noun, plural: metacentric chromosomes
The centromere is the dense, constricted region in a chromosome. It contains highly-specialized repetitive DNA sequences (e.g. satellite DNA) that are packaged into heterochromatin. The centromere serves as the site for kinetochore assembly and therefore it is essential during the alignment of the chromosomes at the metaphase plate and the subsequent segregation of chromosomes during cellular division. The presence of the centromere results in the characterization of the chromosomal arms. The arm that is relatively shorter is called p whereas the one that is longer is called q. Based on the position of the centromere, the chromosome may be described as metacentric, submetacentric, acrocentric, telocentric, subtelocentric, and holocentric.
A metacentric chromosome is a chromosome whose centromere is centrally located. As a result, the chromosomal arms (i.e. p and q arms) are almost equal in length. A metacentric chromosome would have the X shape. A chromosome with slightly unequal chromosomal arm length is referred to as submetacentric chromosome.