After S phase, each single chromosome is made up of two sister chromatids that are identical copies of one another and are joined at the centromere.

What a chromosome is after S phase

  • During S phase, the DNA in each chromosome is replicated, so the chromosome goes from having one DNA double helix to having two identical DNA double helices.
  • Each of these DNA double helices, with its associated proteins, forms a chromatid; together, the two sister chromatids connected at the centromere are still counted as one chromosome.

Why it is still “one” chromosome

  • Chromosome number is based on the count of centromeres, not on how many chromatids or DNA molecules there are.
  • Because the two sister chromatids are attached and share a single centromere, they are considered one chromosome until they separate during anaphase, at which point each chromatid becomes its own chromosome.

TL;DR: After S phase, a single chromosome = one centromere holding two sister chromatids (each a full double-stranded DNA molecule).