What is a centromere? How does the position of the centromere form the basis of the classification of chromosomes?

Chromosomes are found in the nucleus of each cell. It is a thread-like structure that is not visible in the cell’s nucleus. But at the time of cell division, it becomes more tightly packed, and then only it is visible under a microscope. Each chromosome joined at the centromere or the primary constriction and hence consists of two chromatids. These centromeres are the point of attachment of spindle fibers and play a vital role in cell division. On the basis of the position of the centromere, chromosomes are classified into the following types:

(1) Acrocentric chromosome: In this type of chromosome, the centromere is present at the sub-terminal. In the Anaphase stage chromosomes are J-shaped.

(2) Sub-metacentric chromosome: In this type of chromosome, the centromere is sub-median and the anaphasic chromosome appears L-shaped.

(3) Metacentric chromosomes: In this type of chromosome, the centromere is present in the middle and divides the chromosome into two equal parts. The chromosome appears V-shaped.

(4) Telocentric chromosome: In this type of chromosome, the centromere is present at the terminal. The anaphasic stage appears l-shaped.

Depending upon the number of centromeres, a chromosome is of different types.

(i) Monocentric: with a single centromere

(ii) Dicentric: with two centromeres

(iii) Polycentric: with many centromeres

(iv) Acentric chromosome: there is no centromere