VIRUS

VIRUS

GENERAL CHARACTERISTICS:

• Virus is very much smaller than bacteria.
• It is an obligate parasite i.e. it can live only within the host.
• Both DNA and RNA act as a genetic material.
• Study of virus is known as virology.
• The body of virus is made up of lipids and proteins.

• Its body is known as viriods.

Structural properties:

• With the help of electron microscope, x-ray diffraction and some biochemical methods, structure of virus is studied.
• DNA of a virus is covered by capsid.
• Subunits of capsid are known as capsomeres. They are made up of proteins and are also known as proteomers.

Based upon structural properties, viruses are classified into four types:

i. Icosahedral shaped (20 equatorial triangles)

ii. Helical shaped (cylindrical)

iii. Complex shaped (tail, multilayer)

iv. Enveloped (spherical)

Icosahedral shaped virus:

• It is made up of 20 equatorial triangles.
• Outer layer is made up of capsid.
• In the corners, 5 protein subunits are present.
• In the center, 6 protein subunits are present.
• The protein subunits are known as capsomeres.
• Structure of capsomere is ring shaped.
• It consists of 252 capsomeres. Ex: Adenovirus, SV40.

HELICAL SHAPED VIRUS:

• It is a hollow tube shaped virus.

• Hollow tube is made up of proteins.
• Outer covering is known as capsid and is made up of proteins.
• Subunits of capsid are known as capsomeres.
• Diameter of TMV is 10 – 20 nm. 18 nm is occupied by capsomeres.
• Here, RNA acts as a genetic material.
• Generally RNA is helical, flexible and thin.
• Ex: TMV and Influenza virus.

Complex shaped virus:

• Ex: T_even phage l, T₂, T₄, bacteriophage.
• Icosahedral head is present.
• It consists of head, tail, tail pins, sheath and genetic material.
• Collar connects head to the helical sheath.
• Helical sheath is made up of complex proteins. It consists of several nucleotides, 24 rings, 18 proteins (144 copies).
• T1, T5 – Sheathless, base plate is absent.
• T3, T7 – No tail fibers.
• Tail pins contain some chemicals that can degrade cell wall of bacteria.

Enveloped virus:

• Ex: HIV virus, Influenza.
• Outer most layer of virus is called envelope.
• It consists of lipids and proteins.
• Lipid layer is derived from host cell.
• Protein is coded by gene.
• Generally, enveloped viruses have RNA as a genetic material.
• Spike is an external projection present on the envelope. It acts as a receptor for infection and binds to the host.
• They are known as spikes or paplomers.
• Spikes differ from species to species.
• Ex: 1. Haemoglutinin à recognize RBC wall and cause infection.
• 2. Neuraminidase spike à recognize mucous membrane of host.
• When enveloped viruses are treated with any solvents, spikes are easily degraded. Then it becomes non pathogenic.
• Diameter of virus is 10 nm.
• RNA dependent RNA polymerase is present in virus. Here, RNA is synthesized from RNA itself.
• Replicase, RNA transcriptase which are the enzymes necessary for the RNA synthesis are absent in virus. RNA dependent RNA polymerase is used for RNA synthesis.
• Polymerase enzyme undergoes proofreading and cleaves mismatch pairs.
• Infective portion of virus is known as viriods.