DNA complementarity and orientation: If one DNA strand has the sequence 5′-ATTGCCA-3′, what is the 5′→3′ sequence of the complementary strand?

Introduction / Context:
Working with DNA requires careful attention to base pairing and antiparallel strand orientation. Many exam errors occur from writing complements without reversing orientation.

Given Data / Assumptions:

• Given strand: 5′-ATTGCCA-3′.
• Complementary base pairing: A–T and G–C.
• Complement strands are antiparallel: 5′→3′ pairs with 3′→5′.

Concept / Approach:
First write the complement in the opposite direction (3′→5′), then reverse it to express in the conventional 5′→3′ orientation. This avoids orientation mistakes and ensures correctness.

Step-by-Step Solution:
Given: 5′-A T T G C C A-3′.Complement (3′→5′): 3′-T A A C G G T-5′.Write complement 5′→3′ by reversing: 5′-T G G C A A T-3′.Thus, the answer is 5′-TGGCAAT-3′.

Verification / Alternative check:
Align antiparallel strands and verify each base pair: A–T, T–A, T–A, G–C, C–G, C–G, A–T.

Why Other Options Are Wrong:

• 5′-TAACGGT-3′ is the complement written in the wrong 5′→3′ direction for the opposite strand.
• Repeating the same strand is incorrect.
• RNA option with U is irrelevant to DNA complement.

Common Pitfalls:
Forgetting to reverse the direction after complementing; substituting U for T.

Final Answer:
5′ TGGCAAT 3′