Question
During which phase of mitosis do chromosomes first become visible?
(A) Prophase
(B) Metaphase
(C) Anaphase
(D) Telophase
Answer
Choose (A): prophase.
During prophase, chromatin condenses into shorter, thicker structures that become recognizable as individual chromosomes. Each replicated chromosome consists of two sister chromatids joined near the centromere.
The chromosomes did not suddenly come into existence at prophase. Their DNA was already present and had been replicated during S phase, but interphase chromatin is comparatively diffuse. Condensation changes how compactly that DNA is packaged, making the chromosome structures distinguishable with standard microscopy.
Why the later phases are not the first
- Metaphase: chromosomes are already condensed and align at the metaphase plate.
- Anaphase: sister chromatids separate and move toward opposite poles.
- Telophase: chromosomes arrive at the poles and begin to decondense.
Prometaphase is sometimes named separately between prophase and metaphase. Chromosomes become even more condensed and discrete then, but the standard answer to when they first become visible is prophase.
Evidence boundary
This page uses the standard textbook stages of mitosis and ordinary microscopy. First become visible means that chromatin condenses into distinguishable chromosome structures, not that chromosomes or DNA are created in prophase. Prophase and prometaphase describe landmarks within a continuous process.
Sources
These references support the concepts and methods used in the explanation above.