Replication and compaction of chromosomes into pairs of sister chromatids.

(a) Chromosomal replication producing a pair of sister chromatids. While the chromosomes are elongated, they are replicated to produce two copies that are connected and lie parallel. This is a pair of sister chromatids. Later, when the cell is preparing to divide, the sister chromatids condense into more compact structures that are easily seen with a light microscope. (b) A schematic drawing of a metaphase chromosome. This structure has two chromatids that lie side by side. The two chromatids are held together by cohesion proteins in their centromere region.