Question:
The two strands of DNA are not identical, but complementary to each other. Explain.
The two strands of DNA are not identical, but complementary to each other. Explain.
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Use the base pairing rule: A pairs with T and G pairs with C. One strand therefore decides the other, making them complementary, not identical.
Updated On: Jul 10, 2026
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Solution and Explanation
Step 1: DNA is a double helix made of two polynucleotide strands held together by hydrogen bonds between the nitrogen bases that point inward.
Step 2: Base pairing follows fixed rules. Adenine (A) pairs only with Thymine (T) through 2 hydrogen bonds, and Guanine (G) pairs only with Cytosine (C) through 3 hydrogen bonds. A purine always pairs with a pyrimidine.
Step 3: Because of these rules, the base sequence of one strand fixes the sequence of the other. Wherever one strand has A, the partner strand has T; wherever it has G, the partner has C. So the two sequences are different (not identical) but each is the exact partner, or complement, of the other.
Step 4: The two strands also run antiparallel (one 5' to 3', the other 3' to 5'). Therefore the strands are complementary, not identical.
Step 2: Base pairing follows fixed rules. Adenine (A) pairs only with Thymine (T) through 2 hydrogen bonds, and Guanine (G) pairs only with Cytosine (C) through 3 hydrogen bonds. A purine always pairs with a pyrimidine.
Step 3: Because of these rules, the base sequence of one strand fixes the sequence of the other. Wherever one strand has A, the partner strand has T; wherever it has G, the partner has C. So the two sequences are different (not identical) but each is the exact partner, or complement, of the other.
Step 4: The two strands also run antiparallel (one 5' to 3', the other 3' to 5'). Therefore the strands are complementary, not identical.
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