Which one of the following statements is correct?

• A. Adenine forms two hydrogen bonds with guanine; thymine forms three hydrogen bonds with cytosine.
• B. Adenine forms three hydrogen bonds with thymine; guanine forms two hydrogen bonds with cytosine.
• C. Adenine forms two covalent bonds with thymine; guanine forms three covalent bonds with cytosine.
• D. Adenine forms three covalent bonds with thymine; guanine forms two covalent bonds with cytosine.
• E. Adenine forms two hydrogen bonds with thymine; guanine forms three hydrogen bonds with cytosine.

Answer: (E)

Verified step by step guidance

1. Understand the basic structure of DNA, which consists of two strands forming a double helix. The strands are held together by hydrogen bonds between pairs of nitrogenous bases.
2. Identify the four nitrogenous bases in DNA: adenine (A), thymine (T), guanine (G), and cytosine (C). These bases pair specifically: adenine with thymine, and guanine with cytosine.
3. Recall the number of hydrogen bonds formed between these base pairs: adenine and thymine form two hydrogen bonds, while guanine and cytosine form three hydrogen bonds.
4. Evaluate each statement based on the known base pairing rules and the number of hydrogen bonds: Adenine forms two hydrogen bonds with thymine; guanine forms three hydrogen bonds with cytosine.
5. Conclude that the correct statement is: 'Adenine forms two hydrogen bonds with thymine; guanine forms three hydrogen bonds with cytosine.' This aligns with the established base pairing rules in DNA.