As previously mentioned, the centromere is easily visualized as the most constricted region of a condensed mitotic chromosome. Although the word "centromere" is derived from the Greek words centro ("central") and mere ("part"), centromeres are not always found in the center of chromosomes. In fact, only so-called metacentric chromosomes have centromeres at their middle; in other chromosomes, centromeres are located at a variety of positions that are characteristic for each particular chromosome (Figure 2). The position of the centromere, therefore, provides a useful landmark for dividing chromosomes into karyotype groups and for establishing a standardized nomenclature for mapping the positions of genes on chromosomes.
With a few exceptions, eukaryotic chromosomes have a single centromere that ensures their accurate segregation during mitosis. Chromosomes that lack centromeres segregate randomly during mitosis and are eventually lost from cells. At the other extreme, chromosomes with multiple centromeres are subject to fragmentation if the centromeres become attached to opposite spindle poles by way of their kinetochores.