centromere

When a duplicated chromosome is drawn as an X shape, the centromere is the pinched point in the middle where the two halves are joined. It is the chromosome's handle — the spot the cell grabs onto when it needs to pull copies apart during division.

After a chromosome is copied, the two identical strands (sister chromatids) stay attached at the centromere until the cell is ready to separate them. A protein complex called the kinetochore assembles on the centromere and serves as the anchor for spindle fibers, the cables that the cell uses to haul one chromatid toward each pole. The position of the centromere — near the middle, off to one side, or near an end — gives each chromosome a characteristic shape used in identifying it.

If the centromere fails or is missing, the cell loses its grip on the chromosome and the copies may not be shared correctly, leaving daughter cells with the wrong number of chromosomes. Centromeres are therefore essential for accurate cell division, and they are built on special repetitive DNA marked by a distinctive histone variant rather than defined purely by the underlying sequence.