Point mutations

Point mutationsmost frequently consist in the replacement of one nucleotide by another; these are replacement mutations, substitutions.If a nucleotide with a certain type of base, e.g. pyrimidine (C, T), is replaced by a nucleotide with a different type of base – purine (A, G), then this corresponds to transversion; if it is replaced by a nucleotide with a base of the same type, then this is a transition.Point mutations also include deletion and insertion, in which the number of nucleotides is changed at a certain place in the DNA, usually by one; however dinucleotide insertions anddeletions are also quite frequent.The frequencies of the individual types of mutations differ considerably and depend not only on the type of organism (bacteria, eukaryote) and on the genome, in which the mutation occurs (nucleus, mitochondria, plastid), but also on the nucleotides occurring close to the given position.For example, it was found for chloroplast DNA that, if a certain nucleotide is adjoined from both sides A and T, then the probability of transversion is 2.2 times greater at this site than the probability of transition.However, if at least one of the neighbouring nucleotides is G or C, then transitions are 1.5 times more probable at the given site than transversions (Morton & Clegg 1995).However, these ratios differ somewhat for various genes, so that it is apparent that the probability of the individual types of exchanges is not affected only by the immediately neighbouring nucleotides.

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