Question

1. thymine and adenine form a base pair in the dna molecule. these two bases can form a connection between two strands of dna via two hydrogen bonds. which of the following diagrams shows the correct representation of the hydrogen bonding (denoted by dashed lines) between thymine and adenine base pairs? (in each diagram, thymine is shown at the left and adenine is shown at the right. the bases are attached to the backbone portion of the dna strands.) (a) diagram (b) diagram (c) diagram (d) diagram

Explanation:

To determine the correct hydrogen bonding between thymine (left) and adenine (right) in DNA:

1. Thymine Structure: Contains a carbonyl group (\( \text{C}=\text{O} \)) and an \( \text{N}-\text{H} \) group (or relevant H-bond donors/acceptors).
2. Adenine Structure: Contains \( \text{N}-\text{H} \) groups (H-bond donors) and nitrogen atoms (H-bond acceptors).
3. Hydrogen Bonding Rules: H-bonds form between a hydrogen bonded to a highly electronegative atom (N, O, F) (donor) and a lone pair on another electronegative atom (acceptor).

• Option A: Thymine (left) has \( \text{C}=\text{O} \) (acceptor) and \( \text{C}-\text{H} \) (not a donor). Adenine (right) has \( \text{N}-\text{H} \) (donor) and N (acceptor). The dashed lines show H-bonds between \( \text{C}-\text{H} \) (thymine) and N (adenine), and \( \text{C}=\text{O} \) (thymine) and \( \text{N}-\text{H} \) (adenine) — incorrect, as \( \text{C}-\text{H} \) is a weak donor.
• Option B: Thymine’s structure is misrepresented (lacks the correct carbonyl and ring structure for thymine).
• Option C: H-bonds shown between \( \text{C}-\text{H} \) (thymine) and \( \text{N}-\text{H} \) (adenine), and \( \text{N}-\text{H} \) (thymine) and \( \text{C}-\text{H} \) (adenine) — incorrect, as \( \text{C}-\text{H} \) is not a valid donor/acceptor for strong H-bonds.
• Option D: Thymine (left) has \( \text{C}=\text{O} \) (acceptor, bonds to \( \text{N}-\text{H} \) of adenine) and \( \text{N}-\text{H} \) (donor, bonds to N of adenine). Adenine (right) has \( \text{N}-\text{H} \) (donor) and N (acceptor). This matches the correct H-bonding pattern for A-T (two H-bonds: \( \text{C}=\text{O} \cdots \text{H}-\text{N} \) and \( \text{N}-\text{H} \cdots \text{N} \)).

Answer:

D. The diagram showing correct hydrogen bonding between thymine (left) and adenine (right) via two hydrogen bonds, with proper donor-acceptor interactions.