Are Sister Chromatids present in all or part of the S phase?

The replicated chromosomes that are connected to the centromere are called sister chromatids.
Therefore, sister chromatids are present in a chromosome only after the chromosome is fully replicated. Replication starts at the beginning of phase S and ends during S phase (usually at the end of S phase).

The above means that sister chromatids are NOT present in all phase S.
Sister chromatinds are present at a small part of phase S (when the replication of the DNA is finished.)

However, I think that we usually assume that sister chromatids are not present at phase S.

Sister chromatids ARE present in a part of S phase. So, ignore this statement: