Two DNA strands are described as complementary. What does this mean?

• A. Adenine pairs with thymine and cytosine pairs with guanine.
• B. Each strand has identical sequence to the other.
• C. The sugar-phosphate backbones align in opposite directions.
• D. Bases form covalent bonds between strands

Answer: (A)

Complementary DNA strands have base sequences that fit together through specific base pairing: adenine pairs with thymine and cytosine pairs with guanine. This means the sequence on one strand determines the sequence on the other, so the two strands are not identical but complementary. The strands are held together by hydrogen bonds between these base pairs, while the sugar-phosphate backbones form the covalent links within each strand and run in opposite directions (antiparallel). This precise pairing arrangement is what makes the two strands able to come together to form the double helix and to serve as accurate templates during replication.