Q.30 If the template strand of a segment of a gene has the nucleotide sequence
3′– GCTAAGC –5′, what nucleotide sequence will be present in the RNA transcript specified by this gene segment?
The correct answer is option (2): 5′–CGAUUCG–3′.
RNA transcription uses the DNA template strand to synthesize a complementary RNA strand, replacing T with U.
Transcription Basics
DNA transcription copies a gene segment into RNA. The template strand (3′–GCTAAGC–5′) is read 3′ to 5′, while RNA polymerase builds the RNA transcript 5′ to 3′ using base-pairing rules: A pairs with U, T with A, G with C, C with G.
This ensures the RNA sequence is complementary and antiparallel to the template. For example, the template’s first base (3′-G) pairs with C in RNA.
Step-by-Step Sequence Derivation
Start with template: 3′–G C T A A G C–5′.
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Read left to right (3′→5′ direction).
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Complementary RNA (5′→3′): C G A U U C G.
Result: 5′–CGAUUCG–3′.
Option Analysis
| Option | Sequence | Why Incorrect/Correct |
|---|---|---|
| (1) | 3′–GCTAAGC–5′ | Identical to DNA template; RNA replaces T with U and is complementary, not identical. |
| (2) | 5′–CGAUUCG–3′ | Correct: Fully complementary (G→C, C→G, T→A/U, A→U) with proper 5′–3′ RNA polarity. |
| (3) | 3′–CGATTCG–5′ | Uses DNA base T instead of RNA U; wrong polarity for RNA transcript. |
| (4) | 5′–CGATTCG–3′ | Contains T (DNA base) instead of U; matches coding strand, not template-derived RNA. |
The template strand sequence to RNA transcript conversion is a core concept in molecular biology, vital for competitive exams like GATE Life Sciences. This article breaks down the query: “If the template strand of a gene segment is 3′–GCTAAGC–5′, what is the nucleotide sequence in the RNA transcript?” with step-by-step reasoning, matching your expertise in transcription and genetics.
Understanding DNA Template to RNA
In transcription, RNA polymerase reads the template strand (antisense) from 3′ to 5′, synthesizing RNA transcript from 5′ to 3′. Key rule: DNA T → RNA U; others pair complementarily (A-U, G-C). This produces mRNA identical (except U/T) to the coding strand.
For 3′–GCTAAGC–5′:
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G → C
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C → G
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T → A
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A → U
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A → U
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G → C
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C → G
Yields 5′–CGAUUCG–3′.
Why Each Option Fails (Exam-Style Breakdown)
Option (1) copies DNA verbatim—ignores complementarity and U substitution.
Option (3) retains T and reverses polarity—DNA-like, not RNA.
Option (4) uses T, mimicking coding strand 5′–CGATTCG–3′ (template complement).
Only (2) fits: correct bases, direction, U replacement. Ideal for molecular biology MCQs.
Exam Tips for Similar Questions
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Confirm “template strand” (not coding).
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Write template 3′→5′, build RNA 5′→3′.
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Practice: Template 3′-ATGC-5′ → RNA 5′-GC AU-3′.
Master this for GATE Biotechnology or CSIR NET—focus on polarity pitfalls.
