Biochemically, a gene is a segment of DNA with a specific sequence of nitrogenous base. Functionally, genes are segment of DNA which controls the cellular functions by controlling the synthesis of a protein. Thus genes express itself in the form of a protein or enzyme that controls the development of specific character or a specific function.
In other way, expression refers to the molecular mechanism by which genes show its potential in the phenotype of an organism.
One gene one enzyme theory
Regarding the gene expression, the theory “one gene one enzyme” was proposed by Beadle and Tatum of California which working biochemical mutation on red mold Neurospora crassa. They were awarded Nobel Prize in 1958 for this work. Based on their work, they proposed a concept called “one gene one enzyme” hypothesis. It means that in a biosynthetic pathway several steps are involved each step is controlled by a specific enzyme which is synthesized under the control of specific gene. This hypothesis was later modified as one gene one polypeptide theory. Since it was found that the function unit at the genetic level is a polypeptide.
Viral gene expressions
Important characters of a virus
Virus (L. position) is a nucleoprotein entity which uses host machinery for its multiplication. The first virus to be discovered was Tobacco Mosaic Virus (TMV). The characteristic features of a virus are –
I. Virus is the smallest organism known so far.
II. It dose not have cellular structure.
III. It is obligate properties as it multiplies inside the living cells only.
IV. It exhibits properties of both living and non-living things. It has no metabolic activity of its own. It becomes active and multiply when it infects a living host cell.
V. Virus is capable of exhibiting mutation and recombination.
VI. It exhibits high degree of host specificity.
VII. It has very few enzymes like lysozyme, reverse transcriptase, etc.
Classification of viruses
Viruses are highly specific in nature and they have been classified into three categories on the hosts they live in –
I. Plant virus – virus that infects plants. e.g. Potato mosaic virus (PMV), Tobacco mosaic virus (TMV), etc.
II. Animal virus – virus that infects animals. e.g. Polio-myelitis virus, Infuenza virus. Small pox virus, Hepatitis virus, Mumps virus.
III. Bacterial viruses or bacteriophages – virus that infect bacteria.
There are different types of viruses containing DNA or RNA as a hereditary material. Depending on the type of the nucleic acid contained, viruses are placed in two groups –
I. Deoxyvira – virus that possesses DNA as the genetic material, also called as DNA virus. Majority of the animals viruses is DNA virus except polio virus, Rabies virus, Herps, etc.
II. Ribovira – virus that possesses RNA as the genetic material, also called as RNA virus. Majority of the plant viruses is RNA virus, Mumps, Influenza, and Rabies.
Structure of a virus
Structurally, a virus is made up of two components
a) Nucleoid – it is also called core. It is made up of a strand of highly coiled nucleic acid which is either DNA (in DNA virus) or RNA (in RNA virus).
b) Capsid – Capsid forms a covering around the nucleoid. It is made up of proteins or polypeptides. Its proteins are protective in function. They are resistant to proteolytic enzymes of the host. They have enzymatic properties. These also help the viruses in adsorption and penetration inside the host.
Some viruses like influenza or herpes have an additional lipoprotein membrane called enveloped made of lipoproteins.
Structure of bacteriophage
• A bacteriophage has two parts – head and a tail.
• Head is icosahedral and tail cylindrical.
• Head bears genetic material (DNA or RNA)
• Tail contains a hexagonal basal flat plate which bears six long tail fibres.
• These tail fibres remain coiled inside tail but spread out at the time of infection.
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