Mitosis stage determined by chromosome structure and position

Steps:

• Chromosomes with two chromatids form after DNA replication and persist until anaphase.
• In early mitosis, chromosomes condense but do not align until metaphase.
• Alignment at the equator marks metaphase; lack of alignment indicates pre-metaphase.
• Separation of chromatids into single units occurs in anaphase and beyond.

Why A is correct:

• Prophase features condensed chromosomes with two sister chromatids attached, not yet aligned at the metaphase plate.

Why the others are wrong:

• B: Metaphase requires chromosomes lined up single-file at the cell's equator.
• C: Anaphase involves sister chromatids separating, so chromosomes consist of one chromatid each.
• D: Telophase has decondensing chromosomes at opposite poles, each with one chromatid after separation.

Final answer: A