It is clearly important that synthesis of a complementary second strand of DNA should occur with complete accuracy, but occasionally, a non-complementary nucleotide is inserted; this may happen as frequently as once in every 10 000 nucleotides. Cells, however, are able to have a second attempt to incorporate the correct base because of the proofreading activity of the enzymes DNA polymerase I and III. These are able to
This rather long-winded description hardly does justice to a process that incorporates new nucleotides at a rate of around 1000 per second!
Replication bubble joooQ:
Figure 11.3 DNA replication is bidirectional. Two replication forks form simultaneously, moving away from each other and developing a replication bubble cut out the 'wrong' nucleotide and replace it with the correct one. As a result of this monitoring, mistakes are very rare; they are thought to occur at a frequency of around one in every billion (109) nucleotides copied. Mistakes that do slip through the net result in mutations, which are discussed later in this chapter.
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