Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 69

To maintain the stability of a species, replication of the DNA must be almost free of error. Changes in the DNA sequence that are passed on to subsequent generations are called mutations (see chapter 3). Depending on where these changes occur, they can severely alter the protein products of genes or other cellular functions. To avoid such instability, the cell has mechanisms that reduce the error rate.

As DNA replicates, the wrong base is sometimes inserted into the growing DNA chain. For example, Figure 1.11 shows the incorrect incorporation of a T opposite a G. Such a base pair in which the bases are paired wrongly is called a mismatch. Mismatches can occur when the bases take on forms called tautomers, which pair differently from the normal form of the base (see chapter 3). After the first replication shown in Figure 1.11, the mispaired T pairs with an A, causing a GC-to-AT change in the sequence of one of the two progeny DNAs and thus changing the base pair at that position on all subsequent copies of the mutated DNA molecule.

Figure 1.11 Mistakes in base pairing can lead to changes in the DNA sequence called mutations. If a T is mistakenly placed opposite a G during replication (A), it can lead to an AT base pair replacing a GC base pair in the progeny DNA (B to D).