453 14.1 Point mutations ber of copies of a trinucleotide repeat sequence [as when (AGC) 3 becomes (AGC) 5 ]. Mutations can even be caused by the insertion of a transposable element from elsewhere in the genome (Chapter 13). In this chapter we focus on mutations that do not involve transposable elements. We can view DNA as being subjected to a dynamic tug of war between the chemical processes that damage DNA and lead to new mutations and the cellular repair processes that constantly monitor DNA for such damage and correct it. Mutations often arise through the action of certain agents, called mutagens, that increase the rate at which mutations occur. Alternatively, mutations can occur “spontaneously.” Spontaneous mutations are much less frequent (and hence harder to study) than induced mutations, but they are evolutionarily more important. A host of different molecular mechanisms underlie mu-tation, ranging from the reaction of DNA with highly re-active products of cell metabolism to mistakes in the DNA replication process. Cells have evolved sophisticated systems to identify and repair damaged DNA, thereby preventing the occur-rence of mutations. Most notably, there are a variety of re-pair systems, and most of them rely on DNA complemen-tarity. That is, they use one DNA strand as a template for the correction of DNA damage. For example, in the type of repair called excision repair, damage in one strand is cut
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.