Download Virus PPT

Viruses
What Are Viruses?
Viruses are tiny (20 - 400 nm) particles composed
of a nucleic acid core (either DNA or RNA)
surrounded by a protein coat (capsid) and
sometimes a membrane envelope (derived from
its host cell)
Viruses are incapable of reproduction independent
of host cells whose cellular machinery is taken
over by the virus and used as a factory for
producing new viruses
Discovery of Viruses
Mayer
The first virus discovered was the tobacco
mosaic virus (TMV) which causes tobacco
leaves to be mottled and stunts growth
He theorized that a bacteria to small to be
seen with a microscope caused tobacco
mosaic disease
Discovery of Viruses
Ivanowsky
1890s D. Ivanowsky (Russia) filtered sap through
ceramic filters designed to remove bacteria and
found that the filtered sap still caused tobacco
mosaic disease
Ivanowsky theorized that the disease had two
possible causes:
1 Bacteria small enough to pass through his filters
2 A toxin that passed through the filters
Discovery of Viruses
Beijerinck
1897 Martinus Beijerinck (Holland)- Demonstrated that the
mosaic disease agent could reproduce
First sap was filtered, then placed on leaves
When these leaves developed mosaic disease, their sap was
used to cause disease in other plants for several generations
At no point was the ability to cause disease diminished
This eliminated the possibility of a toxin alone being the
cause
Discovery of Viruses
Beijerinck
Beijerinck reasoned that if it was a small bacteria it could be
cultured using nutrient media
This proved to be impossible as the agent would only
reproduce in the tobacco plant
Bacteria should be killed by ethanol, but ethanol did not
destroy the ability to cause mosaic disease
Because of this, the idea of a very small bacteria was
rejected and replaced with a new theory:
Mosaic disease is caused by an agent much smaller
and simpler than bacteria
Discovery of Viruses
Stanley
1935 Wendell Stanley (US) - Crystallized the
Tobacco Mosaic Virus (TMV)
After the advent of electron microscope viruses
could be viewed directly
Do viruses qualify as living
things?
They are much smaller and less complex than cells.
They consist of either DNA or RNA enclosed in a protein
coat called a capsid.
Virons do not grow.
They have no nucleus, cytoplasm, or membranes.
They do not carry out cellular functions.
Virons do not generate metabolic energy.
They are obligate intracellular parasites, meaning that they
require a host cell to reproduce.
Viral Structure
Viruses are made of a maximum of three basic parts:
1 Genetic material:
– DNA viruses - Single or double stranded DNA
– RNA viruses - Single or double stranded RNA
2 Capsid - A protein coat made up of one or a few
capsomere proteins
3 Envelope - Not all viruses have an envelope which is
derived from the host cell’s membrane
The envelope may contain proteins which assist in
attachment and entry into new host cells
Characteristics used to
classify viruses.
Type of nucleic acid: viruses have either single-strand
RNA or double-strand DNA, but never both.
Physical structure: viruses are found in several general
shapes.
– The structure of viruses has been deciphered by electron
microscopy and X-ray diffraction. These techniques identify two
shapes
– Envelope: some viruses use lipids from the cell to form a layer
around the capsid during replication.
– Type of host cell: most viruses are very specific about the type of
cell they invade. There are specific receptor sites that viruses
attach to on a host cell's surface.
Shapes of Viruses
icosahedron (the capsid has
20 triangular faces)
helix (the capsid
resembles a coiled
spring).
Common Viruses
Phage Reproduction:
The Lytic Cycle
Infection
Destruction of
the bacteria’s
DNA
Production of
viral parts
Lysis
Packaging
Replication of
the viral
genome
Size Of Viruses
Viruses are very small, ranging in size from
20 nanometers to 250 nanometers. A
nanometer is equal to 0.00000004 inch
(4X10-8 inch). The smallest of all bacteria
is about the size of the largest virus. This
picture shows round virus particles along
with rod-shaped bacteria cells
Summary
The sequence of events that occurs when you come down
with the flu or a cold is a good demonstration of how a
virus works:
An infected person sneezes near you.
You inhale the virus particle, and it attaches to cells lining
the sinuses in your nose.
The virus attacks the cells lining the sinuses and rapidly
reproduces new viruses.
The host cells break, and new viruses spread into your
bloodstream and also into your lungs. Because you have
lost cells lining your sinuses, fluid can flow into your nasal
passages and give you a runny nose.
Viruses in the fluid that drips down your throat attack the
cells lining your throat and give you a sore throat.
Viruses in your bloodstream can attack muscle cells and
cause you to have muscle aches.