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.