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26.12: DNA and RNA

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  • Linus Pauling was one of the greatest scientists of the twentieth century. A two-time Nobel Prize winner (in chemistry in 1954 and the peace prize in 1962). However, he didn't always come in first. In the 1950s, there was a great deal of interest in the structure of DNA. Pauling spent some time on this puzzle, although he was primarily interested in proteins. The inadequate amount of information he had led him to propose a DNA structure where the bases were on the outside and the phosphate groups were on the inside. It turns out that he was in error about this idea, but it certainly did not take away from his outstanding scientific reputation.

    DNA and RNA

    The three parts of a DNA nucleotide are assembled as shown in the figure below.

    Structure of a DNA monomer
    Figure 26.12.1: Nucleotides are composed of a phosphate group, a sugar, and one of five different nitrogenous bases.

    Every DNA and RNA polymer consists of multiple nucleotides strung together into extremely long chains. The only variation in each nucleotide is the identity of the nitrogenous base. The figure above shows one example of a nitrogenous base, called adenine. There are only five different nitrogenous bases found in all nucleic acids. The four bases of DNA are adenine, thymine, cytosine, and guanine, abbreviated A, T, C, and G respectively. In RNA, the base thymine is not found and is instead replaced by a different base called uracil, abbreviated U. The other three bases are present in both DNA and RNA.

    The specific structure of DNA proved elusive to scientists for many years. In 1953, James Watson and Francis Crick proposed that the structure of DNA consists of two side-by-side polynucleotide chains wrapped into the shape of a double helix. One aspect of this structure is that each nitrogenous base on one of the DNA strands must be paired up with another base on the opposite strand. The figure below illustrates the base pairing. Each adenine base is always paired with a thymine, while each cytosine is paired with a guanine. The bases fit together perfectly from one strand to the other and are also held together by hydrogen bonds. The A-T pairing contains two hydrogen bonds, while the C-G pairing contains three hydrogen bonds. The ends of each strand are labeled either with 3' or 5' based on a numbering of the deoxyribose sugar ring.

    Structure of DNA base pairing
    Figure 26.12.2: Base pairing in DNA.

    The double helical structure of DNA is shown in the figure below.

    Structure of the DNA double helix
    Figure 26.12.3: The DNA double helix.


    • CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.