Question

What is a chromosome? List out the categories of chromosomes basing on the number and organization.

Answer 1

Chromosomes:

• The chromosomes have been considered as the physical bases of heredity because they have a special organization, individuality, functions and are capable of self-reproduction.
• Their main chemical constituent is DNA, an universally accepted genetic or hereditary material, found to carry genetic information from one generation to next generation.
• They occur in all living beings in a specific number and organization and usually fall into following categories.

A. VIRAL CHROMOSOMES:

• The chromosomes of viruses are called viral chromosomes.
• They occur singly in a viral species and chemically may contain either DNA or RNA.
• The DNA containing viral chromosomes may be either of linear shape (e.g., T1, T2, T3, T4 bacteriophages) or circular shape (e.g., most animal viruses and certain bacteriophages).
• The RNA containing viral chromosomes are composed of a linear, single-stranded RNA molecule and occur in some animal viruses (e.g., poliomyelitis virus, influenza virus, etc.); most plant viruses, (e.g., tobacco mosaic virus, TMV) and some bacteriophages.
• Both types of viral chromosomes are either tightly packed within the capsids of mature virus particles (virons) or occur freely inside the host cell.

B. PROKARYOTIC CHROMOSOMES:

• The prokaryotes usually consists of a single, giant and circular chromosome in each of their nucloids.
• Each prokaryotic chromo some consists of a single circular, double-stranded DNA molecule, but has no protein and RNA around the DNA molecule like eukaryotes.
• Different prokaryotic species have different sizes of chromo some.
• Thus, the bacterium Escherichia coli has 100μ long chromo some and Mycoplasma hominis has 265 long chromosome.

C. EUKARYOTIC CHROMOSOMES:

• The eukaryotes (plants and animals) usually contain much more genetic information than the viruses and prokaryotes, therefore, contain a great amount of genetic material, DNA molecule which here may not occur as a single unit, but, as many units called chromosomes.

Chromosomes:

The chromosomes are the structures found in the center (i.e. nucleus) of cells which carry long pieces of DNA.