{"id":4521,"date":"2019-06-24T12:57:39","date_gmt":"2019-06-24T12:57:39","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/5-3-dna-3\/"},"modified":"2023-11-30T17:56:17","modified_gmt":"2023-11-30T17:56:17","slug":"5-3-dna-3","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/5-3-dna-3\/","title":{"raw":"5.3\u00a0DNA","rendered":"5.3\u00a0DNA"},"content":{"raw":" \r\n
\r\n\r\n[caption id=\"attachment_2130\" align=\"aligncenter\" width=\"400\"]\"\" Figure 5.3.1 Woman with natural red hair.<\/em>[\/caption]\r\n

What Makes You...You?<\/h1>\r\n<\/div>\r\nThis 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 [pb_glossary id=\"277\"]DNA[\/pb_glossary].\r\n
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Introducing DNA<\/h1>\r\n<\/div>\r\nToday, it is commonly known that\u00a0[pb_glossary id=\"277\"]DNA[\/pb_glossary]<\/strong>\u00a0is the genetic material that is passed from parents to offspring and determines our traits. For a long time, scientists knew such molecules existed \u2014 that is, they were aware that genetic information is contained within biochemical molecules.\u00a0What\u00a0they\u00a0didn\u2019t<\/em> know was which specific molecules play this role. In fact, for many decades, scientists thought that [pb_glossary id=\"1593\"]proteins[\/pb_glossary] were the molecules that contain genetic information.\r\n
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Discovery that DNA\u00a0is the Genetic Material<\/h1>\r\n<\/div>\r\nDetermining 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\u00a0research\u00a0began in the early part of the 20th century.\r\n\r\n[h5p id=\"493\"]\r\n

Griffith's Experiments with Mice<\/h2>\r\n[caption id=\"attachment_2135\" align=\"alignright\" width=\"500\"]\"Diagram Figure 5.3.2 Griffith\u2019s Experimental Results. Griffith showed that a substance could be transferred to harmless bacteria and make them deadly.<\/em>[\/caption]\r\n\r\nOne of the first important discoveries was made in the 1920s by an American scientist named Frederick Griffith<\/a>.\u00a0 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.\r\n\r\nBased 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?\r\n

Avery and His Colleagues Make a Major Contribution<\/h2>\r\nIn the early 1940s, a team of scientists led by Canadian-American Oswald Avery<\/a> tried to answer the question raised by Griffith\u2019s research\u00a0results. First, they inactivated various substances in the S-strain\u00a0bacteria. 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\u00a0[pb_glossary id=\"1593\"]proteins[\/pb_glossary], 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 [pb_glossary id=\"277\"]DNA[\/pb_glossary] in the S-strain, the R-strain remained harmless. This led to the conclusion that DNA\u00a0\u2014\u00a0and not protein\u00a0\u2014\u00a0is the substance that controls the characteristics of organisms. In other words, DNA is the genetic material.\r\n

Hershey and Chase Confirm the Results<\/h2>\r\nThe conclusion that DNA is the genetic material was not widely accepted until it was confirmed by additional\u00a0research. In the 1950s, Alfred Hershey<\/a> and Martha Chase<\/a> did experiments with viruses and\u00a0bacteria. Viruses are not\u00a0cells. Instead, they\u00a0are basically\u00a0DNA (or RNA)\u00a0inside a\u00a0protein\u00a0coat. To reproduce, a\u00a0virus\u00a0must insert its own genetic material into a cell (such as a bacterium). Then, it uses the cell\u2019s machinery to make more viruses. The researchers used different radioactive elements to label the DNA and\u00a0proteins\u00a0in DNA viruses. This allowed them to identify which molecule the viruses inserted into bacterial\u00a0cells. DNA was the molecule they identified. This confirmed that DNA is the genetic material.\r\n
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Chargaff Focuses on DNA Bases<\/h2>\r\n<\/div>\r\nOther important discoveries about DNA were made in the mid-1900s by Erwin Chargaff.<\/a> He studied DNA from many different\u00a0species\u00a0and 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\u00a0between\u00a0species. Within any given\u00a0species, however, the\u00a0concentration<\/a>\u00a0of 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\u00a0[pb_glossary id=\"2136\"]Chargaff\u2019s rules[\/pb_glossary]<\/strong>. The significance of the rules would not be revealed until the double-helix structure of DNA was discovered.\r\n
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Discovery of the Double Helix<\/h2>\r\n<\/div>\r\n\r\n[caption id=\"attachment_2137\" align=\"alignleft\" width=\"322\"]\"Image Figure 5.3.3 Watson and Crick developed a model of DNA showing its helical shape.<\/em>[\/caption]\r\n\r\nAfter DNA was shown to be the genetic material, scientists wanted to learn more about its structure and function. James Watson<\/a> and Francis Crick<\/a> 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<\/a> and other scientists, who had used X-rays\u00a0to learn more about DNA\u2019s structure. Unfortunately, Franklin and these others have not\u00a0always\u00a0been given credit for their important contributions to the discovery of the double helix.\r\n
\r\n\r\nThe DNA molecule has a double helix shape \u2014 the same basic shape as a spiral staircase. Do you see the resemblance? Which parts of the DNA molecule are like the steps of the spiral staircase?\r\n\r\n<\/div>\r\nThe double helix shape of DNA,\u00a0along\u00a0with [pb_glossary id=\"2136\"]Chargaff\u2019s rules[\/pb_glossary], led to a better understanding of DNA. As a\u00a0nucleic acid, DNA is made from [pb_glossary id=\"518\"]nucleotide[\/pb_glossary] [pb_glossary id=\"5781\"]monomers[\/pb_glossary]. 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.\r\n\r\nScientists 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\u00a0[pb_glossary id=\"528\"]complementary<\/strong>\u00a0<\/strong>base<\/strong>\u00a0<\/strong>pairs<\/strong>[\/pb_glossary].<\/strong>\u00a0 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.\r\n
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5.3 Summary<\/span><\/h1>\r\n<\/header>\r\n
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