Final answer:
The complementary strands of DNA are held together by complementary base pairing, where adenine pairs with thymine and guanine pairs with cytosine, forming hydrogen bonds that maintain the helix's uniform shape. Option number b is correct.
Step-by-step explanation:
The complementary strands of DNA are held together by specific pairings of nitrogenous bases, known as complementary base pairing. Option b. They are held by complementary base pairing; adenine to thymine and guanine to cytosine is correct. Adenine (A) pairs with thymine (T) through two hydrogen bonds, while guanine (G) pairs with cytosine (C) through three hydrogen bonds. This pairing is crucial because it maintains the uniform shape of the DNA molecule, ensuring a constant distance between the two DNA strands, which is essential for the double helix structure.
Furthermore, the logic behind these specific pairings is based on the molecular structures of the nitrogenous bases. Adenine and guanine are purines, which have a two-ring structure, while cytosine and thymine are pyrimidines, with a single-ring structure. This pairing of a purine with a pyrimidine keeps the helix's diameter uniform. The strands of DNA are also anti-parallel, meaning the 3' end of one strand faces the 5' end of the other, adding to the exact fit between nucleotides of the opposing strands.