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Highly expressed genes have a much greater tendency to lose introns (in evolutionary time scale) than genes expressed at very low levels. What molecular mechanism explains this difference?

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This question shows a case of biological cost-effectiveness. First consider the process of transcribing genes. DNA is first transcribed into RNA called the primary transcript. Then the RNA goes through an extensive splicing process, using spliceosomes, that joins the exons together and cuts out the introns. After other modifications (like a methylated cap and poly-A tail), a mature mRNA is produced. In this entire process the introns that are in the primary transcript of RNA are simply extra material and are ...