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LIFE CYCLE OF VIRUSES

INTRODUCTION TO VIRUSES

Viruses are simple, non-cellular entities consisting of one or more molocules of either DNA or RNA enclosed in a coat of protein.Some viruses are enveloped by capsid some are naked. They can reproduce only within living cells and are obligate intra-cellular parasites. Viruses are smaller than prokaryotic cells ranging in size from 0.02 to 0.03 micrometre. A fully assembled infectious virus is called a Virion.The main function of virion is to deliver its DNA or RNA genome into the host cell so that the genome can be expressed by the host cell.Each viral species has very limited host range.

TYPES OF VIRUSES BASED ON HOST

Prokaryotic viruses : Viruses that infect prokaryotes like bacteria.

Eukaryotic viruses : Viruses that infect Eukaryotes like animals and plants.

LIFE CYCLE OF PROKARYOTIC VIRUSES(BACTERIOPHAGE)-  The general life cycle of prokaryotic viruses are examplified by bacteriophage.For a virus to replicate it must induce a living host cell to synthesys all essential components needed to make more virions.These components must then be assembled into new virions and escape from the cell in order to infect other cells. There are to different viral life cycle Lytic and Lysogenic.

LYTIC LIFE CYCLE

Lytic or vegitative life cycle culminates in lysis or rupture of host cell and release numerous viral progeny. These are also called as virulent bacteriophages because they cause death of bacteria.

Lytic life cycle consists of following steps:

• Adsorption or docking with the host receptor protein.

• Entry or peneteration of the viral nucleic acid into the host cytoplasm.

• Biosynthesis of the viral components.

• Assembly (Maturation) of the viral components into complete viral units. After assembly viral proteins cause the lysis of host cell.

• Release of the completed virus from the host cell.

Example of Lytic phage :- T-even phages.

LYSOGENIC LIFE CYCLE

In the Lysogenic cycle the viral DNA is inserted into the host's DNA and it reproduces as the host's DNA replicates. A lysogenis virus can remain in this state for numerous replications of host cell DNA untill it excises itself from the host DNA and undergo a lytic life life cycle.When the genetic material of these phages is inserted into the DNa of host cells it is said to be in the prophage state. A cell that contains a prophage is known as lysogen.These are also called as temperate phages.

Example: lambda phage

SHOWING PROKARYOTIC VIRUSE LIFE CYCLE (LYTIC AND LYSOGENIC)

LIFE CYCLE OF EUKARYOTIC VIRUSES

ENVELOPED VIRUSES

• Adsorption begins with the docking proteins of the virus locking into specific receptor sites on the surface of the target host cells.

• In the case of some enveloped eukaryotic viruses the viral membrane fuses with the cell membrane and in effect opens up so as to allow the virus to enter the cell cytoplasm. Once in the cytoplasm the protein capsid is removed and the viral genome is freed into the cytoplasm. Depending on the nature of the virus the viral genome may stay in the cytoplasm or it may enter the nucleus before beginning the next stage of the life cycle.

• In another case the cell engulfs the entire enveloped eukaryotic virus, taking it into the cytoplasm,called as endocytosis. Once inside the cytoplasm the enveloped eukaryotic virus is said to be in a vesicle. Subsequently, the two lipid membranes of the vesicle fuse, releasing the capsid covered virus. The capsid is then removed and the viral genome is freed for the next stage of viral replication.

• The actual processes from biosynthesis to maturation and release varies depending on the virus type. However, at the core, the viral genomic material uses cellular metabolism for its own selfish purposes. Finally the mature virus or virions. are released into the environment to begin the cycle again.

SHOWING EUKARYOTIC VIRUS (RETROVIRUS)LIFE CYCLE

NAKED VIRUSE

The naked virus replication is less elaborate, but follows the same course. After binding to the host receptors the naked viral capsid penetrates into the host cytoplasm, the genome is released and the steps of viral replication ensue. Since the final release of enveloped eukaryotic viruses involves taking a portion of the cell membrane the process is called budding. For example, the HIV virus buds out of the cell. Whether release involves lysis or budding, a cell eventually dies due to the viral infection and destruction of its metabolism

Examples of eukaryotic viruses are :

Animal viruses - adenovirus, retrovirus

Plant viruses - tobacco mosaic virus

Insect viruses - baculovirus

SUMMARY

Viruses replicate inside the host using host machinary. Different viruses replicate in specific host by differenet mechanisms.General life cycle of viruses include genome entry to host and then replicate in host , assembly of new and then release.