Semi-conservative replication evidence — In the Taylor, Woods, and Hughes autoradiography of Vicia faba, DNA was synthesized in radioactive thymidine during S phase. After one S phase, what labeling pattern was seen for sister chromatids?

Introduction:
Classic experiments in chromosome autoradiography provided direct visual evidence for semi-conservative DNA replication. This question revisits the Taylor, Woods, and Hughes study using Vicia faba (broad bean), in which cells incorporated radioactive thymidine during S phase to track newly synthesized DNA strands.

Given Data / Assumptions:

• Cells were exposed to tritiated thymidine during one S phase.
• Autoradiography detects newly synthesized DNA strands as labeled regions.
• Semi-conservative model: each daughter duplex contains one parental and one newly made strand.

Concept / Approach:
When replication is semi-conservative and labeling occurs during the entire S phase, each newly synthesized strand in both sister chromatids will be radioactive. Each chromatid contains one parental unlabeled strand and one labeled nascent strand, so both chromatids show labeling over their lengths in autoradiographs.

Step-by-Step Solution:

Verification / Alternative check:
Meselson–Stahl density-gradient results are consistent with this logic: after one replication, hybrids are formed; autoradiography simply visualizes nascent strand incorporation directly at the chromosomal level.

Why Other Options Are Wrong:

• Only one chromatid labeled: would imply conservative or partial labeling protocols; not the outcome with labeling throughout S phase.
• Neither labeled: contradicts the experimental setup.
• Both (a) and (b): mutually exclusive in this context.
• Only centromeres labeled: labeling was genome-wide where synthesis occurred.

Common Pitfalls:
Confusing the pattern after the first labeled S phase with that after a second, unlabeled S phase, when a single chromatid per chromosome retains label.

Final Answer:
both chromatids of a chromosome were labeled.