Question

What features of prokaryotic genomes suggest that they are under selection for small genomes and how does this differ from higher eukaryotes?

Answer 1

It is the absence of introns in the prokaryotic genome which makes it smaller.

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Prokaryotes lack introns. Introns are non-coding sequence which are otherwise known as junk dna. Only coding sequence (Exon) is present in prokaryotes. They also lack post-transcriptional modification mechanism to splice out the intron.

Eukaryotic genome consist of both intron and exon sequence. Say for example, human genome consist of 3 billion base pair. But 98 - 99% of human genome does not code for anything. It is only that 2% of the human genome codes for a protein. This is the reason that post transcriptional modification mechanism is present in eukaryotes to remove intronic region so that only coding regions are expressed.

From this example it is very much clear that why prokaryotic genome is smaller in size and eukaryotic genome is bigger.