Question

You obtain the sequence of a gene containing 10 exons, 9 introns, and a 3' UTR containing a polyadenylation consensus sequence. The fifth intron also contains a polyadenylation site. To test whether both polyadenylation sites are used, you isolate mRNA and find a longer transcript from muscle tissue and a shorter transcript from all other tissues. Which of the following is the most likely mechanism underlying these alternative transcripts?

A. The mRNA in muscle cells is edited to eliminate the polyadenylation sequence in exon 5

B. An alternative CPSF protein is expressed in muscle cells with lower affinity for the polyadenylation sequence in exon 5.

C. The fifth exon is spliced out of the mature mRNA transcript in muscle cells

D. Muscle cells have less effective polyadenylation than other tissues

Answer 1

Answer: Choice C

Reason: According to the information, the sequence contains many introns and exons and hence it is suggestive of the fact that the genome is from an eukaryote. As a typical features, eukaryotic cells perform post-transcriptional modification and splicing of the nascent mRNA in which extensive editing is performed. Here, it must be carefully noted that mere presence of the polyadenylation site does not ensure transcription termination and generation of a shorter mRNA, but presence of a stop codon is required. Thus, the only method by which the muscle cells are producing a different length of mRNA could be alternative spicing as compared to other cell types.