MCAT® ExamBiological and Biochemical Foundations of Living SystemsMedium
A genetics researcher is studying a novel eukaryotic gene that produces a protein with a molecular weight of 50 kDa. They discover that the mature mRNA transcript for this protein is significantly shorter than the initial pre-mRNA transcript. This difference in length is primarily due to:
- A5' capping and 3' polyadenylation, which remove untranslated regions.
- BDegradation of mRNA by ribonucleases in the cytoplasm.
- CAlternative splicing, where different exons are included or excluded.
- DThe removal of introns during mRNA processing.
Show answer & explanationAnswer & explanation
Correct answer: D. The removal of introns during mRNA processing.
Eukaryotic genes contain non-coding regions called introns that are transcribed into pre-mRNA but are subsequently removed during splicing. The coding regions, exons, are then ligated together to form the mature mRNA, making it significantly shorter than the initial pre-mRNA.
Why the other options are wrong
- A. 5' capping and 3' polyadenylation add to the ends of the mRNA, they do not shorten the transcript significantly by removing large internal segments.
- B. mRNA degradation occurs after translation and is not the cause of the initial length difference between pre-mRNA and mature mRNA.
- C. Alternative splicing explains how different proteins can be made from one pre-mRNA, but the general shortening is primarily due to intron removal.
Eukaryotic mRNA Processing
In eukaryotes, the primary RNA transcript (pre-mRNA) undergoes several modifications before becoming mature mRNA. These include the addition of a 5' cap, a 3' poly(A) tail, and the removal of non-coding introns through a process called splicing.
- Pre-mRNA is the initial transcript from DNA.
- Splicing removes introns and joins exons.
- 5' cap protects mRNA and aids ribosome binding.
- Poly(A) tail protects mRNA and aids export/translation.
Memory trick: mRNA Processing: 'Intron-Out, Exon-In' for the Mature Message.