sister chromatids
/ SIS-ter KROH-muh-tidz /
Picture making a photocopy of an important page and then clipping the copy to the original so the two stay together until you are ready to file one in each of two folders. A cell does the same thing with each of its chromosomes before dividing. Sister chromatids are the two identical copies of a single chromosome, made during DNA copying and held tightly together until the cell is ready to deal one to each daughter.
When a cell copies its DNA in S phase, each chromosome becomes two identical strands of DNA: the two sister chromatids. They stay joined along their length, most tightly at the pinched centromere region, by a ring-shaped protein called cohesin that literally clamps them together. This is why a duplicated chromosome takes on the familiar X shape — the two sister chromatids are the two arms of the X, crossing at the centromere. They remain a pair through prophase and metaphase, then are pulled apart during anaphase, at which point each separated chromatid becomes a chromosome in its own right.
Sister chromatids matter because keeping the two copies attached is what makes a fair split possible: the cell can line up the joined pair, attach each sister to an opposite pole, and only then cut them apart so exactly one goes to each daughter. A common point of confusion is the difference between sister chromatids and homologous chromosomes — sisters are identical copies of the same chromosome, while homologs are the maternal and paternal versions of a chromosome that carry the same genes but can differ in their exact sequence.
The familiar X-shaped chromosome you see in diagrams is actually a duplicated chromosome made of two sister chromatids clipped together at the center — a single, unduplicated chromosome is just one rod, not an X.
The X shape is two sister chromatids; an unduplicated chromosome is one rod.
Sister chromatids are identical copies, so separating them (as in mitosis) does not create genetic variety — variety comes from separating homologous chromosomes and crossing over, which happen in meiosis.