Final answer:
The correct statement regarding DNA's secondary structure is that bases on complementary strands are held together by hydrogen bonds. Adenine pairs with thymine forming two bonds, and cytosine pairs with guanine forming three. The strands are antiparallel with sugar-phosphate backbones on the outside.
Step-by-step explanation:
The secondary structure of DNA is characterized by the specific pairing of nitrogenous bases and the orientation of the sugar-phosphate backbone. Among the given options, number 3 is correct - 'Bases on complementary strands are held together by hydrogen bonds.' This matches the established knowledge that in the DNA double helix, adenine (A) pairs with thymine (T) with two hydrogen bonds, and cytosine (C) pairs with guanine (G) with three hydrogen bonds. Additionally, it is important to note that the two strands of DNA are antiparallel; that is, one strand runs in the 5' to 3' direction, while the complementary strand runs in the 3' to 5' direction, and the sugar-phosphate groups are on the outside of the molecule.
DNA is composed of nucleotides, consisting of a 5 carbon sugar, a phosphate, and a nitrogenous base. DNA is a double helical structure in which complementary base pairing occurs. Adenine pairs with thymine and guanine pairs with cytosine. Adenine and thymine form two hydrogen bonds and cytosine and guanine form three hydrogen bonds. The two individual strands of DNA are held together by covalent bonds between the phosphate of one nucleotide and sugar of the next. The two strands run antiparallel to each other.