Chromosomes are the physical carriers of genes, which are made up of DNA and associated proteins. The term chromosome was introduced by Waldeyer in 1888. Chromosomes occur in all the living organisms. The bacterial chromosomes are circular. It has closed circular DNA. Linear chromosomes are found in eukaryotes. Bridges in 1916 was the first to prove that the genes are carried on the 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 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.
A satellite is a short chromosomal segment and separated from the main chromosome by a relatively elongated secondary constriction. A chromosome with a satellite is called SAT-chromosome.
Chromatin is a viscous gelatinous substance that contains DNA, RNA, histone and non-histone proteins. H1, H2A, H2B, H3 and H4 are the five types of histones found in the chromatin. The chromatin is formed by a series of repeated units called nucleosomes. Each nucleosome has a core of eight histone subunits.
Telomere is the terminal part of chromosome. It offers stability to the chromosome. DNA of the telomere has specific sequence of nucleotides. Eukaryotic chromosome has DNA, RNA, histones, non- histone proteins and metallic ions like Ca+2, Mg+2, etc.,