Nature of viruses is unclear, because it is difficult to classify them with in the accepted framework of living or non living organisms. Some virologist regard viruses as animate object (when present inside the host cell) whereas other consider them inanimate (when present outside the host cell).
Viruses are living because
- They show growth and multiplication (only inside the host cell).
- They have genetic material i.e. DNA / RNA
- They can direct protein synthesis (though they use host machinery for it).
- They show mutation.
- They can be transmitted from the diseased host to the healthy ones or posses the ability to infect.
- They react to heat, chemicals and radiation and also shows irritability, a character of only living organisms.
- They posses genetic continuity and have definite races / strains.
- Similarity between nucleoproteins of viruses with the protein and nucleic acid of living organisms.
Viruses are non-living because
- They can be crystallized
- They behave as inert chemicals outside the host cell.
- A cell wall or cell membrane of any type is absent in viruses.
- They do not shoe functional autonomy.
- They do not respire or excrete or they do not show any sign of metabolism except reproduction.
- They lack any energy producing enzyme system.
How do virus differ from bacteria and mycoplasmas?
Viruses differ from bacteria and mycoplasmas in:
- Viruses not possessing any cellular organization.
- Viruses do not grow on inanimate media.
- Not multiply by binary fission
- Not possessing both DNA and RNA together.
- Not possessing ribosome
- Not showing any sensitivity to antibiotics
- Showing sensitivity to interferon.
Therefor the contention that ‘viruses are viruses’ and nothing else ‘stands on the top’. According to regressive theory of evolution put forward by Lwoff, some primitive organism (like bacteria) become endoparasitic on some host and gradually lost synthetic enzymes to become today’s viruses. Thus viruses are super parasites.
A single infective particle of virus is called as virion. It consists of nucleic acid core surrounded by a protein coat or capsid. The capsid with enclosed nucleic acid is called nucleocapsid.
These are the smallest infectious agents causing diseases in host. They consist solely of a protein free low molecular weight with 243-360 nucleotides and small fragments of double stranded RNA molecules. They are also known as naked virus, meta virus or pathogen.
“Virusoids are the viroides which require RNA of the supportive virus for replication”.
Hang and clarke discovered a sub viral infectious agent which was later called as prions.
Prions are the causal agent of scrapie disease of sheep and goat. These prions have no nucleic acid (DNA / RNA) but they are made up of only 2-3 molecules of protein only.
Prions are 100 times shorter than viruses and are heterogenous in nature. A single prion rod is made up of about 1000 prion molecules. It is 100 – 200 nm long and 10 – 20 nm in diameter. Other disease caused by prions are Parkinson’s disease, multiple sclerosis, gerstmann stransslar syndrome and creutzfeldt – Jakob disease.