Direct answer first: After S phase in the cell cycle, a single chromosome consists of two sister chromatids held together at the centromere. Each sister chromatid is an identical copy of the original DNA molecule, so a replicated chromosome contains two DNA molecules that will be separated during mitosis or the second meiotic division. In meiosis I, homologous chromosomes are reduced by separating one member of each pair, so each resulting chromosome in the daughter cells still consists of two sister chromatids until anaphase II. Explanation and key points
- Replication during S phase doubles the DNA content of each chromosome, producing two sister chromatids. This changes the observable structure from a single chromatid to a replicated chromosome with two chromatids connected at the centromere. This behavior is consistent across somatic (mitotic) divisions and the early stages of meiosis. [Cited concept: replicated chromosome with sister chromatids after S phase]
- In mitosis, the sister chromatids finally separate in anaphase, giving each daughter cell a complete set of chromosomes, each consisting of one chromatid. In meiosis, homologous chromosomes pair and separate in meiosis I, but the chromosomes entering anaphase II still consist of two sister chromatids until they are finally pulled apart. [Cited concept: separation of chromatids in later stages]
- A chromosome's identity (maternal or paternal copy) is determined by which homolog it originated from, while the physical structure (two sister chromatids per replicated chromosome) is due to DNA replication. [Cited concept: chromosome identity vs replication state]
If you’d like, I can tailor this to a specific organism or visualize the sequence with a simple step-by-step diagram description.
