Q13.The replication on template strand always proceeds in 5′ to 3′ direction. If the sequence of template strand is 5’ATCCCT3′, how would the correct sequence of newly synthesized strand of DNA be read?
(A) ACCGGAT
(B) AGGGAT
(C) TAGGGA
(D) TACGCT
Correct option: (C) TAGGGA
Introduction
DNA replication on template strand always proceeds in 5′ to 3′ direction, meaning DNA polymerase reads the template from 3′ to 5′ while synthesizing the new strand from 5′ to 3′. For template strand 5’ATCCCT3′, the newly synthesized complementary strand follows base-pairing rules (A-T, G-C). This fundamental concept appears frequently in competitive exams like NEET, CSIR NET, and GATE Life Sciences.
Step-by-Step Solution
Template strand: 5′ – A T C C C T – 3′
Reading direction: DNA polymerase reads template 3′ → 5′ (TCCCCTA)
Complementary base pairing:
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Template 3′ end T → New strand 5′ end A
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Template C → New strand G
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Template C → New strand G
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Template C → New strand G
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Template A → New strand T
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Template T → New strand A
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Template 5′ end A → New strand 3′ end T
Newly synthesized strand (5′ → 3′): 5′ – TAGGGA – 3′
Detailed Analysis of All Options
(A) ACCGGAT – Incorrect
This sequence lacks proper base pairing. Template position 1 (5′-A) should pair with T, not A. Contains G which has no template C counterpart. Wrong length and composition.
(B) AGGGAT – Incorrect
Starts correctly with A (pairs with template T), but then mismatches: template C should pair with G (not G at position 2), and lacks final T for template A. Incorrect base pairing throughout.
(C) TAGGGA – Correct
Perfect complementary pairing:
Template: 5'- A T C C C T -3'
New strand: 3'- T A G G G A -5'
(read as 5'-TAGGGA-3')
Every base matches: A↔T, T↔A, C↔G perfectly.
(D) TACGCT – Incorrect
Template C positions should pair with G, not C or T. Position 4 (template C) pairs with G not C; position 6 (template T) pairs with A not C. Multiple mismatches.
Key Memory Points for Exams
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Template read: Always 3′ → 5′ direction
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New strand built: Always 5′ → 3′ direction
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Base pairing: A-T, G-C (template A = new T, template T = new A, etc.)
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Write new strand: From 5′ end to 3′ end as final answer
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Common trap: Students write sequence as read (3′-5′) instead of final 5′-3′ orientation
Quick Reference Table
This systematic approach ensures 100% accuracy for similar DNA replication MCQs.
