XI.7.3 Internal gene periodicity exceeding the borders between introns and exons could probably suggest simultaneous evolutionary emergence of both contemporary gene components
Study of the internal periodicity of genes provided indirect evidence for the existence of introns in primitive genes.It has been found that some sequence motifs that appear in exons can also be detected in the neighboring introns.
It is not apparent why the originally continuous DNA sequence would be differentiated into intron and exon regions.According to one hypothesis, this could entail a mechanism permitting encoding of longer protein chains not interrupted by termination codons.As we already mentioned in the section dealing with the internal periodicity of genes (XI.6.2.1), the origin of long sequences of nucleotides not containing any of the three termination codons is a highly improbable phenomenon.The creation of an enzymatic apparatus that is capable of splicing out the RNA sections with termination codons and constructing new mRNA chains from the remaining sections could be a way of resolving this problem (White 1994; Senapathy 1995).
However, an alternative (and much more prosaic) explanation of the occurrence of identical sequence motifs in neighboring introns and exons could be the already-mentioned propagation of these motifs from the exons to the introns through molecular drive.
- more or less100
- not at all192