Chromosome structure — counting chromatids in a duplicated chromosome When a single chromosome has been duplicated (post-S phase) but before sister chromatids separate, how many chromatids does that single duplicated chromosome contain?

Introduction / Context:
Clear terminology prevents confusion: a chromosome is defined by its centromere. After DNA replication, each chromosome consists of two identical DNA molecules (sister chromatids) joined at a single centromere until anaphase.

Given Data / Assumptions:

• Pre-replication: one chromosome equals one chromatid.
• Post-replication, pre-separation: one chromosome equals two sister chromatids.
• Separation converts each chromatid into an independent chromosome.

Concept / Approach:

In the duplicated state, the chromosome has two chromatids. This number remains constant from the completion of S phase through metaphase until the onset of anaphase, when cohesin cleavage allows individual chromatids to move to opposite poles.

Step-by-Step Solution:

Verification / Alternative check:

Metaphase chromosome spreads show paired sister chromatids forming characteristic X-shaped structures, confirming the two-chromatid state.

Why Other Options Are Wrong:

One applies before S phase; three and four are not normal chromatid counts for a single duplicated chromosome; variability claim is incorrect for the pre-anaphase period.

Common Pitfalls:

Confusing chromatid number with chromosome count per cell; focus on a single chromosome’s structure.

Final Answer:

Two