By the end of which cell-cycle phase does each human chromosome consist of two sister chromatids?

Introduction / Context:
Chromosome duplication produces sister chromatids that remain paired until anaphase. Knowing exactly when, within the interphase subdivisions, this duplicated state is reached is crucial for understanding checkpoints and the timing of DNA repair.

Given Data / Assumptions:

• Interphase includes G1, S, and G2.
• S phase is dedicated to DNA synthesis.
• The phrase “by the end of” asks for the earliest phase completion after which two chromatids per chromosome are present.

Concept / Approach:
As replication proceeds during S phase, each chromosome becomes two sister chromatids. At the end of S phase, all chromosomes have been duplicated. They remain as two chromatids through G2 and into early M phase until separation at anaphase. The earliest correct phase boundary is therefore S phase.

Step-by-Step Solution:

Verification / Alternative check:
Flow cytometry shows DNA content doubling (from 2C to 4C) during S phase; G2 cells maintain 4C content until mitosis.

Why Other Options Are Wrong:

Common Pitfalls:
Answering G2 because chromatids are still present then; however, the earliest correct transition point is the end of S.

Final Answer:
S phase.