The effects of mutations can be reversed

KEY TERMS:

• Revertants are derived by reversion of a mutant cell or organism to the wild-type phenotype.
• Forward mutations inactivate a wild-type gene.
• A back mutation reverses the effect of a mutation that had inactivated a gene; thus it restores wild type.
• A true reversion is a mutation that restores the original sequence of the DNA.
• Second-site reversion occurs when a second mutation suppresses the effect of a first mutation.
• Suppression occurs when a second event eliminates the effects of a mutation without reversing the original change in DNA.
• A suppressor is a second mutation that compensates for or alters the effects of a primary mutation.

KEY CONCEPTS:

• Forward mutations inactivate a gene, and back mutations (or revertants) reverse their effects.
• Insertions can revert by deletion of the inserted material, but deletions cannot revert.
• Suppression occurs when a mutation in a second gene bypasses the effect of mutation in the first gene. 

Figure 1.22 shows that the isolation of revertants is an important characteristic that distinguishes point mutations and insertions from deletions:

• A point mutation can revert by restoring the original sequence or by gaining a compensatory mutation elsewhere in the gene.
• An insertion of additional material can revert by deletion of the inserted material.
• A deletion of part of a gene cannot revert.

Mutations that inactivate a gene are called forward mutations. Their effects are reversed by back mutations, which are of two types.

An exact reversal of the original mutation is called true reversion. So if an A·T pair has been replaced by a G·C pair, another mutation to restore the A·T pair will exactly regenerate the wild-type sequence.

Alternatively, another mutation may occur elsewhere in the gene, and its effects compensate for the first mutation. This is called second-site reversion. For example, one amino acid change in a protein may abolish gene function, but a second alteration may compensate for the first and restore protein activity.

A forward mutation results from any change that inactivates a gene, whereas a back mutation must restore function to a protein damaged by a particular forward mutation. So the demands for back mutation are much more specific than those for forward mutation. The rate of back mutation is correspondingly lower than that of forward mutation, typically by a factor of ~10.

Mutations can also occur in other genes to circumvent the effects of mutation in the original gene. This effect is called suppression. A locus in which a mutation suppresses the effect of a mutation in another locus is called a suppressor.