a. Transition mutations are mutations caused by substitution of a purine for a purine (A-G) or substitution of a pyrimidine for a pyrimidine (C-T). The bases are substituted by another base of the same chemical category.
Compared with transversions, transition mutations are generated at higher frequency. Transitions do not result in amino acid alterations and remain as silent mutations.
Transversion mutation is caused by the substitution of a purine by a pyrimidine or substitution of a pyrimidine by a purine. The bases which substitute are from different chemical category. This causes drastic changes in the chemical structure and there are serious consequences. The causes of transversion mutation are ionic radiation or alkylating agents. Transversion is reversed by spontaneous reversion.
b. Synonymous and neutral mutations are base-pair substitutions.
In synonymous mutation, a new codon is formed due to base-pair substitution. This new codon codes for the same amino acid as the wild-type. Hence, in this mutation the amino acid sequence is not altered.
Neutral mutation does not have any significance. The reason is that this mutation results in a different amino acid which has the same function.
c. The missense mutation produces different amino acids and different protein product of the gene. It is a point mutation. In this mutation a single nucleotide is changed. Examples of missense mutations are sickle cell disease and cystic fibrosis. A nonsense mutation, the new nucleotide changes a codon which codes for certain amino acid to one of the stop codons. So, the process of translation is terminated prematurely due to the stop codon formed in the mutation. This results in shortened protein.
d. Frameshift mutations results when one or more bases is deleted or added in the sequence. It results in a sequence in other than multiples of three. So the reading frame for translation is altered. So, the amino acid sequence also will be changed from the site of the mutation to the end. Frameshift mutations produces stop codons in the new reading frame and hence shortens the protein.
Nonsense mutation produces stop codons in the original reading frame, resulting in termination of translation prematurely and results in shortened protein.