Mutations can be reversed

Just as it is possible for a mutation to occur spontaneously, so it is possible for the nucleotide change causing it to be spontaneously reversed - in other words, a mutant can mutate back to being a wildtype. This is known for obvious reasons as a reverse or back mutation. When this happens, the original genotype and phenotype are restored. Whereas a forward mutation results from any change that inactivates a gene, a back mutation is more specific; it must restore function to a protein damaged by a specific

Template strand

TACGAGTCCCTAACCTGA

TACGAGTCCCTAACCTGA

AUGCUCAGGGAUUGGACU

AUGCUCAGGGAUUGGACU

mRNA

TACAGTCCCTAACCTGA

TACAGTCCCTAACCTGA

AUGUCAGGGAUUGGACU

Template strand

AUGUCAGGGAUUGGACU

mRNA

Peptide

Figure 11.18 Frameshift mutations. (a) A short sequence of DNA is transcribed into mRNA and then transcribed into the corresponding amino acids. (b) The fourth nucleotide in the sequence is deleted, upsetting the groups of triplets or reading frame. This alters the sense of the remainder of the message, leading to a completely different sequence of amino acids mutation. Not surprisingly, given this specificity, the rate of back mutations is much less frequent.

It is possible for the wildtype phenotype to be restored, not through a reversal of the original base change, but due to a second mutation at a different location. The effect of this second mutation is to suppress the effects of the first one. These are called suppressor or second site mutations. They are double mutants that produce a pseudowildtype; the phenotype appears to be wildtype, but the genotype differs.

0 0

Post a comment