Question

For a given gene and its encoded protein, explain why greater variation in nucleic acid sequence than amino acid sequence is observed when the sequences from one species are compared with evolutionarily related species.

Answer 1

• To construct a phylogenetic tree, it is necessary to compare a species with other evolutionary related species.
• Molecular techniques are used for such evolutionary trend determination, by comparing the sequences of DNA, RNA or protein.
• The recent evolutionary classifications and phylogenetics are based on:

DNA base sequences + Amino acid bas sequences.

• Natural selection based on mutations, plays an important role in molecular evolution of DNA and proteins.
• Mutations lead to variations in the DNA sequence. In eukaryotes the mutation may change base pair in intros or exons.
• Species that are closely related have more conserved DNA sequences that distantly related species.
• However, most genomic alterations do not change the protein sequences. Most of the mutations are detected and repaired before expression into proteins.
• Evolutionary mutations may be either of:

1. Synonymous substitution: Also called silent substitution. Involves evolutionary substitution of a base with another in the exon of gene, without causing any modification of amino acid sequence (due to degenerative nature of codon for amino acids).

2. Non-Synonymous substitution: Results in change in amino acid sequences. This may be:

a. Conservative- Ami acids generated have same properties.

b. Semiconservative- negative to positive charged amino acids etc.

c. Radical- different amino acid produced.

Due to these circumstances, there are more alteration of DNA sequences than protein sequences.