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About This Drill
AP Biology — Unit 6 — Transcription & RNA Processing — Drill 24 is a practice drill. It contains 5 original questions created by Brian Stewart, a Barron's test prep author with over 20 years of tutoring experience.
Test your understanding of transcription, mRNA processing, and the steps that transform a pre-mRNA into a mature, export-ready transcript.
Passage
A molecular biology laboratory is studying gene expression in human liver cells. Researchers isolate a gene encoding a metabolic enzyme with four exons and three introns. The DNA template strand sequence at the transcription start site reads 3'-TACGCCATG-5'. After transcription, the pre-mRNA undergoes processing before export from the nucleus. Note that in living cells, the 5' cap is added co-transcriptionally; the table below represents processing stages for instructional clarity.
RNA Processing Stages:
| Pre-mRNA: 2,400 nucleotides | 5' Cap: No | Poly-A Tail: No | Introns Present: Yes |
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| Processed mRNA: 1,100 nucleotides | 5' Cap: Yes | Poly-A Tail: No | Introns Present: No |
| Mature mRNA (export): 1,350 nucleotides | 5' Cap: Yes | Poly-A Tail: Yes | Introns Present: No |
Questions in This Drill
- Based on the data in the table, what is the approximate length of the poly-A tail added during mRNA processing?
- The DNA template strand reads 3'-TACGCCATG-5'. What is the mRNA sequence transcribed from this region?
- A mutation occurs in the splice site at the boundary of intron 2 and exon 3, preventing spliceosome recognition. Which outcome is most likely?
- A researcher claims that the 5' cap is required for ribosome binding during translation initiation. Which finding from the table is most consistent with this claim?
- A second gene in the same liver cell also has four exons but produces three distinct protein isoforms in different cell types. The pre-mRNA length is identical across all three cell types. Which mechanism best explains this observation?