Principles of Genetics and Breeding

1.3.2 Structure of a chromosome

• Each chromosome structurally consists of a limiting membrane called pellicle, anamorphous matrix and two very thin, highly coiled filaments called chromonema or chromonemata.
• The material of chromosomes is called chromatin.
• It contains euchromatin which stains but lightly and heterochromatin which stains darkly. This differential staining is the basic for most of the methods now used for detecting details of the structure of chromosomes.
• Euchromatin is believed to contain the genes in a linear array like beads on a string, whilst heterochromatin is regarded as genetically inert and to have a function in maintaining the structural integrity of the chromosome and perhaps regulating gene expression.
• Heterochromatin is mostly made up of highly repeated simple sequences of DNA, satellite DNA. Heterochromatin is usually found near the centromere and on the ends of each chromosome.
• Two other features of chromosomes are the ends, called telomers, and a constriction called the centromere. The telomeres are stable entity, usually heterochromatic and are for maintaining the integrity of chromosomes threads. During the cell divisions the microtubules of the spindle are get attached with the centromeres and move them towards the opposite poles of the cell.