The kinetochore is the protein structure in eukaryotes which assembles on
the centromere and links the chromosome to microtubule polymers from the mitotic
spindle during mitosis and meiosis. The kinetochore contains two regions:
an inner kinetochore, which is tightly associated with the centromere DNA;
and an outer kinetochore, which interacts with microtubules. "Monocentric"
organisms, including vertebrates, fungi, and most plants, have a single centromeric
region on each chromosome which assembles one kinetochore. "Holocentric" organisms,
including nematodes such as the round worm, assemble a kinetochore along
the entire length of a chromosome. During mitosis, which occurs after chromosomes
are duplicated during S phase, two identical "sister" chromatids are held
together each with its own kinetochore which face in opposing directions and
attach to opposite poles of the mitotic spindle. Following the transition
from metaphase to anaphase, the sister chromatids separate from each other,
and the individual kinetochores on each chromatid drive their movement to
the spindle poles that will define the two new daughter cells. Thus, the kinetochore
is essential for the chromosome segregation that is classically associated
with mitosis and meiosis
