Answer: Hybrid density (15N-14N).
- A Largely light (14N-14N)
- B Largely heavy (15N-15N)
- C Hybrid density (15N-14N)
- D A mixture of light and heavy, with little hybrid
Correct answer: C. Hybrid density (15N-14N)
Explanation: After one generation, all DNA molecules showed an intermediate (hybrid) density, consistent with each molecule having one heavy (15N) parental strand and one light (14N) new strand, confirming semiconservative replication.
Helicase unwinds the double helix at the replication fork; one new strand (leading) is synthesised continuously toward the fork, while the other (lagging) must be made in short, separate Okazaki fragments because DNA polymerase only adds nucleotides 5'→3'.
Concept context
DNA structure, replication, transcription, translation, the genetic code, and gene regulation (lac operon). The single most important chapter for NEET molecular biology.