DNA base pairing and direction
Write a complementary strand while preserving antiparallel orientation.
What you will learn
- Apply complementary base-pairing rules.
- Distinguish complement from reverse complement.
Bases pair specifically
In DNA, adenine pairs with thymine and cytosine with guanine. Hydrogen bonds hold complementary bases across the strands, while each strand's sugar–phosphate backbone is joined by covalent bonds.
Complementarity means the sequence of one strand determines the other. It does not mean the strands have identical sequences.
The strands run in opposite directions
A DNA strand has a 5′ end and a 3′ end. In the double helix, paired strands are antiparallel. If the top strand is written 5′ to 3′ from left to right, its aligned complement reads 3′ to 5′ from left to right.
If asked to write the complementary strand in the 5′ to 3′ direction, reverse the aligned complement. Direction labels prevent ambiguity.
Worked example
Write the complement of 5′-GATCA-3′ in the 3′ to 5′ direction.
Show the worked solution
- Replace G with C, A with T, T with A, C with G and A with T.
- Keep the aligned order because the requested direction is 3′ to 5′.
- Include the direction labels.
Answer 3′-CTAGT-5′
Common mistakes
- DNA uses thymine; RNA uses uracil instead.
- Do not reverse the sequence unless the requested writing direction requires it.
What can you explain now?
Write the same complementary strand in the 5′ to 3′ direction.
Compare with the explanation
5′-TGATC-3′
Reverse the aligned complement CTAGT to write it from its 5′ end.
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