Virus
Structure
and
Classification
Virus
Structure
and
Classification -
After
the
discovery
by
Louis
Pasteur
and
Robert
Koch
that
infectious
diseases
were
caused
by
minute
living
organisms
or
'germs',
it
was
expected
that
the
germs
for
all
infectious
diseases
would
be
discovered.
However,
bacteriological
techniques
failed
to
demonstrate
the
causative
organisms
for
many
diseases
like
measles,
small
pox,
rabies
and
mumps.
The
Russian
botanist
Ivanovski
(lwanowsky)
(1892)
was
the
first
to
give
clear
cut
evidence
of
a
virus.
Mayer
(1886)
had
demonstrated
that
when
juice
from
tobacco
plants
infected
with
the
'mosaic'
disease
was
injected
into
healthy
plants,
it
reproduced
the
mosaic
disease.
Boiling
the
juice
destroyed
the
infectivity.
Mayer
thought
that,
the
causative
agent
was
a
bacterium.
Inoculation
of
tobacco
plants
with
a
variety
of
bacteria,
however,
failed
to
produce
the
tobacco,
mosaic
disease.
Ivanovski
confirmed
the
observations
of
Mayer,
and
also
made
another
very
important
one.
Even
after
filtering
through
the
finest
bacterial
filters,
the
juice
still
remained
infective.
Ivanovski
concluded
that
the
agent
was
smaller
than
any
known
bacterium,
but
he
still
considered
it
to
be
a
bacterium.
This
agent
was
later
called
a
virus.
Bacteriophages
(
viruses
that
parasitise
bacteria)
were
discovered
by
the
French
scientist
d'Herelle
(1917),
who
found
that
some
agent
was
destroying
his
cultures
of
bacilli.
Schelsinger
(1933)
was
the
first
to
determine
the
composition
of
a
virus.
He
showed
that
a
bacteriophage
consists
of
only
protein
and
DNA.
In
1935
Stanley
crystallized
the
virus
causing
tobacco
mosaic
disease,
and
demonstrated
that
the
crystals
retained
their
infectivity
when
inoculated
into
healthy
plants.
He
thus
showed
that
viruses
were
not
like
typical
cells.
In
1952
Hershey
and
Chase
studied
-
the
T2
bacteriophage
and
demonstrated
that
(1)
the
genetic
information
is
carried
in
the
phage
DNA,
and
that
(2)
infection
is
the
result
of
penetration
of
viral
DNA
into
cells.
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