes? And how do genes make you the person you are? Let's find out!

Chromosomes are coiled structures made of DNA and proteins. They are encoded with genetic instructions for making RNA and proteins. These instructions are organized into units called genes. There may be hundreds (or even thousands!) of genes on a single chromosome. Genes are segments of DNA that code for particular pieces of RNA. Once formed, some RNA molecules go on to act as blueprints for building proteins, while other RNA molecules help regulate various processes inside the cell. Some regions of DNA do not code for RNA and serve a regulatory function, or have no known function.

Each species is characterized by a set number of chromosomes. Humans cells normally have two sets of chromosomes in each of their cells, one set inherited from each parent. Because chromosomes occur in pairs, these cells are called diploid or 2N. There are 23 chromosomes in each set, for a total of 46 chromosomes per diploid cell. Each chromosome in one set is matched by a chromosome of the same type in the other set, so there are 23 pairs of chromosomes per cell. Each pair consists of chromosomes of the same size and shape, and they also contain the same genes. The chromosomes in a pair are known as homologous chromosomes.

https://www.youtube.com/watch?v=veB31XmUQm8&feature=youtu.be

Secrets of the X chromosome - Robin Ball, TED-Ed, 2019.

Autosomes

Of the 23 pairs of human chromosomes, 22 pairs are called autosomes (pairs 1-22 in the Figure 5.2.2), or autosomal chromosomes. Autosomes are chromosomes that contain genes for characteristics that are unrelated to biological sex. These chromosomes are the same in males and females. The great majority of human genes are located on autosomes.

Sex Chromosomes

The remaining pair of human chromosomes consists of the sex chromosomes, X and Y (Pair 23 in Figure 5.2.2 and in Figure 5.2.3). Females have two X chromosomes, and males have one X and one Y chromosome. In females, one of the X chromosomes in each cell is inactivated and known as a Barr body. This ensures that females, like males, have only one functioning copy of the X chromosome in each cell.

As you can see from Figure 5.2.3, the X chromosome is much larger than the Y chromosome. The X chromosome has about two thousand genes, whereas the Y chromosome has fewer than 100, none of which is essential to survival. Virtually all of the X chromosome genes are unrelated to sex. Only the Y chromosome contains genes that determine sex. A single Y chromosome gene, called SRY (which stands for sex-determining region Y gene), triggers an embryo to develop into a male. Without a Y chromosome, an individual develops into a female, so you can think of female as the default sex of the human species.

Humans have an estimated 20 thousand to 22 thousand genes. This may sound like a lot, but it really isn’t. Far simpler species have almost as many genes as humans. However, human cells use splicing and other processes to make multiple proteins from the instructions encoded in a single gene. Only about 25 per cent of the nitrogen base pairs of DNA in human chromosomes make up genes and their regulatory elements. The functions of many of the other base pairs are still unclear, but with more time and research their roles may become understood.

The majority of human genes have two or more possible versions, called alleles. Differences in alleles account for the considerable genetic variation among people. In fact, most human genetic variation is the result of differences in individual DNA base pairs within alleles.

Genes that are located on the same chromosome are called linked genes. Linkage explains why certain characteristics are frequently inherited together. For example, genes for hair colour and eye colour are linked, so certain hair and eye colours tend to be inherited together, such as dark hair with dark eyes and blonde hair with blue eyes. Can you think of other human traits that seem to occur together? Do you think they might be controlled by linked genes?

Genes located on the sex chromosomes are called sex-linked genes. Most sex-linked genes are on the X chromosome, because the Y chromosome has relatively few genes. Strictly speaking, genes on the X chromosome are X-linked genes, but the term sex-linked is often used to refer to them. The diagram below is called a linkage map: a linkage map shows the locations of specific genes on a chromosome. The linkage map below (Figure 5.2.4) shows the locations of a few of the genes on the human X chromosome.

Figure 5.2.4 Linkage Map for the Human X Chromosome. This linkage map shows the locations of several genes on the X chromosome. Some of the genes code for normal proteins. Others code for abnormal proteins that lead to genetic disorders.

Attributions

Figure 5.2.1

Twins5 [photo] by Bùi Thanh Tâm on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 5.2.2

Human_male_karyotype by National Human Genome Research Institute/ NIH  on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain). (Original from the Talking Glossary of Genetics.)

Figure 5.2.3

Comparison between X and Y chromosomes byJonathan Bailey, National Human Genome Research Institute, National Institutes of Health [NIH] Image Gallery, on Flickr is used under a CC BY-NC 2.0 (https://creativecommons.org/licenses/by-nc/2.0/) license.

Figure 5.2.4

Linkage Map of Human X Chromosome by Christine Miller is used under a
CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

 

References

Atomi. (2019, October 27). WACE Biology: Coding and Non-Coding DNA. YouTube. https://www.youtube.com/watch?v=M4ut72kfUJM&feature=youtu.be

Seeker. (2015, July 26). How Sex Genes Are More Complicated Than You Thought. YouTube. https://www.youtube.com/watch?v=jhHGCvMlrb0&feature=youtu.be

TED-Ed. (2017, April 18). Secrets of the X chromosome - Robin Ball. YouTube. https://www.youtube.com/watch?v=veB31XmUQm8&feature=youtu.be

 

This young woman has naturally red hair (Figure 5.3.1). Why is her hair red instead of some other colour? In general, what gives her the specific traits she has? There is a molecule in human beings and most other living things that is largely responsible for their traits. The molecule is large and has a spiral structure in eukaryotes. What molecule is it? With these hints, you probably know that the molecule is DNA.

Today, it is commonly known that DNA is the genetic material that is passed from parents to offspring and determines our traits. For a long time, scientists knew such molecules existed — that is, they were aware that genetic information is contained within biochemical molecules. What they didn’t know was which specific molecules play this role. In fact, for many decades, scientists thought that proteinsno post were the molecules that contain genetic information.

Determining that DNA is the genetic material was an important milestone in biology. It took many scientists undertaking creative experiments over several decades to show with certainty that DNA is the molecule that determines the traits of organisms. This research began in the early part of the 20th century.

Griffith's Experiments with Mice

One of the first important discoveries was made in the 1920s by an American scientist named Frederick Griffith.  Griffith was studying mice and two different strains of a bacterium, called R (rough)-strain and S (smooth)-strain. He injected the two bacterial strains into mice. The S-strain was virulent and killed the mice, whereas the R-strain was not virulent and did not kill the mice. You can see these details in Figure 5.3.2. Griffith also injected mice with S-strain bacteria that had been killed by heat. As expected, the dead bacteria did not harm the mice. However, when the dead S-strain bacteria were mixed with live R-strain bacteria and injected, the mice died.

Based on his observations, Griffith deduced that something in the dead S-strain was transferred to the previously harmless R-strain, making the R-strain deadly. What was this "something?" What type of substance could change the characteristics of the organism that received it?

Avery and His Colleagues Make a Major Contribution

In the early 1940s, a team of scientists led by Canadian-American Oswald Avery tried to answer the question raised by Griffith’s research results. First, they inactivated various substances in the S-strain bacteria. Then they killed the S-strain bacteria and mixed the remains with live R-strain bacteria. (Keep in mind that the R-strain bacteria normally did not harm the mice.) When they inactivated proteinsno post, the R-strain was deadly to the injected mice. This ruled out proteins as the genetic material. Why? Even without the S-strain proteins, the R-strain was changed (or transformed) into the deadly strain. However, when the researchers inactivated DNA in the S-strain, the R-strain remained harmless. This led to the conclusion that DNA — and not protein — is the substance that controls the characteristics of organisms. In other words, DNA is the genetic material.

Hershey and Chase Confirm the Results

The conclusion that DNA is the genetic material was not widely accepted until it was confirmed by additional research. In the 1950s, Alfred Hershey and Martha Chase did experiments with viruses and bacteria. Viruses are not cells. Instead, they are basically DNA (or RNA) inside a protein coat. To reproduce, a virus must insert its own genetic material into a cell (such as a bacterium). Then, it uses the cell’s machinery to make more viruses. The researchers used different radioactive elements to label the DNA and proteins in DNA viruses. This allowed them to identify which molecule the viruses inserted into bacterial cells. DNA was the molecule they identified. This confirmed that DNA is the genetic material.

Other important discoveries about DNA were made in the mid-1900s by Erwin Chargaff. He studied DNA from many different species and was especially interested in the four different nitrogen bases of DNA: adenine (A), guanine (G), cytosine (C), and thymine (T). Chargaff found that concentrations of the four bases differed between species. Within any given species, however, the concentration of adenine was always the same as the concentration of thymine, and the concentration of guanine was always the same as the concentration of cytosine. These observations came to be known as Chargaff’s rules. The significance of the rules would not be revealed until the double-helix structure of DNA was discovered.

After DNA was shown to be the genetic material, scientists wanted to learn more about its structure and function. James Watson and Francis Crick are usually given credit for discovering that DNA has a double helix shape, as shown in Figure 5.3.3. In fact, Watson and Crick's discovery of the double helix depended heavily on the prior work of Rosalind Franklin and other scientists, who had used X-rays to learn more about DNA’s structure. Unfortunately, Franklin and these others have not always been given credit for their important contributions to the discovery of the double helix.

The double helix shape of DNA, along with Chargaff’s rules, led to a better understanding of DNA. As a nucleic acid, DNA is made from nucleotide monomers. Long chains of nucleotides form polynucleotides, and the DNA double helix consists of two polynucleotide chains. Each nucleotide consists of a sugar (deoxyribose), a phosphate group, and one of the four bases (adenine, cytosine, guanine, or thymine). The sugar and phosphate molecules in adjacent nucleotides bond together and form the "backbone" of each polynucleotide chain.

Scientists concluded that bonds between the bases hold together the two polynucleotide chains of DNA. Moreover, adenine always bonds with thymine, and cytosine always bonds with guanine. That's why these pairs of bases are called complementary base pairs.  Adenine and guanine have a two-ring structure, whereas cytosine and thymine have just one ring. If adenine were to bond with guanine, as well as thymine, for example, the distance between the two DNA chains would vary. When a one-ring molecule (like thymine) always bonds with a two-ring molecule (like adenine), however, the distance between the two chains remains constant. This maintains the uniform shape of the DNA double helix. The bonded base pairs (A-T and G-C) stick into the middle of the double helix, forming the "steps" of the spiral staircase.

 

Attributions

Figure 5.3.1

Redhead [photo] by Hichem Dahmani on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 5.3.2

Griffith’s mice by Mariana Ruiz Villarreal [LadyofHats] for CK-12 Foundation is used under a
CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

©CK-12 Foundation Licensed under  • Terms of Use • Attribution

Figure 5.3.3

DNA_Overview by Michael Ströck [mstroeck] on Wikimedia Commons is used under a CC BY SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/) license.

References

Brainard, J/ CK-12. (2012). Concentration. In Physical Science [website]. CK12.org. https://www.ck12.org/c/physical-science/concentration/?referrer=crossref

OpenMind. (2017, September 11). The discovery of the structure of DNA. YouTube. https://www.youtube.com/watch?v=V6bKn34nSbk&feature=youtu.be

SciShow. (2013, July 9). Rosalind Franklin: Great minds. YouTube. https://www.youtube.com/watch?v=JiME-W58KpU&feature=youtu.be

Wikipedia contributors. (2020, June 27). Alfred Hershey. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Alfred_Hershey&oldid=964789559

Wikipedia contributors. (2020, June 5). Erwin Chargaff. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Erwin_Chargaff&oldid=960942873

Wikipedia contributors. (2020, June 29). Francis Crick. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Francis_Crick&oldid=965135362

Wikipedia contributors. (2020, July 6). Frederick Griffith. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Frederick_Griffith&oldid=966352134

Wikipedia contributors. (2020, July 5). James Watson. In Wikipedia. https://en.wikipedia.org/w/index.php?title=James_Watson&oldid=966111944

Wikipedia contributors. (2020, March 31). Martha Chase. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Martha_Chase&oldid=948408219

Wikipedia contributors. (2020, July 2). Oswald Avery. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Oswald_Avery&oldid=965632585

Wikipedia contributors. (2020, June 30). Rosalind Franklin. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Rosalind_Franklin&oldid=965334881

 

Created by CK-12 Foundation/Adapted by Christine Miller

The many chimneys on these houses are one way that the inhabitants of the home get rid of the wastes they produce. The chimneys expel waste gases that are created when they burn fuel in their furnace or fireplace. Think about the other wastes that people create in their homes and how we dispose of them. Solid trash and recyclables may go to the curb in a trash can, or in a recycling bin for pick up and transport to a landfill or recycling centre. Wastewater from sinks, showers, toilets, and the washing machine goes into a main sewer pipe and out of the house to join the community’s sanitary sewer system.

Like a busy home, your body also produces a lot of wastes that must be eliminated. Like a home, the way your body gets rid of wastes depends on the nature of the waste products. Some human body wastes are gases, some are solids, and some are in a liquid state. Getting rid of body wastes is called excretion, and there are a number of different organs of excretion in the human body.

Excretion is the process of removing wastes and excess water from the body. It is an essential process in all living things, and it is one of the major ways the human body maintains homeostasis. It also helps prevent damage to the body. Wastes include by-products of metabolism — some of which are toxic — and other non-useful materials, such as used up and broken down components. Some of the specific waste products that must be excreted from the body include carbon dioxide from cellular respiration, ammonia and urea from protein catabolism, and uric acid from nucleic acid catabolism.

Organs of excretion include the skin, liver, large intestine, lungs, and kidneys (see Figure 16.2.2). Together, these organs make up the excretory system. They all excrete wastes, but they don’t work together in the same way that organs do in most other body systems. Each of the excretory organs “does its own thing” more-or-less independently of the others, but all are necessary to successfully excrete the full range of wastes from the human body.

Skin

The skin is part of the integumentary system, but it also plays a role in excretion through the production of sweat by sweat glands in the dermis. Although the main role of sweat production is to cool the body and maintain temperature homeostasis, sweating also eliminates excess water and salts, as well as a small amount of urea. When sweating is copious, as in Figure 16.2.3, ingestion of salts and water may be helpful to maintain homeostasis in the body.

Liver

The liver (shown in Figure 16.2.4) has numerous major functions, including secreting bile for digestion of lipids, synthesizing many proteins and other compounds, storing glycogen and other substances, and secreting endocrine hormones. In addition to all of these functions, the liver is a very important organ of excretion. The liver breaks down many substances in the blood, including toxins. For example, the liver transforms ammonia — a poisonous by-product of protein catabolism — into urea, which is filtered from the blood by the kidneys and excreted in urine. The liver also excretes in its bile the protein bilirubin, a byproduct of hemoglobin catabolism that forms when red blood cells die. Bile travels to the small intestine and is then excreted in feces by the large intestine.

Large Intestine

The large intestine is an important part of the digestive system and the final organ in the gastrointestinal tract. As an organ of excretion, its main function is to eliminate solid wastes that remain after the digestion of food and the extraction of water from indigestible matter in food waste. The large intestine also collects wastes from throughout the body. Bile secreted into the gastrointestinal tract, for example, contains the waste product bilirubin from the liver. Bilirubin is a brown pigment that gives human feces its characteristic brown colour.

Lungs

The lungs are part of the respiratory system (shown in Figure 16.2.5), but they are also important organs of excretion. They are responsible for the excretion of gaseous wastes from the body. The main waste gas excreted by the lungs is carbon dioxide, which is a waste product of cellular respiration in cells throughout the body. Carbon dioxide is diffused from the blood into the air in the tiny air sacs called alveoli in the lungs (shown in the inset diagram). By expelling carbon dioxide from the blood, the lungs help maintain acid-base homeostasis. In fact, it is the pH of blood that controls the rate of breathing. Water vapor is also picked up from the lungs and other organs of the respiratory tract as the exhaled air passes over their moist linings, and the water vapor is excreted along with the carbon dioxide. Trace levels of some other waste gases are exhaled, as well.

Kidneys

The paired kidneys are often considered the main organs of excretion. The primary function of the kidneys is the elimination of excess water and wastes from the bloodstream by the production of the liquid waste known as urine. The main structural and functional units of the kidneys are tiny structures called nephrons. Nephrons filter materials out of the blood, return to the blood what is needed, and excrete the rest as urine. As shown in Figure 16.2.6, the kidneys are organs of the urinary system, which also includes the ureters, bladder, and urethra — organs that transport, store, and eliminate urine, respectively.

By producing and excreting urine, the kidneys play vital roles in body-wide homeostasis. They maintain the correct volume of extracellular fluid, which is all the fluid in the body outside of cells, including the blood and lymph. The kidneys also maintain the correct balance of salts and pH in extracellular fluid. In addition, the kidneys function as endocrine glands, secreting hormones into the blood that control other body processes. You can read much more about the kidneys in section 16.4 Kidneys.

Figure 16.2.3

Created by: CK-12/Adapted by Christine Miller

This young woman has naturally red hair (Figure 5.3.1). Why is her hair red instead of some other colour? In general, what gives her the specific traits she has? There is a molecule in human beings and most other living things that is largely responsible for their traits. The molecule is large and has a spiral structure in eukaryotes. What molecule is it? With these hints, you probably know that the molecule is DNA.

Today, it is commonly known that DNA is the genetic material that is passed from parents to offspring and determines our traits. For a long time, scientists knew such molecules existed — that is, they were aware that genetic information is contained within biochemical molecules. What they didn’t know was which specific molecules play this role. In fact, for many decades, scientists thought that proteinsno post were the molecules that contain genetic information.

Determining that DNA is the genetic material was an important milestone in biology. It took many scientists undertaking creative experiments over several decades to show with certainty that DNA is the molecule that determines the traits of organisms. This research began in the early part of the 20th century.

Griffith's Experiments with Mice

One of the first important discoveries was made in the 1920s by an American scientist named Frederick Griffith.  Griffith was studying mice and two different strains of a bacterium, called R (rough)-strain and S (smooth)-strain. He injected the two bacterial strains into mice. The S-strain was virulent and killed the mice, whereas the R-strain was not virulent and did not kill the mice. You can see these details in Figure 5.3.2. Griffith also injected mice with S-strain bacteria that had been killed by heat. As expected, the dead bacteria did not harm the mice. However, when the dead S-strain bacteria were mixed with live R-strain bacteria and injected, the mice died.

Based on his observations, Griffith deduced that something in the dead S-strain was transferred to the previously harmless R-strain, making the R-strain deadly. What was this "something?" What type of substance could change the characteristics of the organism that received it?

Avery and His Colleagues Make a Major Contribution

In the early 1940s, a team of scientists led by Canadian-American Oswald Avery tried to answer the question raised by Griffith’s research results. First, they inactivated various substances in the S-strain bacteria. Then they killed the S-strain bacteria and mixed the remains with live R-strain bacteria. (Keep in mind that the R-strain bacteria normally did not harm the mice.) When they inactivated proteinsno post, the R-strain was deadly to the injected mice. This ruled out proteins as the genetic material. Why? Even without the S-strain proteins, the R-strain was changed (or transformed) into the deadly strain. However, when the researchers inactivated DNA in the S-strain, the R-strain remained harmless. This led to the conclusion that DNA — and not protein — is the substance that controls the characteristics of organisms. In other words, DNA is the genetic material.

Hershey and Chase Confirm the Results

The conclusion that DNA is the genetic material was not widely accepted until it was confirmed by additional research. In the 1950s, Alfred Hershey and Martha Chase did experiments with viruses and bacteria. Viruses are not cells. Instead, they are basically DNA (or RNA) inside a protein coat. To reproduce, a virus must insert its own genetic material into a cell (such as a bacterium). Then, it uses the cell’s machinery to make more viruses. The researchers used different radioactive elements to label the DNA and proteins in DNA viruses. This allowed them to identify which molecule the viruses inserted into bacterial cells. DNA was the molecule they identified. This confirmed that DNA is the genetic material.

Other important discoveries about DNA were made in the mid-1900s by Erwin Chargaff. He studied DNA from many different species and was especially interested in the four different nitrogen bases of DNA: adenine (A), guanine (G), cytosine (C), and thymine (T). Chargaff found that concentrations of the four bases differed between species. Within any given species, however, the concentration of adenine was always the same as the concentration of thymine, and the concentration of guanine was always the same as the concentration of cytosine. These observations came to be known as Chargaff’s rules. The significance of the rules would not be revealed until the double-helix structure of DNA was discovered.

After DNA was shown to be the genetic material, scientists wanted to learn more about its structure and function. James Watson and Francis Crick are usually given credit for discovering that DNA has a double helix shape, as shown in Figure 5.3.3. In fact, Watson and Crick's discovery of the double helix depended heavily on the prior work of Rosalind Franklin and other scientists, who had used X-rays to learn more about DNA’s structure. Unfortunately, Franklin and these others have not always been given credit for their important contributions to the discovery of the double helix.

The double helix shape of DNA, along with Chargaff’s rules, led to a better understanding of DNA. As a nucleic acid, DNA is made from nucleotide monomers. Long chains of nucleotides form polynucleotides, and the DNA double helix consists of two polynucleotide chains. Each nucleotide consists of a sugar (deoxyribose), a phosphate group, and one of the four bases (adenine, cytosine, guanine, or thymine). The sugar and phosphate molecules in adjacent nucleotides bond together and form the "backbone" of each polynucleotide chain.

Scientists concluded that bonds between the bases hold together the two polynucleotide chains of DNA. Moreover, adenine always bonds with thymine, and cytosine always bonds with guanine. That's why these pairs of bases are called complementary base pairs.  Adenine and guanine have a two-ring structure, whereas cytosine and thymine have just one ring. If adenine were to bond with guanine, as well as thymine, for example, the distance between the two DNA chains would vary. When a one-ring molecule (like thymine) always bonds with a two-ring molecule (like adenine), however, the distance between the two chains remains constant. This maintains the uniform shape of the DNA double helix. The bonded base pairs (A-T and G-C) stick into the middle of the double helix, forming the "steps" of the spiral staircase.

 

Attributions

Figure 5.3.1

Redhead [photo] by Hichem Dahmani on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 5.3.2

Griffith’s mice by Mariana Ruiz Villarreal [LadyofHats] for CK-12 Foundation is used under a
CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

©CK-12 Foundation Licensed under  • Terms of Use • Attribution

Figure 5.3.3

DNA_Overview by Michael Ströck [mstroeck] on Wikimedia Commons is used under a CC BY SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/) license.

References

Brainard, J/ CK-12. (2012). Concentration. In Physical Science [website]. CK12.org. https://www.ck12.org/c/physical-science/concentration/?referrer=crossref

OpenMind. (2017, September 11). The discovery of the structure of DNA. YouTube. https://www.youtube.com/watch?v=V6bKn34nSbk&feature=youtu.be

SciShow. (2013, July 9). Rosalind Franklin: Great minds. YouTube. https://www.youtube.com/watch?v=JiME-W58KpU&feature=youtu.be

Wikipedia contributors. (2020, June 27). Alfred Hershey. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Alfred_Hershey&oldid=964789559

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