Viruses

Viruses  
They did not find a place in classification. Take over the machin
ery of host cell on entering it but as such they have inert
crystalline structure. So, difficult to call them living or non-living.
• Pasteur gave the term ‘Virus’ i.e., poisonous fluid.
• D. J. Ivanowsky found out that certain microbes caused
Tobacco Mosaic Disease in tobacco plant.
• M. W. Beijerinek called fluid as ‘Contagium vivum fluidum’ as
Extracts of infected plants of tobacco could cause infection in
Healthy plants.
• W. M. Stanely showed viruses could be crystallized to form
Crystals of protein which are inert outside their specific host.

Living property:-
They Can multiply
They possess genetic material i.e. Dna and Rna
They causes diseases.
Rich in protein
They possess enzymes

Nonliving property:-
They don’t have cellular structure.
They cannot reproduce outside host cell.
They don’t perform metabolism.
They can be crystallized into fixed geometrical shape .

Structure of Virus :
• Its a nucleoprotein made up of protein called
Capsid. Capsid is made up of capsomeres arranged
in helical or polygeometric forms. Have either
DNA or RNA as genetic material which may be
single or double stranded.
• Usually plant viruses have single stranded RNA;
bacteriophage have double stranded DNA and
animal viruses have single or double stranded
RNA or double stranded DNA.

Diseases caused : Mumps, Small pox, AIDS,hepatitis,meseals etc.

Viroids :
• Infectious agent, free RNA (lack protein coat)
• RNA has low molecular weight.
• Causes potato spindle tuber disease.
• Discovered by T. O. Diener

The virus which attacks:-
Bacteria- bacteriophage
Algae-phycophage
e.coli-coliophage
fungi-mycophage
yeast-xymophage

Some fact:-

Smallest virus-coliophage f2
Smallest plant virus-satellite tobacco necrosis virus
Longest plant virus-citrus tristeza
Smallest animal virus-foot and mouth virus
Largest animal virus-pox virus.