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General structure of virus

Updated: Feb 14

General Structure of Virus

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

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

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

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

Structurally viruses are made of

  • Nucleic acid,

  • Capsid

  • Envelope

  • Enzyme.

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

Nucleic Acid:

  • NA present in virus is called nucleiod (genome)

  • Amount depends on the size of virion.

  • Infective part of virus

  • Utilize host cell machinery and metabolism for synthesis and assembly of viral components

  • 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

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

  • Capsid consists of several identical protein subunits called capsomeres.

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

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

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

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


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

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

  • Subunit of envelope : Peplomeres

  • Have projections called Spikes

  • Naked virus (Lacking envelope)

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

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