Conversation so far:
when does a chromosome go from one chromatid to two sister chromatids
A full set of sister chromatids is created during the synthesis phase of interphase, when all the chromosomes in a cell are replicated.
What is interphase as it relates to DNA replication?
Interphase is the phase of the cell cycle in which a typical cell spends most of its life.
What is a chromatid?
A chromatid is a chromosome that has been newly copied or the copy of such a chromosome.

Question: How does the copy form?
Self-contained rewrite: How does a chromatid form?