Why are mistakes made every so often during DNA replication? One source of erroneous base incorporation is a phenomenon called tautomerism. Some of the nucleotide bases occur in rare alternative forms, which have different base-pairing properties. For example, as you will recall from Chapter 2, cytosine normally has an amino group that provides a hydrogen atom for bonding with the complementary keto group of gua-nine. Occasionally, however (once in about every 104-105 molecules), the cytosine may undergo a rearrangement called a tautomeric shift, which results in the amino group changing to an imino group (==NH), and this now behaves in pairing terms as if it were thymine, and therefore pairs with adenine (Figure 11.19a). Similarly, thymine might undergo a tautomeric shift, changing its usual keto form (C==O) to the rare enol form (COH). This then takes on the pairing properties of cytosine and pairs with guanine (Figure 11.19b). The result of such mispairing is that at subsequent rounds of replication,
Figure 11.19 Rare forms of nucleotides may cause mispairing. If DNA replication takes place whilst a base is in its rare alternative form, an incorrect base will be incorporated into the newly synthesised second strand. (a) In its normal keto form, thymine pairs with adenine in keeping with the rules of base-pairing. In its rare enol form, however, it preferentially pairs with guanine. (b) Cytosine can likewise assume the rare imino form and mispair with adenine one half of the DNA molecules will contain a wrong base pair at that point. The fact that spontaneous mutations occur much less frequently than the rate just quoted is due to the DNA repair mechanisms discussed earlier in this chapter.
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