BIOLOGY NOTES FOR NEET EXAM-VIRUS
Non-living characters of viruses
- Absence of protoplasm
- Absence of enzyme system.
- No respiration.
- They can be crystallized like chemicals.
- They do not grow in culture medium.
- They are inert out side the host cells.
- They are autocatalytic and lack fuctional autonomy.
Living characters of viruses
- They contain nucleic acid as a result of which they are capable of synthesizing proteins for their coat, although they use ribosomes of the host for the purpose.
- They can multiply inside living host cell.
- They have antigentic properties and shows mutation and specifing to the particular host.
Size of Viruses :
- TMV – 300 mµ × 20 mµ or 300 × 20 nm
- Smallest plant virus : Tobacco necrosis satellite virus (17 nm)
- Longest plant virus : Citrus tristeza virus – (2000 × 12 nm)
- Smallest animal virus : – Foot & mouth virus – 10 nm
- Largest animal virus – Small pox virus (Variola virus) – 400 nm
Brick shaped – Small pox virus
Spherical – Influenza virus, Myxo, Polio, HIV
Rod shaped – TMV
Tadpole like – Bacteriophages.
Bullet shaped –Rabies virus
Chemical composition :
Chemically there are two components of viruses :
(1) Nucleic acid-core
(2) Protein coat.
(1) Nucleic acid : Either RNA or DNA
- Generally in plant viruses single stranded RNA is present but in Cauliflower mosaic virus and Potato leaf roll virus, DNA is present.
- Generally in animal viruses either single or double stranded RNA or double stranded DNA is present.
In following animal viruses, RNA is present :
(i) Inluenza virus : Single stranded RNA.
(ii) Rous Scarcoma virus : Single stranded RNA.
(iii) AIDS virus : Single stranded RNA.
(iv) Poliomyelitis virus : Single stranded RNA.
(v) Reovirus : Double stranded RNA.
Single stranded RNA viruses (e.g., AIDS virus) which carrry few molecules of reverse transcriptase enzyme (which copies RNA into DNA, i.e., reverse transcription), are called retroviruses.
Generally RNA is single stranded but in Reovirus, Wound tumour virus and Rice dwarf virus RNA is double stranded.
(2)Protein coat :
- It is known as capsid and made up of structural units called capsomeres. (Number, size and structure of capsomeres are vary and these capsomeres are arranged in different manners to form different types of symmetry).
- Central core & Capsid are collectively known as nucleocapsid.
Note : An additional covering is also present in some viruses around the caspids. It is composed of Lipo protein. Such type of vriruses are known as lipovirus.
Example : Myxo Virus and Herpes Virus.