Q.12 The complementary sequence for the DNA stretch ‘GATCAATCGAACT, would be
1. CTAGTTAGCTTGA
2. CTAGWTCGUUCT
3. AGTTCGATTGATC
4. AGUUCGAWGAUC
The complementary sequence for the DNA stretch ‘GATCAATCGAACT’ is CTAGTTAGCTTGA, so the correct answer is option 1.
Option Analysis
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Option 1: CTAGTTAGCTTGA. Correct. DNA base pairing follows Watson-Crick rules: G pairs with C, A with T, T with A, C with G. Reading the given strand 5′ to 3′ (G-A-T-C-A-A-T-C-G-A-A-C-T), the complementary antiparallel strand (written 5′ to 3′) is C-T-A-G-T-T-A-G-C-T-T-G-A.
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Option 2: CTAGWTCGUUCT. Incorrect. Contains invalid bases ‘W’ and ‘U’; DNA uses only A, T, C, G. ‘U’ is for RNA.
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Option 3: AGTTCGATTGATC. Incorrect. This is the reverse sequence of the original (not complementary), as it mirrors the order without base swapping (e.g., starts with A instead of C for G).
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Option 4: AGUUCGAWGAUC. Incorrect. Mixes RNA bases (U for T) and invalid ‘W’; not a valid DNA sequence.
Introduction to DNA Complementary Sequence
The DNA complementary sequence for GATCAATCGAACT is CTAGTTAGCTTGA, determined by Watson-Crick base pairing where A pairs with T and G with C. This ensures the double helix’s stability and accurate replication, vital for exams like GATE Life Sciences. Understanding this process helps solve sequence-based questions precisely.
Step-by-Step Base Pairing
Write the original 5′-GATCAATCGAACT-3′. Pair complements: G→C, A→T, T→A, etc., yielding 3′-CTAGTTAGCTTGA-5′. Conventionally written 5′ to 3′: 5′-CTAGTTAGCTTGA-3′. Antiparallel orientation is key—no reverse without pairing.
| Position | Original (5′-3′) | Complement (5′-3′) |
|---|---|---|
| 1 | G | C |
| 2 | A | T |
| 3 | T | A |
| … | … | … |
| 13 | T | A |
Common Errors in Sequences
Options with U (RNA base) or invalid letters like W fail DNA rules; reverses ignore pairing specificity. Always verify A=T, G=C ratios per Chargaff’s rules.
Exam Relevance
Mastering DNA complementary sequences aids replication, PCR, and mutation analysis in molecular biology. Practice with tools like sequence aligners for competitive prep.
