Review this diagram: in which phase or phases of mitosis would you see the structure below in this X shaped form?
Locus (genetics) - Wikipedia
a
end of anaphase and telophase
b
interphase
c
end of prophase and metaphase
d
early prophase, only
c) end of prophase and metaphase
This X shaped structure, called a locus or centromere, is where the sister chromatids are held together during mitosis. It is most visible at the end of prophase when the chromosomes have condensed and are clearly visible as X shapes, and during metaphase when the chromosomes line up at the equator of the cell.