Virus | Definition, Nature, Structure, & its Reproduction

Viruses are the infectious agents, with simple, acellular organisation. They are exception to the cell theory. The study of virus is called virology. Viruses are connecting link between living and non-living entities. They have the properties of both living and non-living things. Viruses can reproduce only within living cells and are obligatory intracellular parasites.

Non-living Nature of Virus:

1. Lacking protoplast
2. Ability to get crystallized, e.g., TMV, poliomyelitis virus
3. Inability to live independent of a living cell. (Lack functional autonomy)
4. Absence of respiration.
5. Absence of energy storing system.
6. Absence of growth and division.

Living Nature of Virus:

1. Being formed of organic macromolecules.
2. Presence of genetic material.
3. Ability of multiply.
4. Occurrence of mutations
5. Occurrence of certain enzymes like, neuraminidase (first discovered), transcriptase and lysozyme in certain viruses.
6. Infectivity and host specificity.
7. Viruses can be ‘killed’ by autoclaving and ultraviolet rays.
8. Viruses are responsible for a number of infectious disease like common cold, epidemic influenza, chicken pox, mumps. poliomyelitis, rabies, herpes, AIDS, SARS etc.

Structural Components of Viruses

The structural components of viruses are envelope, capsid and nucleoid:

1. Envelope : It is the outer thin loose covering composed of proteins (from virus), lipids and carbohydrates (both from host). This layer may or may not be present. Envelope is present in HIV, Herpes virus.
2. Capsid : It is the outer protein coat made up of small subunits called capsomeres for the protection of nucleic acid (their genetic material).
3. Nucleoid : Viruses contain either DNA or RNA:

(A)    DNA containing viruses are called deoxyviruses. There are of two types:

1. Double stranded DNA (dsDNA) viruses e.g. Pox virus, Cauliflower mosaic virus, Herpes virus.
2. Single stranded DNA (ssDNA) e.g. Coliphage ɸ ×  174, M13 phage.

(B)    RNA containing viruses or riboviruses are of two types:

1. Double stranded RNA (dsRNA) virus e.g. Reovirus, Wound tumour virus.
2. Single stranded RNA (ssRNA) virus e.g. TMV, Foot and Mouth disease virus, Influenza virus, Retroviruses (HIV).

On the basis of host specificity viruses are divided into three groups:

Group	Common type of genetic material
1. Phytophagineae / Plant viruses 2. Zoophagineae / Animal viruses 3. Bacteriophages / Bacterial viruses	ssRNA ss or dsRNA or dsDNA dsDNA

Structure of Some Viruses

1. Tobacco Mossaic Virus (TMC) is elongated rod like, 3000 Å long, 180 Å in diameter with molecular weight 39.4  ×  10⁶  dalton. 2130 capsomeres are arranged helically to form the capsid. RNA strand is helical. ssRNA consists of 6400 nucleotides. Thus, the ratio of nucleotides  :  capsomeres  =  3 : 1
2. Bacteriophage (or bacterial viruses) are the viruses that infect the bacteria. Bacteriphage usually have double stranded DNA. T4 Bacteriophage has a tadpole like structure with polyhedral head connected to a helical tail (binal). The head consists of nucleic acid surrounded by a protein coat or capsid. Nucleic acid is double stranded DNA. Tail is proteinaceous tube-like, core surrounded by sheath. At one end, tube is joined to the head by thin collar. At the other end, it has a hexagonal base plate with six small tail pins and six tail fibres which help in atttachment of the phage to the host cell.

Reproduction in Viruses

It is of main two types : Phagic and Pinocytic

(a)    Phagic Reproduction  :  Only nucleic acid of virus enters the host cell. It is further of two types:

1. Lytic Cycle : Occurs in virulent phages, e.g., T4 bacteriphages.
2. Lysogenic cycle : Occurs in temperate viruses such as λ phage.

(b)   Pinocytic Reproduction  :  It is found in viruses like TMV, HIV, Hepatitis B etc. in which whole particle of virus enters host cell except envelope (if present).

List of some Diseases caused by viruses

Table  :  Viral Disease of Man

Name of the disease	Causal Agent
1. Influenza 2. Small pox 3. Mumps 4. AIDS 5. Poliomyelitis 6. German measles 7. Measles	Influenza virus Variola virus Paramyxovirus Retroviruses Polio virus Rubella virus Measles virus

 Table  :  Viral Disease of Plants

Name of the disease	Causal agent
1. Tobacco mosaic 2. Cucumber mosaic 3. Potato leaf roll 4. Bunchy top of banana	TMV Cucumber mosaic virus Potato leaf roll virus Banana bunchy top virus