Definition: “A virus is a submicroscopic infectious particle that is an obligate intracellular parasite, replicates only inside the living host cells. These are considered at the borderline of living and nonliving. These are acellular and do not have their own metabolic system. These characteristics of viruses make them nonliving. But their reproduction in the host cell and causing disease in their host make them living”. The important characteristics of this mysterious organism are given below.
General characteristics of the viruses
- The term ‘virus’ is derived from Latin which means “slimy poison fluid” or “venom”.
- The virus is an ultramicroscopic, infectious agent that is metabolically inert so require a living host or cell to multiply.
- Viruses are obligate intracellular parasites.
- Viruses cannot make energy or proteins on their own so these are dependent on their host cell.
- Viruses multiply inside the living cells using host cell machinery.
- The virus has different strains or types.
- The virus has its own genetic material either DNA or RNA which may be single or double-stranded.
- A virus can undergo Mutation.
- They can be destroyed by Ultraviolet Rays.
- These are not composed of cells. They lack cellular structures such as plasma membrane, nucleus, organelles, etc.
- They do not respire or perform a gaseous exchange.
- They do not move, grow in size but can reproduce by using the metabolism of their hosts.
- They can be crystallized and stored in bottles like chemicals.
- They lack the enzyme system and do not have the metabolic activity of their own.
- Viruses are not able to survive without a host cell, so active viruses reside inside a host body. They are present either in a bacterial cell, animal cell or plant cell.
- Size: Viruses are much smaller than Bacteria. Their Size ranges from 20 – 1400nm. The poliovirus is 30nm. Giant Mimi viruses are up to 800 nm.
- Different shapes of Viruses: Viruses are of different shapes. They are rod-shaped, bullet-shaped, filament shaped, icosahedral in shape and tadpole-shaped.
Viral Structure: Virus consists of nucleic acid and a protein. Genome or the nucleic acid is covered by a protein coat called the capsid. Some viruses have an envelope outside the capsid. A virus without the envelope is called the Naked virus.
- VIRUS Genome: It consists of either deoxyribonucleic acid(DNA) or ribonucleic acid ( RNA). It may be a single-stranded form or double-stranded, Either circular or linear.
- Viral Capsid: Viral nucleic acids surrounded by Protein coat called Capsid. Viral capsid is of 3 types. Helical, Icosahedral Complex.