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General structure of virus

General Structure of Virus

v They are very small infectious agents with size ranging from 20-300 nm in diameter.

v Non-cellular entities so they are also called as particles.

v Cannot replicate outside the host cell and dependent on host cell machinery metabolism (obligate intracellular parasites)

v Viruses are classified into four groups based on shape: filamentous, isometric (or icosahedral), enveloped, and head and tail.

Structurally viruses are made of

v Nucleic acid,

v Capsid

v Envelope

v Enzyme.

Only few viruses contain enzymes Lysozyme in Bacteriophage and Reverse Transcriptase in Retroviruses.

Nucleic Acid:

i. NA present in virus is called nucleiod (genome)

ii. amount depends on the size of virion.

iii. infective part of virus

iv. utilize host cell machinery and metabolism for synthesis and assembly of viral components

v. genetic material are of 4 types

a. ds DNA (Hepatitis B Virus)

b. ss DNA (Bacteriophage)

a. ds RNA (Reo virus)

b. ss RNA (TMV, Covid-19)

Protein Coat / Capsid

i. The NA of virus is protected by a protein coat called capsid.

ii. Capsid consists of several identical protein subunits called capsomeres.

iii. The number of protein subunits and arrangement are specific and useful in identification and classification.

iv. Structure of capsid give the symmetry to the virus and may be either cubical or helical or binal or complex symmetry.

v. Capsid also helps to introduce viral genome into host cell during infection.

vi. NA and protein coat of a virus particle is called as virus nucleo-capsid.


i. In some complex form the capsid is covered by envelope.

ii. Usually made of some combination of lipid (H), protein (V) and carbohydrates (H)

iii. Subunit of envelope : Peplomeres

iv. Have projections called Spikes

v. Naked virus (Lacking envelope)

Non-enveloped viruses can be more resistant to changes in temperature, pH, and some disinfectants than are enveloped viruses.