anaphase
/ AN-uh-fayz /
Imagine a starting gun finally fires after everyone has been held at the line: in an instant, the two halves of every chromosome are yanked apart and rush to opposite ends of the cell. That sudden, decisive separation is anaphase. After the long, careful lining-up of metaphase, anaphase is the brief moment of action where the cell commits irreversibly to splitting its DNA into two equal sets.
Anaphase is the stage of mitosis (and meiosis) in which the duplicated chromosomes are pulled apart toward opposite poles of the cell. It begins when the protein glue (cohesin) holding the sister chromatids together is suddenly cut by an enzyme called separase. Freed, each former sister chromatid — now counting as a full chromosome in its own right — is reeled in toward a pole as the spindle fibers attached to its kinetochore shorten, while other fibers push the two poles farther apart. By the end, two complete, identical sets of chromosomes sit at opposite ends of the cell.
Anaphase matters because it is the point of no return: cutting cohesin is irreversible, so the cell only triggers it after the spindle assembly checkpoint has confirmed every chromosome is correctly attached. If a chromosome is pulled to the wrong pole, or fails to separate (a mistake called nondisjunction), a daughter cell ends up with the wrong number of chromosomes — the root of conditions like Down syndrome and a hallmark of many cancers. Anaphase is dramatic precisely because there is no undoing it once it starts.
If a single chromosome fails to separate during anaphase of egg formation, the resulting egg can carry an extra copy — when such an egg with an extra chromosome 21 is fertilized, the child has Down syndrome.
A failure to separate in anaphase leads to the wrong chromosome count.
Anaphase has two events: chromatids separating (anaphase A, as spindle fibers shorten) and the poles themselves moving apart (anaphase B) — both contribute to driving the two sets of DNA to opposite ends.