An overview of chromosomes:
During the cell division the chromatin of the nucleus transforms into chromosomes transform into the chromosomes by the process of condensation and contraction so we can say that the condensed and the contracted form of the chromatin material is the chromosome. The chromosomes show maximum contraction and condensation in the metaphase stage of the process of cell division and thus can be easily viewed and studied at this stage. In the following is provided some detail about the features of the chromosomes.
Structure of chromosome:
A typical metaphase chromosome appears to consist of two arms and a primary constriction called the centromere (Kinetochore). The centromere lies between the arms of the chromosome. The two arms of chromosome are called chromatids and the centromere is the region where the spindle fibers are attached during cell division. A chromosome having no centromere cannot participate in the cell division. Specifically, a chromosome having no centromere cannot complete the anaphase stage of the process of cell division. The position of centromere on chromosomes shows variations and hence there are different types of chromosomes. Apart from primary constriction (centromere) other morphological features like “secondary constrictions” and satellite may also be present on chromosomes. These morphological features are important for identifying specific chromosomes.
Chemical composition of chromosome:
The major components of the chromosome are DNA, traces of RNA, proteins and lipids. In addition to all these major components some polysaccharides and metal ions are also present in little quantities. The chromosomes of most organisms are composed of about 90% of DNA and proteins (Deoxyribonucleo-proteins) and 10% of residual chromosome. The 90% DNA and protein part of the chromosome contains 45% DNA and the remaining 55% part contains Histone proteins and protamines (basic proteins)
The residual chromosome is made up of 12 to 14% RNA, 2 to 3% DNA and 83 to 86% residual protein. The residual protein is an acid protein rich in amino acids like tryptophane and tyrosine. The proteins especially the residual protein makes the back bone of the chromosome to which the nucleic acids are attached, thereby help to maintain the structure of the chromosome.
The metallic ions which are constituents of the chromosome are bivalent. These metallic ions are Ca, Mg and Fe. The metallic ions form the linkages between the DNA and protein, or between DNA groups. The ionic linkages are also known as salt linkages.
Types of chromosomes:
The position of centromere on the chromosome is not fixed. It shows variations and as a consequence there are different shapes of chromosomes. The centromere may occupy any position along the chromosome. The position of the centromere determines the shape of the chromosomes. Based on the positions of the centromere, 4 types of chromosomes have been recognized.
1) Metacentric chromosomes
2) Sub-Metacentric chromosomes
3) Telocentric chromosomes
4) Acrocentric chromosomes
The centromere is situated in the center of the chromosome or median in position. So the two arms are equal in length.
In this type the centromere is sub-median in position. Thus the arms are unequal in length.
In this type the centromere is located at one end of the chromosome. Such chromosomes are rod shaped in appearance.
In this type the centromere is located near one side of the chromosome i.e. the centromere is sub terminal in position.