Viruses

How a virus differs from a bacterium ?

A virus differs from a bacterium in the following features:

(1) There is no cellular organisation like cell wall and cell membrane in viruses but a bacterium have .

(2) Viruses are submicroscopic & smaller than bacteria .

(3)A virus never be free living , it is always  obligate parasite , but a bacterium may be free living .

(4) A virus can not keep DNA and RNA at the same time , but a bacteria can .

(5) A virus does not have  own metabolic enzymes , protein synthetic mechinery but a bacteria has all these .

(6) A virus can not perform binary fission but it is a common phenomenon for a bacteria .

Bacteriophages :  Viruses parasitic on bacteria are termed as Bacteriophages. They may contain ssDNA, dsDNA,ssRNA or dsRNA.

Types of bacteriophages on the basis of morphology 

Morphologically bacteriophages are of six types:

(1) A phage with a head , rigid tail having contractile sheath and fibres .

(2) A phage with a head ,flexible tail but no contractile sheath .

(3) A phage with a short tail  but no contractile apparatus .

(4) A head with large capsomeres at each apex of hexagon and no tail 

(5) A head without capsomeres and a tail .

(6) A long flexible tail only without a head.

The term "bacteriophage" was coined by W.M.Stanley.

Size of virus

Viruses are submicroscopic ,can be seen only by electron microscope , meausring unit is nm .

   1 nm=1/10⁹ mt

Whereas, in case of bacteria it is micrometre.

   1 micro mt=1/10³ mt.

#Smallest virus .....Rhino virus or foot-and-mouth disease virus ...10 nm in diameter.

# Largest animal virus ....Vaccinia,Variola ...300--350 nm.

# Largest plant virus ...Beet yellow virus..1250 x 40 nm.

Chemical structure of virus:

(1) The nucleic acid core either DNA or RNA is sorrounded by a protein coat or capsid .

 Nucleic acid + capsid =nucleocapsid.

(2) A capsomere is the morphological unit of capsid.

 e.g. , TMV capsid=2130 capsomeres

(3) Sometimes carbohydrate, fat, biotin , riboflavin ,Cu ,etc . are added to the capsid protein .

(5) A few viruses like HIV and Influenza virus have 10 --15 micro mt thick layer of lipoprotein or glycoprotein  envelope   around the capsid .

(6) The unit of envelop is called Pelpomere

(7) Sometimes hemoglutinin of  the spikes from lipid layer act as antigen to destroy RBC of the host ,e.g., Influenza virus .

It is to be noted that TMV is a Naked helical virus but HIV , Influenza virus  ,etc.  are the enveloped .

Obligate parasitism of virus:

Viruses behave as nonliving outside the host but  have to enter in host cell to conduct repoduction ,hence , viruses are obligate parasite.

Viruses are antibiotic resistant, why?

Viruses do not have their  own ribosome , so , they make the use of host cell's ribosome for protein synthesis .This is why viruses are resistant to antibiotics.

Non-living features of virus :

(1) Viruses can be crystalized like chemicals (Stanley first crystalised TMV in 1935).

(2)The basic units of a cell like cell wall and cell membrane are lacking in virus.

(3) Viruses are inert when out of the host cell .

(4) Viruses are lack of cell organelles ,so , can not perform any metabolic activities which run life processes in all of the living organisms .

(5) For cellular activities atleast 5000A° is required , but viruses have 100A° - 2000A°  since  it's submicroscopic nature .

Hence, virus is called non-cellular.

Living features of viruse:

(1) A virus contains either DNA or RNA as genetic material but not at a time .

(2) It  reproduces when intracellular .

(3) It infects the host cell.

(4) It makes the use of host ribosome to synthesise proteins of it's own .

(5) Virus genome mutates .

Due to the above said living as well as non-living features  & extracellular - intracellular phases a virus is called intermediate between living and non-living.