Chromosomes, DNA and Genes

Chromosomes, DNA and Genes

The human body is made up of million cells. The nucleus of each cell contains thin thread like structures called chromosomes. The term ‘chromosomes’ was first coined by Waldeyer in 1888. The chromosomes are the carrier of genetic material which contain the heredity information.

The chromosomes are highly condensed coiled chromatin fibres packed with the DNA (Deoxyribonucleic acid) that forms the genetic material. Genes are segments of DNA, which are responsible for the inheritance of a particular phenotypic character. Each gene is present at a specific position on a chromosome called its locus. During cell division, the genetic information present in the genes are passed from one generation to another.

1. Structure of a Chromosome

The chromosomes are thin, long and thread like structures consisting of two identical strands called sister chromatids. They are held together by the centromere. Each chromatid is made up of spirally coiled thin structure called chromonema. The chromonema has number of bead-like structures along its length which are called chromomeres. The chromosomes are made up of DNA,RNA, chromosomal proteins (histones and non-histones) and certain metallic ions. These proteins provide structural support to the chromosome .

A chromosome consists of the following regions

Primary constriction: The two arms of a chromosome meet at a point called primary constriction or centromere. The centromere is the region where spindle fibres attach to the chromosomes during cell division.

Secondary constriction: Some chromosomes possess secondary constriction at any point of the chromosome. They are known as the nuclear zone or nucleolar organizer (formation of nucleolus in the nucleus).

Telomere: The end of the chromosome is called telomere. Each extremity of the chromosome has a polarity and prevents it from joining the adjacent chromosome. It maintains and provides stability to the chromosomes.

Satellite: Some of the chromosomes have an elongated knob-like appendage at one end of the chromosome known as satellite. The chromosomes with satellites are called as the sat-chromosomes.

2. Types of Chromosomes based on the position of Centromere

Based on the position of centromere, the chromosomes are classified as Telocentric, Acrocentric, Submetacentric and Metacentric

a)              Telocentric– The centromere is found on the proximal end. They are rod shaped chromosomes.

b)             Acrocentric – The centromere is found at the one end with a short arm and a long arm. They are also rod-shaped chromosomes.

c)              Submetacentric – The centromere is found near the centre of the chromosome. Thus forming two unequal arms. They are J shaped or L shaped chromosomes.

d)             Metacentric – The centromere occurs in the centre of the chromosome and form two equal arms. They are V shaped chromosomes

3. Types of Chromosomes based on function

The eukaryotic chromosomes are classified into autosomes and allosomes.

Autosomes contain genes that determine the somatic (body) characters. Male and female have equal number of autosomes.

Allosomes are chromosomes which are responsible for determining the sex of an individual. They are also called as sex chromosomes or hetero-chromosomes. There are two types of sex chromosomes, X and Y- chromosomes. Human male have one X chromosome and one Y chromosome and human female have two X chromosomes.

4. Karyotype

The number of chromosomes in any living organism (animal or plant) is constant. In human, each cell normally contains 23 pairs of chromosomes. Out of which 22 pairs are autosomes and the 23rd pair is the allosome or sex chromosome.

In the body cells of sexually reproducing organisms, the chromosomes generally occur in pairs. This condition is called diploid (2n). The gametes produced by the organisms contain a single set of chromosomes. Hence, the gametes are said to be haploid (n).

Karyotype is the number, size and shape of chromosomes in the cell nucleus of an organism. Idiogram is the diagrammatic representation of karyotype of a species. It consists of all the metaphasic chromosomes arranged in homologous pairs according to decreasing length, thickness, position of centromere, shape etc., with the sex chromosomes placed at the end.