Introduction:
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism, having the capacity to infect all types of life forms, from animals and plants to microorganisms.
It has the smallest size in the microorganisms 20-300nm.
They lack essential metabolic machinery to synthesize their own essential proteins hence has to depend on the hosts.
Viruses hence are called obligate intracellular parasites.
Structure of Viruses:
Viruses are obligate intracellular parasites hence they lack nucleus, cytoplasm, cell membrane and other cellular components.
A virion is a complete and well developed particle composed of a nucleic acid enclosed in a specialized protein coat.
Nucleic Acid:
Viruses contain either DNA or RNA as a nucleic acid material.
Exceptionally leukoviruses contain both as nucleic acid materials.
Nucleic acid may be double stranded or single stranded, may be linear or circular even,it is found in segments in case of some viruses like Influenza virus.
Capsid and Envelope:
Nucleic acid of viruses is surrounded by a special protein called “Capsid”.
Amino acid sequence of capsid is decided by the nucleic acid itself.
The smaller units of capsid are called “Capsomers”.
The capsomers may be made up of single protein or multiple proteins.
In some viruses the capsid is covered with an envelope.
The envelope is made up of some lipids, carbohydrates and proteins.
In case of some viruses the parts of the host cell membrane forms the envelope.
Some viruses show the presence of spikes like carbohydrate protein complexes on envelopes.
The presence of spikes helps the viruses in attachment with the host cells.
Depending on the presence of the envelope they are called naked and enveloped viruses.
Sizes of Viruses:
Viruses are very much smaller than the bacterias.
Viruses being very small were called ultramicroscopic.
Their smaller size has given birth to the new category of infectious agents that is “Filterable Viruses”.
The largest known virus is of size 300nm while smallest known virus is of size 20nm only.
Viral sizes are determined by ultracentrifugation, collodion, membrane filter of graded porosity and electron microscopes.
Morphology of Viruses:
Depending on capsid morphology the viruses can be categorised as follows,
Helical Viruses.
Polyhedral Viruses.
Enveloped Viruses.
Complex Viruses.
Helical Viruses:
They appear as rigid or flexible long rods.
The nucleic acid is present within cylindrical capsid.
The length of the virus is dependent on the length of the nucleic material.
Single unit of the capsid is called a capsomer.
E.g. Rabies Virus.
Tobacco mosaic virus.
Polyhedral Viruses:
Also called “Icosahedral” viruses.
Many viruses belong to this class.
The regular polyhedron contains around 20 triangular faces and 12 corners.
The nucleic acid along with certain proteins is stored in the core.
E.g. Polio virus
Enveloped Viruses:
Roughly spherical and highly pleomorphic (Change in shape).
Polyhedral or helical virus after enclosing in an envelope becomes enveloped viruses.
The envelope of viruses provides protection and impart infectivity to them.
E.f. Influenza virus, Herpes simplex virus.
Complex Viruses:
Some bacteriophages possess characteristic structure hence referred to as complex viruses.
Their nucleic acid is packed in a hexagonal head, which is attached to a terminal tail.
Tail may be surrounded with a contractile sheath.
The terminal end of the tail has a basal plate.
The basal plate has attached six pins and six tail fibers that help in binding to the specific receptor on the bacterial cell.
Commonly Asked Questions:
Draw a well labelled diagram of T-even Virus (Complex Virus).
Write a note on structure of virion.