Biology • Year 12 • Module 5 • Lesson 10
Transcription — From DNA to mRNA
Apply the DNA→mRNA pairing rule to real sequences, real research-style data, and a CFTR mutation scenario.
1. Sequence the steps of transcription
The eight events below describe what happens during transcription of one gene in a eukaryotic cell, but they are shuffled. Write the correct order (1 = first, 8 = last) in the "Order" column. 8 marks
| Order | Event (shuffled) |
|---|---|
| The newly formed mRNA strand separates from the DNA template and carries the coded information away. | |
| Cytosine on the DNA template pairs with guanine on the mRNA, and guanine pairs with cytosine. | |
| The cell requires a polypeptide that a particular gene codes for. | |
| The mRNA carries codons (three-base units) ready for the next stage of polypeptide synthesis. | |
| The double helix of DNA at that gene unwinds, exposing the bases of the template strand. | |
| The DNA template strand re-anneals (rewinds) with its partner strand inside the nucleus. | |
| Adenine on the DNA template pairs with uracil on the new mRNA (instead of thymine). | |
| RNA nucleotides line up against the exposed template strand using complementary base pairing. |
2. Graph interpretation — transcription rate vs RNA polymerase concentration
The figure below is a stylised model based on in vitro transcription assays. It shows how the number of mRNA molecules produced per minute from a single gene changes as the concentration of RNA polymerase II is increased. 7 marks
Stylised in vitro data — adapted from typical eukaryotic transcription assay literature (after Sims et al., 2004; values illustrative).
2.1 Describe the trend shown by the graph. 2 marks
2.2 Estimate the transcription rate at an RNA polymerase concentration of 15 nM, and again at 40 nM. 2 marks
2.3 Explain why the curve plateaus rather than continuing to rise, in terms of transcription needing a DNA template strand and limited gene access. 3 marks
3. Cause-and-effect chain — what a CFTR base-substitution does at the mRNA level
The first four "Cause" boxes are filled in for you. Complete the matching "Effect" boxes by describing what each cause produces, then write the overall outcome (so…) at the end. 5 marks (1 per effect + 1 overall outcome)
| Cause | Effect — what does this directly produce? |
|---|---|
| 3.1 A single base on the CFTR DNA template strand changes from cytosine to thymine. | |
| 3.2 During transcription, RNA nucleotides pair with the exposed bases of the (now altered) template strand using complementary base pairing. | |
| 3.3 The new mRNA strand contains a different RNA base at that position compared with the wild-type mRNA. | |
| 3.4 The mRNA strand separates from the DNA and is read in three-base units. |
3.5 Overall outcome — so… In one sentence, state what this chain shows about how a DNA change can affect the cell even before translation happens.
4. Predict and justify — translating a DNA template
A student is given the following DNA template strand for a short section of a gene:
3' — T A C G G A T T C A C T — 5'
4.1 Write the mRNA sequence (in codons) that this template will produce during transcription. 2 marks
4.2 Predict what the mRNA sequence would become if the third base on the template strand changed from cytosine (C) to thymine (T). Write the new mRNA in codons and identify which codon changes. 2 marks
4.3 Justify why only one codon changes (not all four), using the language of complementary base pairing. 2 marks
Q1 — Sequence the steps (8 marks)
Correct order:
- The cell requires a polypeptide that a particular gene codes for.
- The double helix of DNA at that gene unwinds, exposing the bases of the template strand.
- RNA nucleotides line up against the exposed template strand using complementary base pairing.
- Adenine on the DNA template pairs with uracil on the new mRNA (instead of thymine).
- Cytosine on the DNA template pairs with guanine on the mRNA, and guanine pairs with cytosine.
- The newly formed mRNA strand separates from the DNA template and carries the coded information away.
- The DNA template strand re-anneals (rewinds) with its partner strand inside the nucleus.
- The mRNA carries codons (three-base units) ready for the next stage of polypeptide synthesis.
1 mark per correctly placed event (steps 4 and 5 may be swapped — they are simultaneous applications of the same pairing rule — accept either order for those two).
Q2.1 — Trend (2 marks)
As RNA polymerase II concentration increases from 0 to about 30 nM, the rate of mRNA production rises steeply [1]. Above 30 nM the rate plateaus at approximately 17–18 mRNA molecules per minute and does not increase further with more enzyme [1].
Q2.2 — Estimates (2 marks)
At 15 nM, transcription rate ≈ 13 molecules min⁻¹ (accept 11–14) [1]. At 40 nM, transcription rate ≈ 18 molecules min⁻¹ (accept 17–18) [1].
Q2.3 — Why the curve plateaus (3 marks)
Transcription requires the enzyme to bind to a DNA template strand at the gene [1]. There is only a limited number of copies of the gene's template strand available at any one moment, so adding more polymerase past that point cannot speed up transcription further — every available template is already being used [1]. The plateau therefore reflects template availability becoming the limiting factor, not enzyme concentration [1].
Q3 — Cause-and-effect chain (5 marks)
3.1 Effect: The base at that position on the template strand is now thymine. Adjacent bases on the rest of the template are unchanged. [1]
3.2 Effect: RNA nucleotides still pair complementarily with each exposed template base — A with U, T with A, C with G, G with C — but the altered base on the template will now pair with a different RNA nucleotide than before. [1]
3.3 Effect: The mRNA contains an altered base at exactly one position, which sits inside one specific codon of the mRNA. [1]
3.4 Effect: When the mRNA is read in three-base units, one codon carries different transferable information from the wild-type mRNA, while the other codons are unchanged. [1]
3.5 Overall outcome: A change in the DNA base sequence can change the codons carried by the mRNA at the transcription step itself, before translation even begins — so the message that leaves the nucleus is already different. [1]
Q4.1 — mRNA from the template (2 marks)
Template (3'→5'): T A C G G A T T C A C T
mRNA (5'→3'): A U G C C U A A G U G A [2 marks — 1 for correct base pairing throughout, 1 for grouping into the correct four codons]
Q4.2 — After the C→T substitution (2 marks)
The third base of the template changes from C to T, so the matching mRNA base at that position changes from G to A. The new mRNA reads: A U A C C U A A G U G A [1]. The first codon has changed from AUG to AUA [1].
Q4.3 — Why only one codon changes (2 marks)
Complementary base pairing acts on each base independently — every other base on the template still pairs with the same RNA nucleotide as before [1]. Only the one altered template base produces a different RNA base in the mRNA, and that altered base happens to sit inside one codon, so only that codon is affected [1].