Kinetochore

A kinetochore is a protein structure assembled on the centromere of a chromosome that serves as the attachment point for spindle fibers (microtubules) during cell division. Each duplicated chromosome (with two sister chromatids) has two kinetochores - one on each chromatid - which anchor the chromatids to spindle microtubules so they can be pulled apart in mitosis or meiosis.

• The kinetochore forms at the centromere region of a chromatid. It's often described as a "protein handle" that the spindle fiber grabs onto.
• Do not confuse centromere and kinetochore: the centromere is the DNA region of the chromosome that is constricted, and the kinetochore is the protein complex that assembles on that region. The spindle fibers attach to the kinetochore (not directly to DNA).
• Also, don't confuse kinetochore with centrosome. The centrosome is the microtubule-organizing center at the cell poles, whereas kinetochores are on the chromosomes. (Centrosomes send out the spindle fibers, which attach to kinetochores.)

• A frequent question: "Where do the spindle fibers attach to chromosomes?" The correct answer is kinetochores (at the centromeres). Recognize this as a fundamental mechanism in mitosis/meiosis.
• If a diagram of mitosis labels the connection between microtubules and the chromosome, be prepared to identify it as the kinetochore. Some questions may also ask what happens if kinetochores don't attach properly (e.g., chromatids wouldn't segregate evenly).