centromere
/ SEN-troh-meer /
When a cell divides, it has just copied every chromosome and now must hand exactly one copy of each to each daughter cell — no more, no less. To do that cleanly, it needs a handle on every chromosome that the cell's pulling-apart machinery can grab. The centromere is that handle: the pinched-in 'waist' you see on a condensed chromosome, the spot where the two identical copies are joined and where the cell hooks on.
Physically, the centromere is a specialized region of the chromosome made of particular repetitive DNA and special proteins. After a chromosome is copied, the two identical copies (sister chromatids) stay clamped together at the centromere until it's time to separate them. On top of the centromere the cell builds a protein platform called the kinetochore, which is what the spindle fibers — the cell's pulling ropes — actually attach to during division.
Getting the centromere right is a matter of life and death for the cell. If the fibers don't attach properly, or attach to both copies from the same side, a daughter cell can end up with too many or too few chromosomes — an error that causes miscarriages, conditions like Down syndrome, and is rampant in cancer cells. Interestingly, what marks a spot as 'the centromere' is largely an inherited protein pattern, not just the DNA sequence, so it is partly an epigenetic feature.
On a textbook X-shaped chromosome, the point where the two arms cross is the centromere — the clasp holding the two copied halves together until the cell yanks them into separate daughter cells.
The crossing point of the X: the chromosome's grab-handle.
Don't confuse the centromere (the chromosome's waist) with the centrosome or centriole (separate structures that organize the spindle) — the similar names trip up nearly everyone.