- Classify mutations in coding regions as silent, nonsense, missense, or frameshift.
The effect of a mutation will depend upon the function of the DNA sequence. Mutations in non-protein coding regions could occur in regulatory regions that affect genes or have no effect on a cell or organism.
When mutations occur in coding sequences, we can predict the effect on the primary structure of the protein using the codon table.
- Silent mutations: do not change the encoded amino acid
- Nonsense mutations: change a codon to a STOP codon
- Missense mutations: change a codon to a codon for a DIFFERENT amino acid
- Frameshift mutations: add or remove bases to change all downstream codons
Multiples of three
Adding or removing bases in multiples of three, will not cause a frameshift mutation. Why not?
Would mutations like this affect the protein? What additional information would you need to make a prediction about such a mutation?
Do mutations affect protein function?
Both frameshift and nonsense mutations are likely to affect protein function by producing a shortened or extremely altered peptide that is unlikely to retain enough structure to perform the intended function. However, a frameshift or nonsense mutation that occurs near the 3' end of the gene might allow the protein to retain the ability to function.
Silent mutations are least likely to affect protein function.
Missense mutations are often difficult to predict. If the change is to a similar amino acid or in a non-critical amino acid, the mutation protein may continue to function. However, if the amino acid is changed to a different type of amino acid or removes a critical amino acid (in terms of structure or activity), then the mutant protein may change in activity.
Mutations that reduce the function of a gene product are called loss-of-function. If the gene product is completely non-functional, the allele is termed a null allele. Loss-of-function and null alleles are usually (but not always) recessive in diploid organisms, because one functional allele can usually supply enough protein product.
Some mutations actually make proteins more active, for example, by altering an inhibitory domain; these mutations are called gain-of-function. Gain-of-function alleles are often dominant alleles because their new function or activity is produced independently of the other allele.