The two chromatids of a chromosome are held together at a region called the centromere which is important for the movement of chromosomes during cell division.
The centromere has the function of holding the two chromatids together until they separate during cell division (a certain phase of mitosis when the centromere splits).
The positions of centromere form the basis of classification of chromosomes.
Based on the position of the centromere, the chromosomes can be classified into four types:
(1) Metacentric chromosome
(2) submetacentric chromosome
(3) Acrocentric chromosome
(4) Telocentric chromosome