STRUCTURE OF CHROMOSOME

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Structure of chromosome:
Each chromosome consists of similar structures called chromatids. They are identical and are called sister chromatids. A typical chromosome has narrow
zones called constrictions. There are two types of constrictions namely primary constriction and secondary constriction.
The primary constriction is made up of centromere and kinetochore. Both the chromatids are joined at centromere, which is essential for the movement of
chromosomes at anaphase. If the centromere of the chromosomes is damaged, such chromosome fails to move at anaphase. The number of
centromeres varies from chromosome to chromosome. The monocentric chromosome has one centromere and the polycentric chromosome has many centromeres.
The centromere contains a complex system of fibres called kinetochore. Each centromere has two kinetochores lined with chromosomal arms. The kinetochore is made up of protein fibres and microtubules which assist in the formation of spindles during mitosis and meiosis.
All constrictions other than primary are called secondary constrictions. In a given set of chromosomes only one or two chromosomes have secondary constrictions. The nucleoli develop from secondary constrictions and such secondary constrictions are called nucleolar organisers.

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