It has been recently determined that the gene for Duchenne muscular dystrophy (DMD) is more than 2000 kb (kilobases) in length; however, the mRNA produced by this gene is only about 14 kb long. What is a likely cause of this discrepancy?

(A) More amino acids are coded for by the DNA than by the mRNA.

(B) The exons have been spliced out during mRNA processing.

(C) When the mRNA is produced, it is highly folded and therefore less long.

(D) The introns have been spliced out during mRNA processing.

(E) The DNA represents a double-stranded structure, whereas the RNA is single-stranded.

Answer. D

Explanation:

After the messenger RNA (mRNA) is formed, it has to go trough different steps before being translated into proteins, also known as maturation of the mRNA. One of the most important steps is splicing, a process that removes the introns (regions of the sequence that do not codify for any particular amino acid sequence or protein). In other words, the splicing process removes sequences that do not generate any protein, leaving only the exons to be translated into protein. However, the genes (within our DNA) will contain exons and introns; Therefore, the gene sequence will have more kilobases compared to mature mRNA as the introns in this macromolecule have been spliced out.

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