A genetics researcher is studying a novel eukaryotic gene that produces a protein with a molecular weight of 50 kDa. However, the initial mRNA transcript detected in the nucleus is significantly longer than what would be required to encode a 50 kDa protein. Which process explains this discrepancy?
- AAlternative splicing
- BPost-translational modification
- CRNA editing
- DDNA replication errors
Show answer & explanationAnswer & explanation
Correct answer: A. Alternative splicing
In eukaryotes, genes contain both coding regions (exons) and non-coding regions (introns). The initial mRNA transcript, known as pre-mRNA or hnRNA, contains both. During splicing, introns are removed, and exons are ligated together to form the mature mRNA. Alternative splicing allows different combinations of exons to be included or excluded, leading to different protein products from a single gene. The initial longer transcript containing introns, which are later removed, explains why the nuclear mRNA is longer than the final coding sequence.
Why the other options are wrong
- B. Post-translational modification occurs after protein synthesis and involves changes to the protein itself, not the mRNA transcript length.
- C. RNA editing involves changes to individual nucleotides, not large-scale removal of non-coding regions, which explains the significant length difference.
- D. DNA replication errors affect the DNA sequence, not directly the length discrepancy between nuclear mRNA and coding protein.
Eukaryotic mRNA Processing
Eukaryotic pre-mRNA undergoes several processing steps in the nucleus, including 5' capping, 3' polyadenylation, and splicing, to become mature mRNA ready for translation.
- 5' capping adds a modified guanine nucleotide
- 3' polyadenylation adds a poly-A tail
- Splicing removes introns and joins exons
- Occurs in the nucleus before export to cytoplasm
Memory trick: Cap, Tail, Splice, Ship.