Gene mutations can arise during DNA replication. Explain why different types of gene mutation may have different effects on the encoded polypeptide.

Gene mutations arise due to errors in DNA replication and include insertion, deletion, substitution, inversion, duplication and translocation of bases. Each triplet of bases encodes one amino acid. Therefore, base mutation can change the amino acid sequence of the polypeptide encoded by the gene. However, the effect on the polypeptide depends on the type of mutation. For example, due to the degenerate nature of the genetic code, a substitution of one base may result in a triplet that encodes the same amino acid as was encoded by the original base sequence, in which case there will be no effects on the encoded polypeptide. However, some mutations could drastically affect the encoded amino acid. For example, a substitution may result in a triplet that encodes a premature stop codon. This would result in a truncated amino acid sequence - which is the primary structure of a polypeptide - which would in turn affect the secondary and tertiary structures of the polypeptide and could impair or abolish its function.

Another type of mutation that could significantly affect the encoded polypeptide is an indel - an insertion or deletion - as these can cause a frameshift. In a frameshift, the sequence of bases that encodes the amino acid chain is shifted to another reading frame. For example, a region of the gene may be read in the following triplets: GCG CAA GAT. If the first G is deleted, the whole base sequence shifts so the triplets are now read as CGC AAG AT… and so on. Indels can therefore severely alter the amino acid sequence and hence the secondary and tertiary polypeptide structures, which again may significantly reduce or abolish the function of the polypeptide.