Properties of virus

• Some general properties of virus are;

1. Size:

• The size of virus ranges from (20-300) nm in diameter.
• Parvovirus is the smallest virus with size 20nm whereas Poxvirus is largest being 400nm.

2. Shape:

• The overall shape of virus varies in different groups of virus.
• Most of animal viruses are spherical shape, Pox virus is rectangular shape, TMV is rod shape, Poliovirus is bullet shape etc
• Some virus are irregular and pleomorphic in shape.

3. Symmetry:

• Morphological protein subunits of capsid are arranged together to from a symmetrical structure of the virus.
• Two basic symmetry are recognized in virus, they are helical symmetry and icosahedral symmetry.
• In some virus, symmetry is more complex, which is other than helical or icosahedral.

4. Structure and Chemical composition:

i. Genome:

• Viral genome or nucleic acid  contains either DNA or RNA but not both.
• The genome can be either ds DNA or ss DNA or ds RNA or ss RNA
• The genome can exist as single piece or segmented. Eg, Influenza virus contains 8 segments of ss RNA genome.
• The genome may be linear or circular. Most virus possess linear genome except Papova virus which contains circular ss DNA.
• Genome helps replication of virus in host cell.

ii. Capsid:

• Capsid is the outer shell of a virus.
• It is chemically a viral protein.
• Capsid is composed of capsomere.
• Structure of capsid gives the symmetry of virus.
• Capsid protects the nuceic acid and also helps in attachments on host cell surface during infection.

iii. Envelope:

• Some virus contains phospholipid bilayer known as envelope.
• Virus lacking envelope is called naked virus.
• Envelope is a lipid bilayer which is acquired from host cell membrane

iv. Glycoprotein spike:

• Envelope of some virus contains viral coded spike projected outside the envelope called glycoprotein spike or peplomers.
• Glycoprotein spike are viral coded protein with carbohydrate head.
• Glycoprotein spikes is an important antigenic structure.
• Neuraminidase and Haemagglutinin are glycoprotein spikes which helps in virus attachment to cellular receptor on host cell to establish infection.

v. Enzymes:

• Some virus possess their own enzymes.
• Retrovirus possess reverse transcriptase

5. Viral replication:

• Virus only replicates inside host cell

6. Metabolism:

• Viruses are metabolically inert outside host cell.
• They are also called as obligate intracellular parasite

7. Resistance:

i. Temperature:

• Most viruses are heat labile.
• Viruses are inactivated by heating at 60°C for 30 minutes or 100°C for few seconds.

ii. Cold:

• Viruses are stable and resistant to cooling.
• Virus can be stored for long duration at -40°C to -70°C by lyophilization or freeze drying.

iii. Radiation:

• Both non-ionizing and ionizing radiation can kill virus.
• UV rays causes pyrimidine dimer formation while ionizing radiation eg, X-rays causes lethal break of viral genome.

iv. Organic solvent:

• Chloroform, ether and bile salt can destroy all viruses by lipid solubiliation.

v. Disinfectant:

• Most viruses are destroyed by oxidizing agents such as chlorine, H2O2, iodine etc.
• Many viruses are resistant to phenol and chlorination. The phenol and chlorine do not always inactivates enterovirus, particularly if they are present in faecal materials.

vi. Antibiotics:

• Viruses are resistant to antibiotics.