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Cell
structure 7. The structure and function of a typical virus - reproduction and
effects on cells
Doc Brown's GCSE level Biology exam study revision notes
There are various sections to work through, after 1 they can be read and studied in any order.
Sub-index
of biology notes on cell structure
(7a) Notes on the
structure of viruses (which are NOT
classified as living organisms)
Viruses are not living organisms.
They are particles
smaller than bacteria; viruses are parasitic and can only reproduce
inside living cells; they can infect every type of living organism.
Viruses have a wide variety of shapes and sizes and have no cellular
structure but do have a protein coat and only contain one type of
nucleic acid, either DNA or RNA.
Examples include the tobacco mosaic
virus that causes discolouring of the leaves of tobacco plants by
preventing the formation of chloroplasts, the influenza virus that
causes ‘flu’, the common cold virus and the HIV virus that causes AIDS
The basic structure of a virus
Viruses are not considered to be a living
organism such as a plant, animal, bacteria or archaea.
Viruses are not considered to alive
because they do not fulfil the seven life processes, namely: movement,
respiration, sensitivity, nutrition, excretion, reproduction and growth.
Biological science uses the phrase 'strains' of virus and not species.
Viruses are the smallest agents of
infectious disease and are exceedingly small (about 20 - 500 nm diameter)
and essentially round in shape.
Viruses are consist of a relatively short
length of genetic material (DNA or RNA) which is enclosed in a thin protein coat,
which is sometimes surrounded by an extra thin fatty coating or envelope.
A typical size of a virus is about 1/50th
of a red blood cell, but they can vary in size from 20 to 500 nm).
Within the protein shell the DNA/RNA
nucleic acid can be single- or double stranded.
The entire infectious virus particles are unable to grow or reproduce
without a host.
They have non of the usual sub-cellular structures seen
in most plant or animal cells.
The above diagram of the structure of typical virus
includes the DNA or RNA, enclosed in a protein coat/sheath, which itself
is surrounded by an envelope.
The virus has a head, tail fibres and spikes that can
attach to protein receptor cells of the host, in which it induces the
host cell molecular replication apparatus to reproduce the virus!
Viruses are different from all other
infectious microorganisms because they are the only group of microorganisms
that cannot replicate outside of a host cell.
Viruses do not consume food, but they
obtain materials and energy from host cells by hijacking their host
cell's cellular machinery.
Specific types of viruses only infect specific cells and
persuades them to reproduce the invading virus.
Some scientists argue that they are
more like complex molecules than living creatures.
Viruses are known to infect nearly
every type of organism on Earth and some viruses, called bacteriophages,
even infect bacteria - nothing is safe from some virus or other!
A virus is sometimes considered
a non-cellular form of life, but it really isn't a living organism.
Check on the characteristics of
'living organisms' in Part 1.
For more on virus infection
mechanism see
communicable diseases -
pathogen infections
(7b)
Viruses - reproduction and killing cells
The basic structure of a virus
Viruses are NOT cells, but usually consist of a strand of genetic
material enclosed in a protein coat.
The genetic material, nucleic acid can be
either DNA or RNA.
Sometimes the virus has an outer coat or
envelope.
Viruses come in all shapes and sizes e.g.
spherical (on the left) or various other geometrical shapes with a
tail and trailing fibres (on the right).
All viruses have spikes of protein that can
attach to receptors of host cells - prior to invasion!
How do viruses reproduce?
They cannot reproduce on their and have to 'invade' i.e. infect a
living cell that acts as a host.
Specific types of viruses only infect specific cells and
persuades them to reproduce the invading virus.
The sequence of events is:
The virus particle attaches to a host
cell when its protein spikes recognise the receptors.
The virus releases its genetic
instructions into the host cell.
The injected genetic material takes over the
host cell's enzymes.
These same enzymes make parts for more new
virus particles.
The new particles assemble forming new
viruses.
Specific types of virus will only infect
specific cells, known as host cells.
The life cycle pathways of a virus
The life cycle of a virus begins when it gets through a cell
membrane to infect a host cell.
Most viruses reproduce by the lytic pathway and but others
involve the lysogenic pathway first.
The lytic cycle pathway
 In the lytic pathway the virus attaches itself to a
specific
host cell and injects its genetic material (viral DNA)
into the cytoplasm through the host cell's membrane.
The host cell now contains the viral DNA.
 The virus then uses the proteins and enzymes of the host cell
to replicate its genetic material (DNA instructions) and
so produce the material for the new viruses.
 The virus components then assemble to form lots of new
viruses.
Eventually so many viruses are produced that the cell bursts open,
killing it - thus releasing lots
more viruses to invade more host cells to create more viruses!
Therefore back to square
and the cycles begin all over again, not good for the host
cells!
The lysogenic cycle pathway
As with the lytic cycle, the virus attaches itself to a specific
host cell and injects its genetic material (viral DNA)
into the cytoplasm through the host cell's membrane.
and
The host cell now contains the viral DNA, but unlike in the
lytic cycle, the viral DNA becomes incorporated into the host
cell's genome..
 ,
and
 Therefore
the viral genetic material gets replicated along with the
host DNA every time the host cell divides, but for the time
being, the virus stays dormant - doesn't do anything - so no new
viruses are formed.
BUT, unfortunately, all the new
cells produced in the lysogenic pathway are now carrying the
viral genetic information and therefore, at a later time,
when triggered, produce lots of viruses.
 Eventually, some kind of trigger e.g. from a chemical
stimulation, causes the viral genetic material to leave the
genome and become separated in the host cell.
The virus then reproduces using the lytic pathway
to
described above.
Note: The lytic cycle can happen in a
few minutes. but the viral DNA can stay in the lysogenic cycle
for many years. See also
communicable diseases
Summary of learning
objectives and key words or phrases
Know how a virus kills cells by multiplying in host cell. Be
able to describe details of reproduction lytic cycle of viruses and the lysogenic cycle
of virus reproduction. In the virus life
cycles, the virus takes over the host DNA. From a diagram, understand the basic structure
and function of a typical virus including features like strands of DNA and RNA, protein
coat sheath, the head and tail fibres, the outer envelope of spikes of protein receptor cells.
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