Radioactively labelled nucleotides were incubated with an unlabelled molecule of DNA - VCE - SSCE Biology - Question 11 - 2011 - Paper 1

In the first cycle of DNA replication, each strand of the original DNA molecule serves as a template. The incorporation of radioactively labelled nucleotides would result in two daughter DNA molecules, each containing one strand from the original unlabelled DNA and one newly synthesized strand that incorporates the labelled nucleotides.

In the second cycle, the newly synthesized strands from the first cycle will again serve as templates for replication. Consequently, half of the resulting DNA molecules will contain labelled strands and half will contain unlabelled strands. Thus, after two cycles, the expected result is that both strands of some DNA molecules will be labelled, while others will remain unlabelled.

Examining the options:

• Option A shows some labelled strands but fails to depict the correct distribution.
• Option B incorrectly suggests a complete labelled state.
• Option C accurately represents the mixture of labelled and unlabelled DNA strands, confirming that both scenarios are present after two cycles.
• Option D shows an incorrect label distribution.

Thus, the correct response is C.