Question

“Introns and Exons” essay question

Explain why the average rate of evolution differs between introns and exons, and why this is important for choosing genetic markers to be used in phylogeny construction.

Answer 1

Introns exhibit a greater genetic variation (Sequence variation) compared exons. The conservation of a sequence depends upon its importance. The source and frequency of mutation origin are same for both introns and exons. However, exons are under selection pressure due to their functional significance compared to introns. Hence exons exhibit greater sequence conservation compared to introns.

Exons are the coding sequences. So, if there are any mutation in exons, they might negatively affect the coded product (RNA or protein). So, even if a deleterious mutation arises in an exon, it will be eliminated from the population. So, its allelic frequency is always low.

Introns are non-coding sequences. Except for few intronic sequences (Which are involved in gene regulation and splicing), most of the introns can tolerate mutations as they do not code for a trans-factor.

If we want to construct a phylogeny based on recent evolutionary advances, we should use intronic sequences for phylogeny construction. This is because of the fact that exons require a large amount of time for mutation accumulation.

If we want to construct a phylogeny based on evolutionary advances that occurred over a long period of time, we should use exonic sequences for phylogeny construction. This is because of the fact that exons accumulate mutations and these mutations are subjected to selection pressure.