Chromosomes





What is chromosome?

In a cell nucleus the chromatin reticulum gets dissociated into a definite number thread like structures during cell division, called chromosome. A complete set of chromosomes in a organism carry all the genetic information and it is called the diploid set, but in the gametes half of the diploid number, namely the haploid number is noted. The nucleic acid and proteins are the major chemical components of the chromosome. The DNA and RNA are the main nucleic acid and the proteins are either of histone or non-histone type. The functional fragment of the DNA within the chromosome is called gene. The manipulation of these genes after isolation in living organisms is called genetic engineering. Chromosomes of the prokaryotic cell are simple with only DNA, non-complexed with histone. But the chromosomes of the eukaryotic cell are complex with all the components.




Chromosomes are self-reduplicating nucleoprotein filaments composed of hereditary units called genes. They form the intricate chromatin network of the interphase nucleus.

Chromosome number and its importance: –

chromos
Chromosome

The number of chromosomes in a given species remains constant and it was first discovered by Brendan and Boveri. This is most significant and of great importance in assigning the phylogeny of the species. The somatic cells have the chromosomes of diploid number (2n) but the gametes have half diploid set and there have haploid (n) number of chromosome. In an organism the haploid set of chromosomes is called genome also. The fusion of two haploid gametes one male and other female during sexual reproduction again restores the diploid number of chromosomes.
What is the lowest and highest chromosome number?

In the plant kingdom the plant Mucor hemialis (Bread mould) have the lowest numbered of chromosomes i.e. 2n=2. And in the animal kingdom Ascaris megalocephala univalens (horse roundworm) have the lowest numbered of chromosomes i.e. 2n=2.

Again in the plant kingdom the plant Ophioglossum reticulatum (fern) have the highest number of chromosomes i.e. 2n=1260. And in the animal kingdom the animal Aulacantha sdp (protozoa) have the highest numbered of chromosome 2n=1600.

The organism with less number and longer size chromosomes are primitive and those with greater number and shorter size of chromosomes are comparatively more advanced.

Types of chromosome: –

Broadly chromosomes are classified into two types. Those are Autosomes and Allosomes. Allosomes are also known as sex chromosome.



i) Autosomes: – the chromosomes which determine the somatic characters of an organism are known as autosomes. The human beings have 22 pairs of autosomes.
ii) Allosomes: - the chromosomes other than autosomes are called Allosomes or heterochromosomes. It includes accessory chromosome or B chromosome, or L chromosome, minute or m chromosome, S chromosome and E chromosome and the sex chromosome. The sex chromosomes normally determine the sex of an organism. They are either homomorphic, e.g. XX in human female and XY in human male, and in case of birds ZZ in male birds and ZW in the female birds.

Size of chromosome: –

In any organism the size of chromosome is an important characteristic and it varies from species to species. Chromosome’s diameter may vary from 0.2 to 2 µm, and the length may vary from 0.1 to 50 µm. In general monocots among plants have large chromosome while orthopetra and amphibia among animal have larger chromosome.
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