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.

Question: what are the sisters that are created during interphase
Self-contained rewrite: What is a chromatid?