In this article we will discuss about the structure of chromosomes with the help of suitable diagram.
Chromosome is present as individual bodies in the interphase as well as in the mitosis. The predominant component in the chromosome is DNA molecule.
The genes are located in chromosome of the nucleus and can be called as the discrete unit of transmission of hereditary character, because it is the specific locus or spot on a chromosome carrying the genetic material or information for a specific character. The gene is a part of the DNA molecule.
Each chromosome consists of one to four coiled threads called chromonema and also contains juxtaposed minute particles known as chromomeres (Fig. 1.15) which are rich in DNA. Most of the chromosomes possess usually two constrictions – primary (kinetochore) and secondary.
Chromosomes having terminal or almost terminal kinetochore are called acrocentric chromosomes. While chromosomes having unequal arms and almost centrally situated kinetochore are known as metacentric chromosomes. Chromosomes possessing secondary constriction in addition to primary one are called submetacenteric chromosomes (Fig. 1.17).
Autosomes and Sex Chromosomes:
In human being, there are 46 chromosomes, arranged in pairs, in the nucleus of each cell. In each individual somatic cell nucleus, there are 22 pairs of somatic chromosomes, also called autosomes, which are homologous and concerned with the transmission of ordinary hereditary characteristics and the remaining pair is concerned with the determination of sex.
In the female the sex chromosomes consist of a pair of identical large X chromosomes, whereas in the male the pair consists of an X chromosome and a Y chromosome which is small and has no influence on sex determination. One of the X chromosome, present in the female, is tightly coiled and can be seen under microscope in the nuclei of squamous epithelial cells and neutrophil granulocytes. This X chromosome may also be present in sexually abnormal cases.
On the other hand, in females germ cells (gametes) have 22 autosomes and one X chromosome; in males it – may be 22 autosomes and either an X or a Y chromosome. It is these chromosomes which determine the specific characteristics of the cell and it is it is through them than that the hereditary qualities pass from one generation to the other.
Nucleus is the most essential part of the cell. Upon it depends the power of morphological and chemical synthesis. The non-nucleated fragment of the cell quickly dies; because it possesses no power of synthesis and regeneration. While that fragment of a cell which contains the nucleus, rapidly resynthesise the lost parts and carries on normal life.
If the nucleus of a cell is punctured, the cell dies. From the above facts -it seems that the cell, although a very tiny body, yet in itself, is a highly organised structure. The whole cell life appears to be divided up into a number of functional compartments and that each one of the so called organoids possibly acts as the presiding officer of a particular department of cell life.