The centromere is a region of a eukaryotic chromosome where the kinetochore is assembled. Thus, it is the site where spindle fibers of the mitotic spindle attach to the chromosome during mitosis. It is also the site of the primary constriction visible in microscopy images of chromosomes.
In most eukaryotes, the centromere has no defined DNA sequence. It typically consists of large arrays of repetitive DNA where the sequence within individual repeat elements is similar but not identical. In humans, the centromeric repeat unit is called α-satellite. However, repeats of α-satellite are not sufficient to cause the assembly of a kinetochore, and there are functioning centromeres with no α-satellite DNA.
Epigenetic inheritance plays a major role in specifying the centromere in most organisms. The daughter chromosomes will assemble centromeres in the same place as the parent chromosome, independent of sequence. However, there must still be some original way in which the centromere is specified, even if it is subsequently propagated epigenetically. In rare cases in humans, neocentromeres can form at new sites on the chromosome. This must be coupled with the inactivation of the previous centromere since chromosomes with two functional centromeres (dicentric chromosomes) will result in chromosome breakage during mitosis.
The centromeric DNA is normally in a heterochromatin state, which is probably essential for its function. In this chromatin, the normal histone H3 is replaced with CENP-A, a centromere-specific variant. The presence of CENP-A is believed to be important for the assembly of the kinetochore on the centromere and may play a role in the epigenetic inheritance of the centromere site.
In the yeast Schizosaccharomyces pombe (and likely in other eukaryotes) the formation of centromeric heterochromatin is connected to RNAi*. In nematodes such as Caenorhabditis elegans and in some plants, chromosomes are "holocentric" indicating that there is not a primary site of microtubule attachments or a primary constriction, and a "diffuse" kinetochore assembles along the entire length of the chromosome.