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.
How does a chromatid form?
Chromatids are produced from chromatin fibers during both meiosis and mitosis.

Question: what are the fibers
Self-contained rewrite: What is chromatin?