Sister Chromatids in S and G2 Phases

Steps:

• DNA replication in S phase (stage 1) produces identical sister chromatids joined at the centromere.
• Sister chromatids remain joined throughout G2 phase (stage 2) for checkpoint verification.
• Separation occurs later in mitosis (stage 3), ending the joined state.
• Thus, stages 1 and 2 feature chromosomes as sister chromatids at the centromere.

Why A is correct:

• Stages 1 (S phase) and 2 (G2 phase) define the period post-replication when sister chromatids form and persist joined, per cell cycle definitions.

Why the others are wrong:

• B: Stage 3 (mitosis) includes separation in anaphase, so not fully joined.
• C: Stage 3 lacks consistent joining due to separation.
• D: Excludes stage 1, where replication first creates joined chromatids.

Final answer: A