Question

when do sister chromatids become visible?

Answer 1

Sister chromatids forms when they go through Synthesis Reaction or S-Phase of interphase which is basically the beginning of cell growth. Sythensis is the actual replication of DNA . BUT they are not visible until prophase; this is when the chromatin condenses and the double-stranded chromosomes or sister chromatids become visible.

Answer 2

during M phase .

Answer 3

during last prophase