As long as the old and the new DNA remain attached at the centromere, the whole structure is still called one chromosome and each half is called a chromatid . To determine the number of chromosomes at a particular stage of nuclear division, determine the number of centromeres. DNA replication occurs. Nuclear membrane dissolves. Centrioles migrate to opposite sides of cell.
Spindle fibers run from the centromeres to the poles.
Splitting of the centromeres marks the end of metaphase and the beginning of anaphase. Cytokinesis involves the formation of a cleavage furrow in animal cells and a cell plate in plant cells. Nuclear membrane reappears. Chromosomes begin to uncoil and form chromatin. You will arrest the onion cells in various stages of mitosis.