Introduction to virus

 Definition of Virus

Viruses may be defined as acellular organisms whose genome consist of nucleic acid, and which obligately replicate inside host cells using host metabolic machinery and ribosomes to form a pool of components which assemble into particles called VIRIONS, which serve to protect the genome and to transfer it to other cells.

• Virus is from Latin referring to “poison”.

• Viruses are non-living entities.

• Infectious agents.

• Virion is the complete, structurally mature infectious unit of virus particle.

Size of Viruses

• A small virus has a diameter of about 20 nm.

– Parvovirus

• A large virus has a diameter of up to 400 nm.

– Poxvirus 

Structural Components of Viruses

• Nucleic acid: Either DNA or RNA.

• Capsid: Protective coat of protein surrounding NA.

• Nucleocapsid: Nucleic acid + Capsid proteins.

• Envelope: Lipid containing membrane surrounds some viral particles.

• Spikes: Envelope in some viruses contain glycoprotein spikes.

Viral Nucleic Acid

• RNA or DNA

• Segmented or non-segmented

• Segmented genome confers evolutionary advantages.

• Linear or circular

• Single-stranded or double-stranded

• If single-stranded RNA – is genome positive sense or negative sense.

Viral Capsid

• Protein shell of a virus.                                                            

• Made up of units called capsomere.

• Protect fragile Nucleic acid from:

– Physical damage.

– Chemical damage.

– Enzymatic damage

• Various type, two are most common

– Icosahedral

– Helical

Complex capsid

• Neither purely helical nor icosahedral.

• May possess extra structures.

Viral Envelope

• Many viruses have viral envelope covering their protein capsid.

• Derived from the portions of host cell membrane, but carrying some viral glycoproteins.

• Capsid inside envelope may be based on helical or icosahedral symmetry.

• Antigenecity

• Infectivity

Viral Envelope Spikes

• Many enveloped viruses contain protein spikes protruding from the surface.

Influenza virus further classified by subtypes on the basis of two main surface glycoprotein spikes.

1) Hemagglutinin (HA) 17 varieties – Adheres to the cells in the respiratory tract.

2) Neuraminidase (NA) 09 varieties – Remove neuraminic acid from mucin results in the release of new virus particles

• Envelope of HIV carries two glycoprotein spikes gp120 and gp41.

• Enables the virus to attach and fuse with target cells to initiate infectious cycle.