3.4 Carbohydrates
The Cellulose of Our Lives
Where would we be without our jeans? They have been the go-to pants for many people for decades, and they are still as popular as ever. Jeans are made of denim, a type of cotton fabric. Cotton is a soft, fluffy fibre that grows in a protective case around the seeds of cotton plants. The fibre is almost pure cellulose. Cellulose is the single most abundant biochemical compound found in Earth’s living things, and it’s one of several types of carbohydrates.
What Are Carbohydrates?
are the most common class of biochemical compounds. They include sugars and starches. Carbohydrates are used to provide or store , among other uses. Like most biochemical compounds, carbohydrates are built of small repeating units, or monomers, which form bonds with each other to make larger molecules, called polymers. In the case of carbohydrates, the small repeating units are known as . Each monosaccharide consists of six carbon atoms, as shown in the model of the monosaccharide glucose shown in Figure 3.4.2.
Sugars
are the general name for sweet, short-chain, soluble carbohydrates, which are found in many foods. Their function in living things is to provide . The simplest sugars consist of a single . They include glucose, fructose, and galactose. is a simple sugar that is used for energy by the cells of living things. Fructose is a simple sugar found in fruits, and galactose is a simple sugar found in milk. Their chemical structures are shown in Figure 3.4.3. All monosaccharides have the formula C6H12O6.
Other sugars contain two molecules and are called . These include sucrose (table sugar), maltose, and lactose. Sucrose is composed of one fructose molecule and one glucose molecule, maltose is composed of two glucose molecules, and lactose is composed of one glucose molecule and one galactose molecule. Lactose occurs naturally in milk. Some people are lactose intolerant because they can’t digest lactose. If they drink milk, it causes gas, cramps, and other unpleasant symptoms, unless the milk has been processed to remove the lactose.
Complex Carbohydrates
Some carbohydrates consist of hundreds — or even thousands! — of bonded together in long chains. These carbohydrates are called (“many saccharides”). Polysaccharides are also referred to as . Complex carbohydrates that are found in living things include starch, glycogen, cellulose, and chitin. Each type of complex has different functions in living organisms, but they generally either store energy or make up certain structures in living things.
Starch
is a complex carbohydrate that is made by plants to store energy. For example, the potatoes pictured in Figure 3.4.4 are packed full of starches that consist mainly of repeating units of and other simple sugars. The leaves of potato plants make sugars by , and the sugars are carried to underground tubers where they are stored as starch. When we eat starchy foods such as potatoes, the starches are broken down by our digestive system into sugars, which provide our with energy. Starches are easily and quickly digested with the help of digestive such as amylase, which is found in the saliva. If you chew a starchy saltine cracker for several minutes, you may start to taste the sugars released as the starch is digested.
Glycogen
Animals do not store energy as starch. Instead, animals store extra energy as the complex carbohydrate glycogen. Glycogen is a of . It serves as a form of energy storage in fungi (as well as animals), and it is the main storage form of glucose in the human body. In humans, glycogen is made and stored primarily in the cells of the liver and muscles. When energy is needed from either storage area, the glycogen is broken down to glucose for use by cells. Muscle glycogen is converted to glucose for use by muscle cells, and liver glycogen is converted to glucose for use throughout the rest of the body. Glycogen forms an energy reserve that can be quickly mobilized to meet a sudden need for glucose, but one that is less compact than the energy reserves of lipids, which are the primary form of energy storage in animals.
Glycogen plays a critical part in the of glucose levels in the blood. When blood glucose levels rise too high, excess glucose can be stored in the liver by converting it to glycogen. When glucose levels in the blood fall too low, glycogen in the liver can be broken down to glucose and released into the blood.
Cellulose
is a polysaccharide consisting of a linear chain of several hundred to many thousands of linked units. Cellulose is an important structural component of the cell walls of plants and many algae. Human uses of cellulose include the production of cardboard and paper, which consist mostly of cellulose from wood and cotton. The cotton fibres pictured are about 90 per cent cellulose.
Certain animals, including termites and ruminants such as cows, can digest cellulose with the help of microorganisms that live in their gut. Humans cannot digest cellulose, but it nonetheless plays an important role in our diet. It acts as a water-attracting bulking agent for feces in the digestive tract and is often referred to as “dietary fibre.” In simpler terms, it helps you poop.
Chitin
is a long-chain polymer of a derivative of . It is found in many living things. For example, it is a component of the cell walls of fungi; the exoskeletons of arthropods, such as crustaceans and insects ; and the beaks and internal shells of animals, such as squids and octopuses. The structure of chitin is similar to that of cellulose.
In Figure 3.4.7, both the exoskeleton of the ladybug and the cell walls of the mushroom are made partly of the complex carbohydrate chitin.
The Right Molecule for the Job
Starch, glycogen, cellulose and chitin are all made from the . So how are they all so different? Their difference in structure and function is related to how they are linked together. Starch is linked in long chains with a small amount of branching, glycogen is linked in many branching chains, and chitin and cellulose form long single chains that pack together tightly. Each of these variations of linking the same monomer, glucose, together creates a different way the molecule can be used. As shown in the Figure 3.4.8 diagram, starch and glycogen have many exposed “ends” of their chains. These are areas where a glucose molecule can easily be removed for use as energy, whereas cellulose does not. For this reason, glycogen and starch are well-suited for energy storage in organisms while cellulose is not. Conversely, cellulose packs many monomers together in a sort of mesh that is very strong — this is why it is a great option for building strong cell walls.
Feature: My Human Biology
You probably know that you should eat plenty of fibre, but do you know how much fibre you need, how fibre contributes to good health, or which foods are good sources of fibre? Dietary fibre consists mainly of cellulose, so it is found primarily in plant-based foods, including fruits, vegetables, whole grains, and legumes. Dietary fibre can’t be broken down and absorbed by your digestive system. Instead, it passes relatively unchanged through your gastrointestinal tract and is excreted in feces (otherwise known as poop). That’s how it helps keep you healthy.
Fibre in food is commonly classified as either soluble or insoluble fibre.
- Soluble fibre dissolves in water to form a gel-like substance as it passes through the gastrointestinal tract. It lowers blood levels of cholesterol and glucose, which is beneficial for your health. Good sources of soluble fibre include whole oats, peas, beans, and apples.
- Insoluble fibre does not dissolve in water. This type of fibre increases the bulk of feces in the large intestine, and helps keep food wastes moving through, which may help prevent or correct constipation. Good sources of insoluble fibre include whole wheat, wheat bran, beans, and potatoes.
How much fibre do you need for good health? That depends on your age and gender. The Institute of Medicine recommends the daily fibre intake for adults shown in Table 3.4.1 below. Most dietitians further recommend a ratio of about three parts of insoluble fibre to one part of soluble fibre each day. Most fibre-rich foods contain both types of fibre, so it usually isn’t necessary to keep track of the two types of fibre as long as your overall fibre intake is adequate.
Table 3.4.1
Recommended Daily Fibre Intake for Males and Females
| Recommended Daily Fibre Intake for Males and Females | ||
| Gender | Age 50 or Younger | Age 51 or Older |
| Male | 38 grams | 30 grams |
| Female | 25 grams | 21 grams |
Use food labels like the one shown below in Figure 3.4.10 and online fibre counters to find out how much total fibre you eat in a typical day. Are you consuming enough fibre for good health? If not, consider ways to increase your intake of this important substance. For example, substitute whole grains for refined grains, eat more legumes (such as beans), and try to consume at least five servings of fruits and vegetables each day.
Table 3.4.2
Carbohydrate Comparison
Name |
Class |
Function |
Location |
| Glucose | Monosaccharide | Energy for cells | Cells |
| Starch | Polysaccharide | Energy storage | Plant cells |
| Glycogen | Polysaccharide | Energy storage | Animal cells |
| Cellulose | Polysaccharide | Structural component in cell walls | Plant cells |
| Chitin | Polysaccharide | Structural component in cell walls and exoskeletons | Fungi and arthropods |
3.4 Summary
- are the most common class of biochemical compounds. The basic building block of carbohydrates is the monosaccharide, which consists of six carbon atoms.
- Sugars are sweet, short-chain, soluble carbohydrates that are found in many foods and supply us with energy. Simple sugars, such as , consist of just one . Some sugars, such as sucrose (or table sugar), consist of two monosaccharides. These are called .
- , or , consist of hundreds — or even thousands — of monosaccharides. They include , glycogen, , and . They generally either store energy or form structures, such as cell walls, in living things.
- Starch is a complex carbohydrate that is made by plants to store energy. Potatoes are a good food source of dietary starch, which is readily broken down into its component sugars during digestion.
- Glycogen is a complex carbohydrate that is made by animals and fungi to store energy. Glycogen plays a critical part in the homeostasis of blood glucose levels in humans.
- Cellulose is the single most common biochemical compound in living things. It forms the cell walls of plants and certain algae. Like most other animals, humans cannot digest cellulose, but it makes up most of the crucial dietary fibre in the human diet.
- Chitin is a complex carbohydrate, similar to cellulose, that makes up organic structures, such as the cell walls of fungi and the exoskeletons of insects and other arthropods.
3.4 Review Questions
- What are carbohydrates? Describe their structure.
- Compare and contrast sugars and complex carbohydrates.
- If you chew on a starchy food (such as a saltine cracker) for several minutes, it may start to taste sweet. Explain why.
- True or False: Glucose is mainly stored by lipids in the human body.
- Name three carbohydrates that contain glucose as a monomer.
- Jeans are made of tough, durable cotton. Based on what you know about the structure of carbohydrates, explain how you think this fabric gets its tough qualities.
- Which do you think is faster to digest — simple sugars or complex carbohydrates? Explain your answer.
- True or False: Cellulose is broken down in the human digestive system into glucose molecules.
- ___________ fibre dissolves in water, __________ fibre does not dissolve in water.
- What are the similarities and differences between muscle glycogen and liver glycogen?
- Which carbohydrate is used directly by the cells of living things for energy?
- Which of the following is not a complex carbohydrate?
- Chitin
- Starch
- Disaccharide
- None of the above
3.4 Explore More
How do carbohydrates impact your health? – Richard J. Wood, TED-Ed, 2016
Why is cotton in everything? – Michael R. Stiff, TED-Ed, 2020
Attributions
Figure 3.4.1
Jeans by Maude Frédérique Lavoie on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 3.4.2
e-from-xtal-1979-Alpha-D-glucose-from-xtal-1979-3D-balls by Ben Mills [Benjah-bmm27] on Wikimedia Commons, is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 3.4.3
Monosasccharides by OpenStax College on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.
Figure 3.4.4
Potatoes by Jean Beaufort, on Public Domain Pictures.net, is used under a CC0 1.0 Universal Public Domain Dedication license (https://creativecommons.org/publicdomain/zero/1.0/).
Figure 3.4.5
Homeostasis_of_blood_sugar by Christine Miller [christinelmiller] Is used under a CC0 1.0 Universal Public Domain Dedication license (https://creativecommons.org/publicdomain/zero/1.0/).
Figure 3.4.6
Cotton by David Nance for Agricultural Research Service, the research agency of the United States Department of Agriculture, on Wikimedia Commons, is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 3.4.7
Ladybug on a mushroom /Fungi in the Woods by Benjamin Balázs on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 3.4.8
Carbohydrate structure comparison [Three Important Polysaccharides] by OpenStax College is on Wikimedia Commons, used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 3.4.9
Beans by Milada Vigerova on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 3.4.10
FDA Nutrition Facts Label 2014, by US Food and Drug Administration, on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Table 3.4.1
Recommended Daily Fibre Intake for Males and Females is from OpenStax, used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license..
Table 3.4.2
Carbohydrate Comparison is from OpenStax. used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.
References
Betts, J.G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 2.18. Five important monosaccharides [image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/1-introduction
Betts, J.G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 2.20. Three important polysaccharides [image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/1-introduction
Mayo Clinic. (n.d.). Lactose intolerance [online article]. Mayo Foundation for Medical Education and Research (MFMER). https://www.mayoclinic.org/diseases-conditions/lactose-intolerance/symptoms-causes/syc-20374232
TED-Ed. (2016, January 11). How do carbohydrates impact your health? – Richard J. Wood. YouTube. https://youtu.be/wxzc_2c6GMg
TED-Ed. (2020, January 23). Why is cotton in everything? – Michael R. Stiff. https://www.youtube.com/watch?v=tKLJ6KQAcjI
A biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1. Complex carbohydrates are polymers made from monomers of simple carbohydrates, also termed monosaccharides.
The ability to do work.
The simplest form of sugar and the most basic units of carbohydrates, also called simple sugars.
Glucose (also called dextrose) is a simple sugar with the molecular formula C6H12O6. Glucose is the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using energy from sunlight.
A sugar composed of two linked monosaccharides.
Polysaccharides are carbohydrate molecules composed of long chains of monosaccharide units bound together. They range in structure from linear to highly branched.
A polysaccharide (such as starch, cellulose or chitin) consisting of usually hundreds or thousands of monosaccharide units.
A stored form of glucose used by plants.
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that can later be released to fuel the organisms' activities.
The smallest unit of life, consisting of at least a membrane, cytoplasm, and genetic material.
Biological molecules that lower amount the energy required for a reaction to occur.
Created by: CK-12/Adapted by Christine Miller
Case Study: Cancer in the Family
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.
Chapter Overview: Genetics
At the end of this chapter, you will find out Rebecca’s test results. By then, you will have learned how traits are inherited from parents to offspring through genes, and how mutations in genes such as BRCA1 and BRCA2 can be passed down and cause disease. Specifically, you will learn about:
- The structure of DNA.
- How DNA replication occurs.
- How DNA was found to be the inherited genetic material.
- How genes and their different alleles are located on chromosomes.
- The 23 pairs of human chromosomes, which include autosomal and sex chromosomes.
- How genes code for proteins using codons made of the sequence of nitrogen bases within RNA and DNA.
- The central dogma of molecular biology, which describes how DNA is transcribed into RNA, and then translated into proteins.
- The structure, functions, and possible evolutionary history of RNA.
- How proteins are synthesized through the transcription of RNA from DNA and the translation of protein from RNA, including how RNA and proteins can be modified, and the roles of the different types of RNA.
- What mutations are, what causes them, different specific types of mutations, and the importance of mutations in evolution and to human health.
- How the expression of genes into proteins is regulated and why problems in this process can cause diseases, such as cancer.
- How Gregor Mendel discovered the laws of inheritance for certain types of traits.
- The science of heredity, known as genetics, and the relationship between genes and traits.
- How gametes, such as eggs and sperm, are produced through meiosis.
- How sexual reproduction works on the cellular level and how it increases genetic variation.
- Simple Mendelian and more complex non-Mendelian inheritance of some human traits.
- Human genetic disorders, such as Down syndrome, hemophilia A, and disorders involving sex chromosomes.
- How biotechnology — which is the use of technology to alter the genetic makeup of organisms — is used in medicine and agriculture, how it works, and some of the ethical issues it may raise.
- The human genome, how it was sequenced, and how it is contributing to discoveries in science and medicine.
As you read this chapter, keep Rebecca’s situation in mind and think about the following questions:
- BCRA1 and BCRA2 are also called Breast cancer type 1 and 2 susceptibility proteins. What do the BRCA1 and BRCA2 genes normally do? How can they cause cancer?
- Are BRCA1 and BRCA2 linked genes? Are they on autosomal or sex chromosomes?
- After learning more about pedigrees, draw the pedigree for cancer in Rebecca’s family. Use the pedigree to help you think about why it is possible that her mother does not have one of the BRCA gene mutations, even if her grandmother, aunt, and uncle did have it.
- Why do you think certain gene mutations are prevalent in certain ethnic groups?
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
The ability of an organism to maintain constant internal conditions despite external changes.
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 and have all the same genes. Unless you have an identical twin, no one else in the world has exactly the same 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!
Introducing Chromosomes and Genes
are coiled structures made of and . They are encoded with genetic instructions for making and . These instructions are organized into units called . 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.
Human Chromosomes
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 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 .
All human cells (except gametes, which are sperm and egg cells) have the 23 pairs of chromosomes as shown in Figure 5.2.2.
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. 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 . 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.
Human Genes
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 . 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.
Linkage
Genes that are located on the same chromosome are called . 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 . 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 , 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.
5.2 Summary
- are coiled structures made of and proteinsno post that are encoded with genetic instructions for making and proteins. The instructions are organized into units called , which are segments of DNA that code for particular pieces of RNA. The RNA molecules can then act as a blueprint for proteins, or directly help regulate various cellular processes.
- Each species is characterized by a set number of chromosomes. The normal chromosome complement of a human cell is 23 pairs of chromosomes. Of these, 22 pairs are , which contain genes for characteristics unrelated to sex. The other pair consists of (XX in females, XY in males). Only the Y chromosome contains genes that determine sex.
- Humans have an estimated 20 thousand to 22 thousand genes. The majority of human genes have two or more possible versions, which are called .
- Genes that are located on the same chromosome are called . Linkage explains why certain characteristics are frequently inherited together. A linkage map shows the locations of specific genes on a chromosome.
5.2 Review Questions
- What are chromosomes and genes? How are the two related?
- Describe human chromosomes and genes.
- Explain the difference between autosomes and sex chromosomes.
- What are linked genes, and what does a linkage map show?
- Explain why females are considered the default sex in humans.
- Explain the relationship between genes and alleles.
- Most males and females have two sex chromosomes. Why do only females have Barr bodies?
5.2 Explore More
https://www.youtube.com/watch?v=M4ut72kfUJM
WACE Biology: Coding and Non-Coding DNA, Atomi, 2019.
https://www.youtube.com/watch?time_continue=3&v=jhHGCvMlrb0&feature=emb_logo
How Sex Genes Are More Complicated Than You Thought, Seeker, 2015.
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
A molecule that can undergo polymerization, creating macromolecules. Large numbers of monomers combine to form polymers in a process called polymerization.
![Carbohydrate structure comparison [Three Important Polysaccharides]](https://commons.wikimedia.org/wiki/File:219_Three_Important_Polysaccharides-01.jpg){kind=link}
