Biological Classification
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The sexual
reproduction is by
copulation of male
and female followed
by embryological
development.
07
VÏRÜSËS, VÏRØÏDS ÅÑD LÏÇHËÑ
In the five-kingdom classification, acellular organisms (viruses, viroid
and prions) and lichens are not mentioned.
Viruses are not truly ‘living’. So they are not included in five-kingdom
classification.
Viruses are non-cellular organisms having an inert crystalline structure
outside the living cell.
Viruses are obligate parasites. (NEET 2019) When they infect a cell, they
take over the machinery of the host cell to replicate themselves, killing
the host. Thus, they are a connecting link between living and non living.
D.J. Ivanowsky (1892) discovered virus. He recognized certain microbes
that cause mosaic disease of tobacco. They were smaller than bacteria
because they passed through bacteria-proof filters.
M.W. Beijerinek (1898) demonstrated that the extract of the infected
tobacco plants cause infection in healthy plants and called the fluid as
Contagium vivum fluidum (infectious living fluid).
W.M. Stanley (1935) showed that viruses could be crystallised and
crystals consist largely of proteins.
A virus is a nucleoprotein, i.e., it has a protein coat (capsid) & genetic
material (RNA or DNA). (NEET 2014)
The genetic material is infectious.
No virus contains both RNA & DNA.
Generally, viruses that infect plants have single stranded RNA. Viruses
that infect animals have either single or double stranded RNA or double
stranded DNA. Bacteriophages (viruses that infect bacteria) usually
have double stranded DNA.
The capsid is made of small subunits (capsomeres) protects nucleic
acid. Capsomeres are arranged in helical or polyhedral geometric
forms.