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AP Biology: Unit 6, Transcription & RNA Processing (Drill 24)

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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 nucleotides5' Cap: NoPoly-A Tail: NoIntrons Present: Yes
Processed mRNA: 1,100 nucleotides5' Cap: YesPoly-A Tail: NoIntrons Present: No
Mature mRNA (export): 1,350 nucleotides5' Cap: YesPoly-A Tail: YesIntrons Present: No

Questions & Explanations

Question 1. Based on the data in the table, what is the approximate length of the poly-A tail added during mRNA processing?

  • A) 150 nucleotides
  • B) 250 nucleotides ✓
  • C) 1,300 nucleotides
  • D) 2,400 nucleotides

Explanation: The processed mRNA (after splicing and 5' capping but before poly-A addition) is 1,100 nucleotides. The mature mRNA ready for export is 1,350 nucleotides. The difference -- 250 nucleotides -- represents the poly-A tail added by poly-A polymerase. A is incorrect because 150 is not the difference between the two measured values. C is incorrect because 1,300 approximates the mature mRNA length, not the tail addition. D is incorrect because 2,400 is the pre-mRNA length before any processing.

Question 2. The DNA template strand reads 3'-TACGCCATG-5'. What is the mRNA sequence transcribed from this region?

  • A) 5'-ATGCGGTAC-3'
  • B) 5'-UACGCCAUG-3'
  • C) 5'-AUGCGGUAC-3' ✓
  • D) 3'-AUGCGGUAC-5'

Explanation: RNA polymerase reads the template strand 3' to 5' and synthesizes mRNA 5' to 3', substituting uracil for thymine. Reading 3'-TACGCCATG-5' gives mRNA 5'-AUGCGGUAC-3'. A uses DNA nucleotides (T instead of U) -- a DNA strand, not mRNA. B copies the template strand sequence directly rather than producing the complementary strand, reflecting a wrong-strand misconception. D contains the correct sequence but written in the wrong orientation (3' to 5' instead of 5' to 3'), reflecting a directionality misconception.

Question 3. 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) The pre-mRNA is degraded immediately and no protein is produced.
  • B) The mature mRNA includes intron 2 sequence, potentially altering the reading frame or inserting a premature stop codon. ✓
  • C) The ribosome skips intron 2 during translation, producing a slightly shorter but functional protein.
  • D) Exon 3 is duplicated to compensate for the missing splice signal.

Explanation: Splice site mutations prevent the spliceosome from recognizing the intron-exon boundary, so intron 2 is retained in the mature mRNA. Introns typically contain stop codons or frameshifting sequences, so their retention disrupts the open reading frame. A is incorrect because unspliced or partially spliced mRNAs are not necessarily degraded immediately -- they may be exported or translated aberrantly. C is incorrect because ribosomes do not recognize or skip intron sequences; they translate whatever sequence is present. D is incorrect because exon duplication is not a compensatory cellular response to splice site mutations.

Question 4. 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) The pre-mRNA is longer than the mature mRNA.
  • B) The processed mRNA lacks a poly-A tail but has a 5' cap.
  • C) The mature mRNA has both a 5' cap and a poly-A tail, and is the form exported for translation. ✓
  • D) Introns are removed before the 5' cap is added.

Explanation: The table shows that only the mature mRNA -- the form exported from the nucleus for translation -- carries a 5' cap. This is consistent with the claim because the exported mRNA, the form available for ribosome access, contains a 5' cap. The table does not prove that the cap is necessary, but the data are consistent with that conclusion. B is a weaker match -- the processed mRNA is an intermediate not yet exported for translation. A describes a length difference that relates to splicing, not cap function. D is contradicted by the table, which shows the 5' cap is present in the processed mRNA after splicing.

Question 5. 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?

  • A) Post-translational modification produces three variants from a single protein.
  • B) Three different promoters drive transcription of three different pre-mRNAs.
  • C) Alternative splicing includes different combinations of exons in the mature mRNA. ✓
  • D) The poly-A tail length varies across cell types, changing which codons are translated.

Explanation: Alternative splicing allows the same pre-mRNA to be processed differently in different cell types by including or excluding specific exons, producing distinct mature mRNAs and therefore distinct protein isoforms. The identical pre-mRNA length across cell types confirms that transcription produces the same initial transcript -- the variation occurs at the splicing stage. A is incorrect because post-translational modification acts on an already-translated protein and does not explain distinct isoforms arising before translation. B is incorrect because different promoters would produce pre-mRNAs that might differ in length or sequence, inconsistent with the identical pre-mRNA observation. D is incorrect because poly-A tail length affects mRNA stability, not which codons are translated.