A piece of mRNA is 660 nucleotides long while the DNA coding strand from which it was transcribed is 870 nucleotides long. Why is the mRNA shorter?

• A. DNA contains introns; mRNA is edited to remove introns and contain only exons.
• B. mRNA length is shorter due to random degradation.
• C. mRNA length is increased by processing.
• D. DNA has introns that are spliced out of mRNA.

Answer: (A)

In gene expression, the initial transcript from a eukaryotic gene includes both exons (coding regions) and introns (noncoding interruptions). During RNA processing, introns are removed and the exons are joined together to form the mature mRNA. That means the mature mRNA contains only the exons and is shorter than the DNA coding region, which includes both exons and introns. So, a 660-nucleotide mRNA can arise from a gene whose DNA coding sequence is 870 nucleotides long because the introns are spliced out during processing. The other ideas don’t fit because random degradation isn’t a regulated, size-specific process, processing doesn’t typically increase length, and intron removal happens in the RNA, not by removing introns from DNA.