Now, what are the causes of mutations to genes and chromosomes? Mutations can be the result of both internal and external factors. Those that are a consequence of natural changes in DNA structure, are termed spontaneous mutations, whereas those that result from changes caused by environmental chemicals or radiation, are known as induced mutations. Let's look at these a little closer:
Spontaneous Replication Errors
DNA replication is impressively accurate, fewer than one in a billion errors are made in the course of DNA synthesis. However, spontaneous replication errors do occasionally occur. Mutations can occur through ‘wobble’, in which non-complementary bases can pair because of the flexibility of the DNA helix. When a mismatched base has been incorporated into a newly synthesized nucleotide chain, an incorporated error is said to have occurred.
Mutations due to small insertions and deletions also may arise spontaneously in replication and crossing over. During normal crossing over, the homologous sequences of the two DNA molecules align, and crossing over produces no net change in the number of nucleotides in either molecule. Misaligned pairing may cause unequal crossing over, which results in one DNA molecule with an insertion and the other with a deletion.
Spontaneous Chemical Changes
In addition to spontaneous mutations that arise in replication, mutations can also result from spontaneous chemical changes in DNA. One such change is called depurination, the loss of a purine base from a nucleotide. This happens when the covalent bond connecting the purine to the carbon atom of the deoxyribose sugar breaks, producing an apurinic site.
Another spontaneous chemical change that can occur, is deamination, the loss of an amino group (NH2) from a base. This may occur spontaneously, but can also be induced by mutagenic chemicals.