{"id":933,"date":"2022-04-20T20:43:39","date_gmt":"2022-04-20T20:43:39","guid":{"rendered":"https:\/\/pressbooks.hcfl.edu\/bio1\/chapter\/historical-basis-of-modern-understanding\/"},"modified":"2025-08-29T19:14:16","modified_gmt":"2025-08-29T19:14:16","slug":"historical-basis-of-modern-understanding","status":"publish","type":"chapter","link":"https:\/\/pressbooks.hcfl.edu\/bio1\/chapter\/historical-basis-of-modern-understanding\/","title":{"raw":"Historical Basis of Modern Understanding","rendered":"Historical Basis of Modern Understanding"},"content":{"raw":"<div class=\"textbox textbox--learning-objectives\"><header class=\"textbox__header\">\n<h2 class=\"textbox__title\">Learning Objectives<\/h2>\n<\/header>\n<div class=\"textbox__content\">\n\nBy the end of this section, you will be able to do the following:\n<ul>\n \t<li>Explain transformation of DNA<\/li>\n \t<li>Describe the key experiments that helped identify that DNA is the genetic material<\/li>\n \t<li>State and explain Chargaff\u2019s rules<\/li>\n<\/ul>\n<\/div>\n<\/div>\nOur current understanding of DNA began with the discovery of nucleic acids followed by the development of the double-helix model. In the 1860s, Friedrich Miescher (Figure 14.2), a physician by profession, isolated phosphate-rich chemicals from white blood cells (leukocytes). He named these chemicals (which would eventually be known as DNA)\u00a0<em data-effect=\"italics\">nuclein<\/em>\u00a0because they were isolated from the nuclei of the cells.\n<div id=\"fig-ch14_01_01\" class=\"os-figure\">\n<figure data-id=\"fig-ch14_01_01\"><span id=\"fs-id2081310\" data-type=\"media\" data-alt=\"Photo of Friedrich Miescher.\"><\/span><\/figure>\n<div class=\"os-caption-container\"><span class=\"os-caption\">\u00a0<\/span><\/div>\n<div>\n\n[caption id=\"attachment_930\" align=\"aligncenter\" width=\"544\"]<img class=\"wp-image-930 size-full\" src=\"http:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2022\/04\/General-Biology-I-Lecture-Lab-1657046460_Page_648_Image_0001.jpg\" alt=\"Photo of Friedrich Miescher.\" width=\"544\" height=\"748\"> Figure\u00a014.2\u00a0Friedrich Miescher (1844\u20131895) discovered nucleic acids.[\/caption]\n\n<\/div>\n<div>\n<div class=\"textbox\">\n<h3 id=\"3\" class=\"os-subtitle\" data-type=\"title\"><span class=\"os-subtitle-label\">Link to Learning<\/span><\/h3>\n<p id=\"fs-id2009505\">To see Miescher conduct his experiment that led to his discovery of DNA and associated proteins in the nucleus, click through\u00a0<a href=\"http:\/\/openstax.org\/l\/miescher_levene\" target=\"_blank\" rel=\"noopener nofollow\">this review<\/a>.<\/p>\n\n<\/div>\n<span style=\"font-size: 1em\">A half century later, in 1928, British bacteriologist Frederick Griffith reported the first demonstration of bacterial <\/span><strong style=\"font-size: 1em\"><span id=\"term531\" data-type=\"term\">transformation<\/span><\/strong><span style=\"font-size: 1em\">\u2014a process in which external DNA is taken up by a cell, thereby changing its morphology and physiology. Griffith conducted his experiments with\u00a0<\/span><em style=\"font-size: 1em\" data-effect=\"italics\">Streptococcus pneumoniae,<\/em><span style=\"font-size: 1em\">\u00a0a bacterium that causes pneumonia. Griffith worked with two strains of this bacterium called rough (R) and smooth (S). (The two cell types were called \u201crough\u201d and \u201csmooth\u201d after the appearance of their colonies grown on a nutrient agar plate.)<\/span>\n\n<\/div>\n<\/div>\n<p id=\"fs-id1430786\">The R strain is non-pathogenic (does not cause disease). The S strain is pathogenic (disease-causing), and has a capsule outside its cell wall. The capsule allows the cell to escape the immune responses of the host mouse.<\/p>\n<p id=\"fs-id1430886\">When Griffith injected the living S strain into mice, they died from pneumonia. In contrast, when Griffith injected the live R strain into mice, they survived. In another experiment, when he injected mice with the heat-killed S strain, they also survived. This experiment showed that the capsule alone was not the cause of death. In a third set of experiments, a mixture of live R strain and heat-killed S strain were injected into mice, and\u2014to his surprise\u2014the mice died. Upon isolating the live bacteria from the dead mouse, only the S strain of bacteria was recovered. When this isolated S strain was injected into fresh mice, the mice died. Griffith concluded that something had passed from the heat-killed S strain into the live R strain and transformed it into the pathogenic S strain. He called this the\u00a0<em data-effect=\"italics\">transforming principle<\/em>\u00a0(Figure 14.3). These experiments are now known as Griffith's transformation experiments.<\/p>\n&nbsp;\n<div id=\"fig-ch14_01_02\" class=\"os-figure\">\n<figure data-id=\"fig-ch14_01_02\"><span id=\"fs-id1631652\" data-type=\"media\" data-alt=\"Four scenarios in an experiment are shown. In the first, living S cells are injected, and the mouse dies. In the second, living R cells are injected, and the mouse is healthy. In the third, heat-killed S cells are injected, and the mouse is healthy. In the fourth, mixture of heat-killed S cells and Living R cells are injected, and the mouse dies and living S cells are recovered.\"><\/span><\/figure>\n<div class=\"os-caption-container\">\n\n&nbsp;\n\n[caption id=\"attachment_931\" align=\"aligncenter\" width=\"800\"]<img class=\"wp-image-931 size-full\" src=\"http:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/68.jpg\" alt=\"Photo on left shows a mouse that was injected with the heat-killed S strain and lived. Image on the left shows a dead mouse on its back. This mouse was injected heat-killed S strain and a live R strain\" width=\"800\" height=\"317\"> Figure 14.3 Two strains of S. pneumoniae were used in Griffith\u2019s transformation experiments. The R strain is non-pathogenic, whereas the S strain is pathogenic and causes death. When Griffith injected a mouse with the heat-killed S strain and a live R strain, the mouse died. The S strain was recovered from the dead mouse. Griffith concluded that something had passed from the heat-killed S strain to the R strain, transforming the R strain into the S strain in the process. (credit \"living mouse\": modification of work by NIH; credit \"dead mouse\": modification of work by Sarah Marriage)[\/caption]\n\n<\/div>\n<\/div>\n<p id=\"fs-id2890946\">Scientists Oswald Avery, Colin MacLeod, and Maclyn McCarty (1944) were interested in exploring this transforming principle further. They isolated the S strain from the dead mice and isolated the proteins and nucleic acids (RNA and DNA) as these were possible candidates for the molecule of heredity. They used enzymes that specifically degraded each component and then used each mixture separately to transform the R strain. They found that when DNA was degraded, the resulting mixture was no longer able to transform the bacteria, whereas all of the other combinations were able to transform the bacteria. This led them to conclude that DNA was the transforming principle.<\/p>\n\n<div class=\"textbox\">\n<h4 id=\"5\" class=\"os-subtitle\" data-type=\"title\"><span class=\"os-subtitle-label\">Career Connection<\/span><\/h4>\n<p id=\"fs-id2115765\"><span data-type=\"title\">Forensic Scientist<\/span><\/p>\n<span style=\"font-size: 1em\">Forensic Scientists used DNA analysis evidence for the first time to solve an immigration case. The story started with a teenage boy returning to London from Ghana to be with his mother. Immigration authorities at the airport were suspicious of him, thinking that he was traveling on a forged passport. After much persuasion, he was allowed to go live with his mother, but the immigration authorities did not drop the case against him. All types of evidence, including photographs, were provided to the authorities, but deportation proceedings were started nevertheless. Around the same time, Dr. Alec Jeffreys of Leicester University in the United Kingdom had invented a technique known as <\/span><strong><span id=\"term532\" style=\"font-size: 1em\" data-type=\"term\">DNA fingerprinting<\/span><\/strong><span style=\"font-size: 1em\">. The immigration authorities approached Dr. Jeffreys for help. He took DNA samples from the mother and three of her children, as well as an unrelated mother, and compared the samples with the boy\u2019s DNA. Because the biological father was not in the picture, DNA from the three children was compared with the boy\u2019s DNA. He found a match in the boy\u2019s DNA for both the mother and his three siblings. He concluded that the boy was indeed the mother\u2019s son.<\/span>\n<div id=\"fs-id1772414\" class=\"career ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\"><section>\n<div class=\"os-note-body\">\n<p id=\"fs-id1227579\">Forensic scientists analyze many items, including documents, handwriting, firearms, and biological samples. They analyze the DNA content of hair, semen, saliva, and blood and compare it with a database of DNA profiles of known criminals. Analysis includes DNA isolation, sequencing, and sequence analysis. Forensic scientists are expected to appear at court hearings to present their findings. They are usually employed in crime labs of city and state government agencies. Geneticists experimenting with DNA techniques also work for scientific and research organizations, pharmaceutical industries, and college and university labs. Students wishing to pursue a career as a forensic scientist should have at least a bachelor's degree in chemistry, biology, or physics, and preferably some experience working in a laboratory.<\/p>\n\n<\/div>\n<\/section><\/div>\n<\/div>\n<p id=\"fs-id2025461\">Although the experiments of Avery, McCarty and McLeod had demonstrated that DNA was the informational component transferred during transformation, DNA was still considered to be too simple a molecule to carry biological information. Proteins, with their 20 different amino acids, were regarded as more likely candidates. The decisive experiment, conducted by Martha Chase and Alfred Hershey in 1952, provided confirmatory evidence that DNA was indeed the genetic material and not proteins. Chase and Hershey were studying a\u00a0<strong><span id=\"term533\" data-type=\"term\">bacteriophage<\/span><\/strong>\u2014a virus that infects bacteria. Viruses typically have a simple structure: a protein coat, called the capsid, and a nucleic acid core that contains the genetic material (either DNA or RNA). The bacteriophage infects the host bacterial cell by attaching to its surface, and then it injects its nucleic acids inside the cell. The phage DNA makes multiple copies of itself using the host machinery, and eventually the host cell bursts, releasing a large number of bacteriophages. Hershey and Chase selected radioactive elements that would specifically distinguish the protein from the DNA in infected cells. They labeled one batch of phage with radioactive sulfur,\u00a0<sup>35<\/sup>S, to label the protein coat. Another batch of phage were labeled with radioactive phosphorus,\u00a0<sup>32<\/sup>P. Because phosphorous is found in DNA, but not protein, the DNA and not the protein would be tagged with radioactive phosphorus. Likewise, sulfur is absent from DNA, but present in several amino acids such as methionine and cysteine.<\/p>\n<p id=\"fs-id1352111\">Each batch of phage was allowed to infect the cells separately. After infection, the phage bacterial suspension was put in a blender, which caused the phage coat to detach from the host cell. Cells exposed long enough for infection to occur were then examined to see which of the two radioactive molecules had entered the cell. The phage and bacterial suspension was spun down in a centrifuge. The heavier bacterial cells settled down and formed a pellet, whereas the lighter phage particles stayed in the supernatant. In the tube that contained phage labeled with<sup>\u00a035<\/sup>S, the supernatant contained the radioactively labeled phage, whereas no radioactivity was detected in the pellet. In the tube that contained the phage labeled with\u00a0<sup>32<\/sup>P, the radioactivity was detected in the pellet that contained the heavier bacterial cells, and no radioactivity was detected in the supernatant. Hershey and Chase concluded that it was the phage DNA that was injected into the cell and carried information to produce more phage particles, thus providing evidence that DNA was the genetic material and not proteins (Figure 14.4).<\/p>\n\n<div id=\"fig-ch14_01_03\" class=\"os-figure\">\n<figure data-id=\"fig-ch14_01_03\"><span id=\"fs-id1879663\" data-type=\"media\" data-alt=\"Illustration shows bacteria being infected by phage labeled with superscript 35 baseline upper case S, which is incorporated into the protein coat, or superscript 32 baseline upper case P, which is incorporated into the D N A. Infected bacteria were separated from phage by centrifugation and cultured. The bacteria that had been infected with phage containing superscript 32 baseline upper P labeled D N A made radioactive phage. The bacteria that had been infected with superscript 35 baseline upper S labeled phage produced unlabeled phage. The results support the hypothesis that D N A, and not protein, is the genetic material.\"><\/span><\/figure>\n<div class=\"os-caption-container\"><span class=\"os-caption\">\u00a0<\/span><\/div>\n<div>\n\n[caption id=\"attachment_932\" align=\"aligncenter\" width=\"1024\"]<img class=\"wp-image-932 size-large\" src=\"http:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/70-1024x889.png\" alt=\"Illustration shows bacteria being infected by phage labeled with superscript 35 baseline upper case S, which is incorporated into the protein coat, or superscript 32 baseline upper case P, which is incorporated into the D N A. Infected bacteria were separated from phage by centrifugation and cultured. The bacteria that had been infected with phage containing superscript 32 baseline upper P labeled D N A made radioactive phage. The bacteria that had been infected with superscript 35 baseline upper S labeled phage produced unlabeled phage. The results support the hypothesis that D N A, and not protein, is the genetic material.\" width=\"1024\" height=\"889\"> Figure\u00a014.4\u00a0In Hershey and Chase's experiments, bacteria were infected with phage radiolabeled with either\u00a035S, which labels protein, or\u00a032P, which labels DNA. Only\u00a032P entered the bacterial cells, indicating that DNA is the genetic material.[\/caption]\n\n<\/div>\n<\/div>\n<p id=\"fs-id1430603\">Around this same time, Austrian biochemist Erwin Chargaff examined the content of DNA in different species and found that the amounts of adenine, thymine, guanine, and cytosine were not found in equal quantities, and that relative concentrations of the four nucleotide bases varied from species to species, but not within tissues of the same individual or between individuals of the same species. He also discovered something unexpected: That the amount of adenine equaled the amount of thymine, and the amount of cytosine equaled the amount of guanine (that is, A = T and G = C). Different species had equal amounts of\u00a0<em data-effect=\"italics\">purines<\/em>\u00a0(A+G) and\u00a0<strong><span id=\"term534\" data-type=\"term\">pyrimidines<\/span>\u00a0<\/strong>(T + C), but different ratios of A+T to G+C. These observations became known as\u00a0<strong><span id=\"term535\" data-type=\"term\">Chargaff\u2019s rules<\/span><\/strong>. Chargaff's findings proved immensely useful when Watson and Crick were getting ready to propose their DNA double helix model! You can see after reading the past few pages how science builds upon previous discoveries, sometimes in a slow and laborious process.<\/p>","rendered":"<div class=\"textbox textbox--learning-objectives\">\n<header class=\"textbox__header\">\n<h2 class=\"textbox__title\">Learning Objectives<\/h2>\n<\/header>\n<div class=\"textbox__content\">\n<p>By the end of this section, you will be able to do the following:<\/p>\n<ul>\n<li>Explain transformation of DNA<\/li>\n<li>Describe the key experiments that helped identify that DNA is the genetic material<\/li>\n<li>State and explain Chargaff\u2019s rules<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<p>Our current understanding of DNA began with the discovery of nucleic acids followed by the development of the double-helix model. In the 1860s, Friedrich Miescher (Figure 14.2), a physician by profession, isolated phosphate-rich chemicals from white blood cells (leukocytes). He named these chemicals (which would eventually be known as DNA)\u00a0<em data-effect=\"italics\">nuclein<\/em>\u00a0because they were isolated from the nuclei of the cells.<\/p>\n<div id=\"fig-ch14_01_01\" class=\"os-figure\">\n<figure data-id=\"fig-ch14_01_01\"><span id=\"fs-id2081310\" data-type=\"media\" data-alt=\"Photo of Friedrich Miescher.\"><\/span><\/figure>\n<div class=\"os-caption-container\"><span class=\"os-caption\">\u00a0<\/span><\/div>\n<div>\n<figure id=\"attachment_930\" aria-describedby=\"caption-attachment-930\" style=\"width: 544px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-930 size-full\" src=\"http:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2022\/04\/General-Biology-I-Lecture-Lab-1657046460_Page_648_Image_0001.jpg\" alt=\"Photo of Friedrich Miescher.\" width=\"544\" height=\"748\" srcset=\"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2022\/04\/General-Biology-I-Lecture-Lab-1657046460_Page_648_Image_0001.jpg 544w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2022\/04\/General-Biology-I-Lecture-Lab-1657046460_Page_648_Image_0001-218x300.jpg 218w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2022\/04\/General-Biology-I-Lecture-Lab-1657046460_Page_648_Image_0001-65x89.jpg 65w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2022\/04\/General-Biology-I-Lecture-Lab-1657046460_Page_648_Image_0001-225x309.jpg 225w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2022\/04\/General-Biology-I-Lecture-Lab-1657046460_Page_648_Image_0001-350x481.jpg 350w\" sizes=\"auto, (max-width: 544px) 100vw, 544px\" \/><figcaption id=\"caption-attachment-930\" class=\"wp-caption-text\">Figure\u00a014.2\u00a0Friedrich Miescher (1844\u20131895) discovered nucleic acids.<\/figcaption><\/figure>\n<\/div>\n<div>\n<div class=\"textbox\">\n<h3 id=\"3\" class=\"os-subtitle\" data-type=\"title\"><span class=\"os-subtitle-label\">Link to Learning<\/span><\/h3>\n<p id=\"fs-id2009505\">To see Miescher conduct his experiment that led to his discovery of DNA and associated proteins in the nucleus, click through\u00a0<a href=\"http:\/\/openstax.org\/l\/miescher_levene\" target=\"_blank\" rel=\"noopener nofollow\">this review<\/a>.<\/p>\n<\/div>\n<p><span style=\"font-size: 1em\">A half century later, in 1928, British bacteriologist Frederick Griffith reported the first demonstration of bacterial <\/span><strong style=\"font-size: 1em\"><span id=\"term531\" data-type=\"term\">transformation<\/span><\/strong><span style=\"font-size: 1em\">\u2014a process in which external DNA is taken up by a cell, thereby changing its morphology and physiology. Griffith conducted his experiments with\u00a0<\/span><em style=\"font-size: 1em\" data-effect=\"italics\">Streptococcus pneumoniae,<\/em><span style=\"font-size: 1em\">\u00a0a bacterium that causes pneumonia. Griffith worked with two strains of this bacterium called rough (R) and smooth (S). (The two cell types were called \u201crough\u201d and \u201csmooth\u201d after the appearance of their colonies grown on a nutrient agar plate.)<\/span><\/p>\n<\/div>\n<\/div>\n<p id=\"fs-id1430786\">The R strain is non-pathogenic (does not cause disease). The S strain is pathogenic (disease-causing), and has a capsule outside its cell wall. The capsule allows the cell to escape the immune responses of the host mouse.<\/p>\n<p id=\"fs-id1430886\">When Griffith injected the living S strain into mice, they died from pneumonia. In contrast, when Griffith injected the live R strain into mice, they survived. In another experiment, when he injected mice with the heat-killed S strain, they also survived. This experiment showed that the capsule alone was not the cause of death. In a third set of experiments, a mixture of live R strain and heat-killed S strain were injected into mice, and\u2014to his surprise\u2014the mice died. Upon isolating the live bacteria from the dead mouse, only the S strain of bacteria was recovered. When this isolated S strain was injected into fresh mice, the mice died. Griffith concluded that something had passed from the heat-killed S strain into the live R strain and transformed it into the pathogenic S strain. He called this the\u00a0<em data-effect=\"italics\">transforming principle<\/em>\u00a0(Figure 14.3). These experiments are now known as Griffith&#8217;s transformation experiments.<\/p>\n<p>&nbsp;<\/p>\n<div id=\"fig-ch14_01_02\" class=\"os-figure\">\n<figure data-id=\"fig-ch14_01_02\"><span id=\"fs-id1631652\" data-type=\"media\" data-alt=\"Four scenarios in an experiment are shown. In the first, living S cells are injected, and the mouse dies. In the second, living R cells are injected, and the mouse is healthy. In the third, heat-killed S cells are injected, and the mouse is healthy. In the fourth, mixture of heat-killed S cells and Living R cells are injected, and the mouse dies and living S cells are recovered.\"><\/span><\/figure>\n<div class=\"os-caption-container\">\n<p>&nbsp;<\/p>\n<figure id=\"attachment_931\" aria-describedby=\"caption-attachment-931\" style=\"width: 800px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-931 size-full\" src=\"http:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/68.jpg\" alt=\"Photo on left shows a mouse that was injected with the heat-killed S strain and lived. Image on the left shows a dead mouse on its back. This mouse was injected heat-killed S strain and a live R strain\" width=\"800\" height=\"317\" srcset=\"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/68.jpg 800w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/68-300x119.jpg 300w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/68-768x304.jpg 768w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/68-65x26.jpg 65w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/68-225x89.jpg 225w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/68-350x139.jpg 350w\" sizes=\"auto, (max-width: 800px) 100vw, 800px\" \/><figcaption id=\"caption-attachment-931\" class=\"wp-caption-text\">Figure 14.3 Two strains of S. pneumoniae were used in Griffith\u2019s transformation experiments. The R strain is non-pathogenic, whereas the S strain is pathogenic and causes death. When Griffith injected a mouse with the heat-killed S strain and a live R strain, the mouse died. The S strain was recovered from the dead mouse. Griffith concluded that something had passed from the heat-killed S strain to the R strain, transforming the R strain into the S strain in the process. (credit &#8220;living mouse&#8221;: modification of work by NIH; credit &#8220;dead mouse&#8221;: modification of work by Sarah Marriage)<\/figcaption><\/figure>\n<\/div>\n<\/div>\n<p id=\"fs-id2890946\">Scientists Oswald Avery, Colin MacLeod, and Maclyn McCarty (1944) were interested in exploring this transforming principle further. They isolated the S strain from the dead mice and isolated the proteins and nucleic acids (RNA and DNA) as these were possible candidates for the molecule of heredity. They used enzymes that specifically degraded each component and then used each mixture separately to transform the R strain. They found that when DNA was degraded, the resulting mixture was no longer able to transform the bacteria, whereas all of the other combinations were able to transform the bacteria. This led them to conclude that DNA was the transforming principle.<\/p>\n<div class=\"textbox\">\n<h4 id=\"5\" class=\"os-subtitle\" data-type=\"title\"><span class=\"os-subtitle-label\">Career Connection<\/span><\/h4>\n<p id=\"fs-id2115765\"><span data-type=\"title\">Forensic Scientist<\/span><\/p>\n<p><span style=\"font-size: 1em\">Forensic Scientists used DNA analysis evidence for the first time to solve an immigration case. The story started with a teenage boy returning to London from Ghana to be with his mother. Immigration authorities at the airport were suspicious of him, thinking that he was traveling on a forged passport. After much persuasion, he was allowed to go live with his mother, but the immigration authorities did not drop the case against him. All types of evidence, including photographs, were provided to the authorities, but deportation proceedings were started nevertheless. Around the same time, Dr. Alec Jeffreys of Leicester University in the United Kingdom had invented a technique known as <\/span><strong><span id=\"term532\" style=\"font-size: 1em\" data-type=\"term\">DNA fingerprinting<\/span><\/strong><span style=\"font-size: 1em\">. The immigration authorities approached Dr. Jeffreys for help. He took DNA samples from the mother and three of her children, as well as an unrelated mother, and compared the samples with the boy\u2019s DNA. Because the biological father was not in the picture, DNA from the three children was compared with the boy\u2019s DNA. He found a match in the boy\u2019s DNA for both the mother and his three siblings. He concluded that the boy was indeed the mother\u2019s son.<\/span><\/p>\n<div id=\"fs-id1772414\" class=\"career ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\">\n<section>\n<div class=\"os-note-body\">\n<p id=\"fs-id1227579\">Forensic scientists analyze many items, including documents, handwriting, firearms, and biological samples. They analyze the DNA content of hair, semen, saliva, and blood and compare it with a database of DNA profiles of known criminals. Analysis includes DNA isolation, sequencing, and sequence analysis. Forensic scientists are expected to appear at court hearings to present their findings. They are usually employed in crime labs of city and state government agencies. Geneticists experimenting with DNA techniques also work for scientific and research organizations, pharmaceutical industries, and college and university labs. Students wishing to pursue a career as a forensic scientist should have at least a bachelor&#8217;s degree in chemistry, biology, or physics, and preferably some experience working in a laboratory.<\/p>\n<\/div>\n<\/section>\n<\/div>\n<\/div>\n<p id=\"fs-id2025461\">Although the experiments of Avery, McCarty and McLeod had demonstrated that DNA was the informational component transferred during transformation, DNA was still considered to be too simple a molecule to carry biological information. Proteins, with their 20 different amino acids, were regarded as more likely candidates. The decisive experiment, conducted by Martha Chase and Alfred Hershey in 1952, provided confirmatory evidence that DNA was indeed the genetic material and not proteins. Chase and Hershey were studying a\u00a0<strong><span id=\"term533\" data-type=\"term\">bacteriophage<\/span><\/strong>\u2014a virus that infects bacteria. Viruses typically have a simple structure: a protein coat, called the capsid, and a nucleic acid core that contains the genetic material (either DNA or RNA). The bacteriophage infects the host bacterial cell by attaching to its surface, and then it injects its nucleic acids inside the cell. The phage DNA makes multiple copies of itself using the host machinery, and eventually the host cell bursts, releasing a large number of bacteriophages. Hershey and Chase selected radioactive elements that would specifically distinguish the protein from the DNA in infected cells. They labeled one batch of phage with radioactive sulfur,\u00a0<sup>35<\/sup>S, to label the protein coat. Another batch of phage were labeled with radioactive phosphorus,\u00a0<sup>32<\/sup>P. Because phosphorous is found in DNA, but not protein, the DNA and not the protein would be tagged with radioactive phosphorus. Likewise, sulfur is absent from DNA, but present in several amino acids such as methionine and cysteine.<\/p>\n<p id=\"fs-id1352111\">Each batch of phage was allowed to infect the cells separately. After infection, the phage bacterial suspension was put in a blender, which caused the phage coat to detach from the host cell. Cells exposed long enough for infection to occur were then examined to see which of the two radioactive molecules had entered the cell. The phage and bacterial suspension was spun down in a centrifuge. The heavier bacterial cells settled down and formed a pellet, whereas the lighter phage particles stayed in the supernatant. In the tube that contained phage labeled with<sup>\u00a035<\/sup>S, the supernatant contained the radioactively labeled phage, whereas no radioactivity was detected in the pellet. In the tube that contained the phage labeled with\u00a0<sup>32<\/sup>P, the radioactivity was detected in the pellet that contained the heavier bacterial cells, and no radioactivity was detected in the supernatant. Hershey and Chase concluded that it was the phage DNA that was injected into the cell and carried information to produce more phage particles, thus providing evidence that DNA was the genetic material and not proteins (Figure 14.4).<\/p>\n<div id=\"fig-ch14_01_03\" class=\"os-figure\">\n<figure data-id=\"fig-ch14_01_03\"><span id=\"fs-id1879663\" data-type=\"media\" data-alt=\"Illustration shows bacteria being infected by phage labeled with superscript 35 baseline upper case S, which is incorporated into the protein coat, or superscript 32 baseline upper case P, which is incorporated into the D N A. Infected bacteria were separated from phage by centrifugation and cultured. The bacteria that had been infected with phage containing superscript 32 baseline upper P labeled D N A made radioactive phage. The bacteria that had been infected with superscript 35 baseline upper S labeled phage produced unlabeled phage. The results support the hypothesis that D N A, and not protein, is the genetic material.\"><\/span><\/figure>\n<div class=\"os-caption-container\"><span class=\"os-caption\">\u00a0<\/span><\/div>\n<div>\n<figure id=\"attachment_932\" aria-describedby=\"caption-attachment-932\" style=\"width: 1024px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-932 size-large\" src=\"http:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/70-1024x889.png\" alt=\"Illustration shows bacteria being infected by phage labeled with superscript 35 baseline upper case S, which is incorporated into the protein coat, or superscript 32 baseline upper case P, which is incorporated into the D N A. Infected bacteria were separated from phage by centrifugation and cultured. The bacteria that had been infected with phage containing superscript 32 baseline upper P labeled D N A made radioactive phage. The bacteria that had been infected with superscript 35 baseline upper S labeled phage produced unlabeled phage. The results support the hypothesis that D N A, and not protein, is the genetic material.\" width=\"1024\" height=\"889\" srcset=\"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/70-1024x889.png 1024w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/70-300x260.png 300w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/70-768x667.png 768w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/70-1536x1333.png 1536w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/70-65x56.png 65w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/70-225x195.png 225w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/70-350x304.png 350w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/70.png 1600w\" sizes=\"auto, (max-width: 1024px) 100vw, 1024px\" \/><figcaption id=\"caption-attachment-932\" class=\"wp-caption-text\">Figure\u00a014.4\u00a0In Hershey and Chase&#8217;s experiments, bacteria were infected with phage radiolabeled with either\u00a035S, which labels protein, or\u00a032P, which labels DNA. Only\u00a032P entered the bacterial cells, indicating that DNA is the genetic material.<\/figcaption><\/figure>\n<\/div>\n<\/div>\n<p id=\"fs-id1430603\">Around this same time, Austrian biochemist Erwin Chargaff examined the content of DNA in different species and found that the amounts of adenine, thymine, guanine, and cytosine were not found in equal quantities, and that relative concentrations of the four nucleotide bases varied from species to species, but not within tissues of the same individual or between individuals of the same species. He also discovered something unexpected: That the amount of adenine equaled the amount of thymine, and the amount of cytosine equaled the amount of guanine (that is, A = T and G = C). Different species had equal amounts of\u00a0<em data-effect=\"italics\">purines<\/em>\u00a0(A+G) and\u00a0<strong><span id=\"term534\" data-type=\"term\">pyrimidines<\/span>\u00a0<\/strong>(T + C), but different ratios of A+T to G+C. These observations became known as\u00a0<strong><span id=\"term535\" data-type=\"term\">Chargaff\u2019s rules<\/span><\/strong>. Chargaff&#8217;s findings proved immensely useful when Watson and Crick were getting ready to propose their DNA double helix model! You can see after reading the past few pages how science builds upon previous discoveries, sometimes in a slow and laborious process.<\/p>\n","protected":false},"author":130,"menu_order":2,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":["jung-choi","mary-ann-clark","matthew-douglas"],"pb_section_license":"cc-by"},"chapter-type":[],"contributor":[92,93,94],"license":[53],"class_list":["post-933","chapter","type-chapter","status-publish","hentry","contributor-jung-choi","contributor-mary-ann-clark","contributor-matthew-douglas","license-cc-by"],"part":925,"_links":{"self":[{"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/pressbooks\/v2\/chapters\/933","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/wp\/v2\/users\/130"}],"version-history":[{"count":1,"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/pressbooks\/v2\/chapters\/933\/revisions"}],"predecessor-version":[{"id":934,"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/pressbooks\/v2\/chapters\/933\/revisions\/934"}],"part":[{"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/pressbooks\/v2\/parts\/925"}],"metadata":[{"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/pressbooks\/v2\/chapters\/933\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/wp\/v2\/media?parent=933"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/pressbooks\/v2\/chapter-type?post=933"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/wp\/v2\/contributor?post=933"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/wp\/v2\/license?post=933"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}