/n 2/9/24, 2:52 PM https://nerd.wwnorton.com/nerd/122222/r/goto/cfi/158!/4 Deep in the DNA Deep in the DNA Each day in the United States, on average 22 people die while waiting for an organ transplant. Over

113,000 men, women, and children are currently on the national transplant waiting list, each hoping their name is called before it's too late. Researchers have explored many ways to grow and store organs for transplantation-from freezing them to building them from scratch-but one of the most promising, if you can look past the mud and flies, is pigs. Our porcine friends have long been considered an excellent potential source of organs because their organs-including the heart, liver, and kidneys-are relatively close in size to human organs and because pigs and humans have similar anatomies (Figure 9.2). In addition, pigs are an easier sell to the public: people tend to prefer the idea of transplants from pigs over transplants from mammals more closely related to us, such as baboons. If we were able to transplant organs from nonhuman animals into humans, a process called xenotransplantation, healthy organs could be available in essentially limitless supply. Kidney Pancreas Liver Heart Lung Figure 9.2 Pig organs and human organs are remarkably similar in size For a nonhuman-to-human organ transplant to succeed, the nonhuman organ must fit into the space where the human organ was removed. Organs that are too big will not fit. Organs that are too small will not function at the level necessary to sustain life. 1/4 2/9/24, 2:52 PM https://nerd.wwnorton.com/nerd/122222/r/goto/cfi/158!/4 Deep in the DNA Yet there has been a barrier to harvesting pig organs for humans: the pig genome is dotted with DNA from a family of viruses called porcine endogenous retroviruses, or PERVS. Because of the presence of this viral DNA in the pig genome, pig cells produce and release PERVS-and two of the three subtypes of PERVS can infect human cells, making it risky to transplant pig organs into people for fear of making the recipients sick. After pigs acquired the viruses, PERV DNA slipped easily into the pig genome because it has the same structure as pig DNA. In fact, all living things share the same DNA structure, and species often share and swap DNA with each other. As discussed in Chapters 3-4, DNA is built from two parallel strands of repeating units called nucleotides. Each nucleotide is composed of the sugar deoxyribose, a phosphate group, and one of four bases: adenine, cytosine, guanine, or thymine. We identify nucleotides by their bases, using "adenine nucleotide" as shorthand for "nucleotide with an adenine base." The nucleotides of a single strand are connected by covalent bonds between the phosphate group of one nucleotide and the sugar of the next nucleotide. The two DNA strands are connected by hydrogen bonds linking the bases on one strand to the bases on the other, like the rungs that connect the two sides of a ladder (Figure 9.3). Covalent bonds are strong, which is important in maintaining the specific order of the nucleotides. The weaker hydrogen bonds allow the two DNA strands to be pulled apart for replication. A pairs only with T The nucleotides in one strand are paired with the nucleotides in the complementary strand. C pairs only with G. The two strands of DNA are held together by hydrogen bonds (dotted lines) between the bases. Nucleotides are linked together by covalent bonds to form one strand of DNA. Q1: Name two base pairs. SHOW ANSWER 0000 SHOW ANSWER Phosphate Sugar (deoxyribose) Sugar-phosphate Base Nucleotide Nucleotide bases: Adenine Figure 9.3 A molecule of DNA consists of two complementary strands of nucleotides that are twisted into a spiral around an imaginary axis, rather like the winding of a spiral staircase. Thymine Guanine Cytosine Q2: Why is the DNA structure referred to as a "ladder"? What part of the DNA represents the rungs of the ladder? What part represents the sides? Q3: Is the hydrogen bond that holds the base pairs together a strong or weak chemical bond? Why is that important? 2/4 2/9/24, 2:52 PM https://nerd.wwnorton.com/nerd/122222/r/goto/cfi/158!/4 SHOW ANSWER Deep in the DNA You can also see Appendix A for answers to the figure questions. The term base pair, or nucleotide pair, refers to two nucleotides held together by bonds between their bases; that is, a base pair corresponds to one rung of the DNA ladder. The ladder twists into a spiral called a double helix (Figure 9.4). Within the long, winding double helix of the pig genome, short sections of DNA from PERVS are scattered about. These PERV sections are made up of the same four nucleotides as the rest of the DNA, but they encode information for viral proteins instead of pig proteins. Figure 9.4 What DNA actually looks like In November 2012, Italian researchers used an electron microscope to directly visualize DNA for the first time. This is the single thread of double-stranded DNA that they saw. Nucleotides do not form base pairs willy-nilly. As shown in Figure 9.3, the adenine (A) nucleotide on one strand can pair only with thymine (T) on the other strand; cytosine (C) on one strand can pair only with guanine (G) on the other strand. These base-pairing rules, which provide complementary base-pairing between two nucleic acid strands, have an important consequence: when the sequence of nucleotides on one strand of the DNA molecule is known, the sequence of nucleotides on the other, complementary strand of the molecule is automatically known as well. The fact that A can pair only with T and that C can pair only with G allows the original strands to serve as "template strands" on which new strands can be built through complementary base-pairing. (In Chapter 10, we delve more deeply into building new DNA strands, including how RNA can pair with DNA, which CRISPR takes advantage of.) Still, the four nucleotides can be arranged in any order along a single strand of DNA, and each DNA strand is composed of millions of these nucleotides, so a tremendous amount of information can be stored in a DNA sequence and in a genome. The genome of the domestic pig, for example, has about 3 billion base pairs, the human genome has about 3.2 billion base pairs, a tomato has only about 900 million base pairs, and the bacterium Escherichia coli (better known as E. coli) has a measly 4.6 million. The sequence of 3/4 2/9/24, 2:52 PM https://nerd.wwnorton.com/nerd/122222/r/goto/cfi/158!/4 Deep in the DNA nucleotides in DNA differs among species and among individuals within a species, and these differences in genotype can result in different phenotypes (Figure 9.5). MIN G A C SHOW ANSWER A Human A Figure 9.5 The sequence of bases in DNA differs among species and among individuals within a species The sequence of bases in a hypothetical gene is compared for two humans (A and B) and a pig. Base pairs highlighted in blue are variant; that is, they differ between the genes of persons A and B and between the same genes in humans and pigs. SHOW ANSWER Human B Q1: If all genes are composed of just four nucleotides, how can different genes carry different types of information? Pig SHOW ANSWER Q2: Would you expect to see more variation in the sequence of DNA bases between two members of the same species (such as humans) or between two individuals of different species (for example, humans and pigs)? Explain your reasoning. Q3: Do different alleles of a gene have the same DNA sequence or different DNA sequences? You can also see Appendix A for answers to the figure questions. In the mid-1990s, scientists became very excited about the idea of using pig organs in humans, but testing stalled because of the fear that humans would become infected with PERVS. Just breeding pigs in sterile conditions can't get rid of the virus; it's integrated right there in the double helix. The Harvard Medical School team believed CRISPR might be able to solve that problem by inactivating the PERV DNA in pig cells once and for all. 4/4