DNA replication and chromosome copies in mitosis

Steps:

• In G1 phase, chromosomes are unreplicated, so each has 1 DNA copy.
• S phase replicates DNA, yielding 2 copies per chromosome (sister chromatids).
• Mitosis separates sister chromatids to opposite poles.
• During cytokinesis, the single cell contains both separated sets, so 2 copies of each DNA molecule.

Why B is correct:

• G1 has unreplicated DNA (1 copy per chromosome, per standard cell cycle definition); cytokinesis occurs after separation, with the cell holding 2 copies until cytoplasmic division.

Why the others are wrong:

• A: Cytokinesis has 2 copies in the dividing cell, not 1.
• C: G1 has 1 copy, not 2 (replication is in S phase).
• D: G1 has 1 copy, not 2.

Final answer: B