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Chapter 38 Viruses
Objectives
Define viruses
Consider whether they are living or non-living
Describe the structure of a virus
List the names of the virus shapes
Describe how replication occurs in viruses
Explain the medical importance of viruses to
humans, plants and animals.
Viruses
• Tiny (They are much smaller (50 times) than a
bacterium.
• Need Electron Microscope to see them
• Non Cellular (not made of cells)
• Cannot reproduce on their own (not alive
according to our rules)
Structure of Viruses
A virus consists of:
• A strand of genetic
material either DNA or
RNA.
• A protein coat capsid.
• Viruses do not have
ribosomes,
mitochondria or other
cell organelles.
• Viruses do not respire.
• They cannot reproduce
outside a host cell.
• They are obligate parasites
Viruses are not living organisms?
Living
Non-living
Possess genetic material
either DNA or RNA
Are non-cellular
Possess a protein coat
Cannot reproduce by
themselves
Can replicate (inside a
living cell)
Only have one type of
nucleic acid (living things
have both DNA and RNA)
RNA or DNA?
Viruses with RNA
– Human immunodeficiency virus (HIV)
– Influenza viruses
– Rabies
Viruses with DNA
– Warts
– Chickenpox
– mononucleosis
Shapes
Viruses can be:
1. Spherical
2. Rod shaped
3. Complex shaped
Bacteriophage
A bacteriophage or phage is a virus that infects
bacteria
Bacteriophages are among the best studied
viruses. T phage takes about
30 minutes to replicate.
Virus Replication
Attachment
Proteins on the virus match up with the wall of the host cell
Entry
The virus forms a hole and the viral DNA is injected into the
bacteria
Synthesis
The host DNA is switched off and the viral DNA takes over.
It makes virus parts.
Assembly
The virus parts are put together and new viruses are made.
Release
The host cell bursts and releases thousands of the viruses.
These move to other cells.
The chart below shows the replication of a bacteriophage. This
virus infects bacteria.
It takes about 30 minutes to complete its replication cycle.
Benefits of Viruses
(need to know two harmful examples and one beneficial example of viruses)
1. Genetic Engineering
Viruses can be used to transfer genes from
one organism to another in genetic
engineering. Such viruses are called vectors
e.g. human insulin gene transferred into E. coli
2. Control of infections
Bacteriophages may be used to control
bacterial infections
Disadvantages of Viruses
1. Human Diseases
Viruses cause many diseases in
Humans e.g. colds, influenza,
measles and cold sores
2. Plant Diseases
Viruses cause many plant diseases
including potato mosaic disease
Viruses can be beneficial…
Bacteriophages – attack & destroy bacteria
Baculovirus – ebola-like virus that attacks insects
– Could use for pest control in crops
• Cabbage loopers eat
cabbage crops
• Virus can kill pests in days
– (it’s really gross)
… and then there are those that are not so
good….
Control & Immunity
• Viruses are controlled by body’s general
defence system e.g. skin
• Immunity to many viral infections can be
produced -by vaccinations or by injecting antibiodies into the body.
• Antibiotics kill bacteria and fungi but have no
effect on viruses.
Interferon
Interferon is a range of substances produced by
virus infected cells to protect healthy cells.
Interferon can be made artificially now and can
be used to treat colds etc.
Viruses Enter Living Cells
Viruses enter bacterial cells by punching a hole
in the cells wall and injecting its DNA
Mutating viruses
Viruses can mutate when they copy the genetic
material
Copy something wrong
Mistake proves useful
More powerful virus (more infectious)
Viruses don’t mutate often, except…
Influenza & HIV
Retroviruses
Retroviruses contain RNA instead of DNA
They have an enzyme that converts the Virus
RNA to DNA
HIV is a Retrovirus
Plant Diseases Caused by Viruses
Potato Mosaic Virus
HIV Virus
AIDS: Acquired Immune Deficiency Syndrome is caused by
the HIV virus
Effects: The virus damages the lymphocyte white blood cells
making them unable to make antibnodies to fight against other
diseases e.g. colds.
A person suffering from AIDS is unable to fight infections and
may die from diseases e.g. cancer or pneumonia.
Spread: Spread through body fluids e.g. blood
Unprotected sex with an infected person or drug users sharing
needles
Treatment: Drugs are given to slow down the progression of
AIDS but there is no cure.
HIV Virus
How Is HIV Spread?
•
•
•
•
•
Sexual contact
Sharing contaminated needles
Blood transfusions
Breast feeding (mother to baby)
Mother to baby during pregnancy or birth
FACTS
• In the US, there is better than a 1/1000 chance of contracting
HIV during unprotected sex
• A person can be contagious for more than 10 years before any
sign of the disease is apparent
• HIV becomes AIDS when the number of immune cells drops
below a predetermined number
• No one dies from HIV or AIDS; people die from secondary
infections (ranging from the common cold to cancer)
• More than 3 million people (size of Chicago) die each year
• There are approx. 14,000 new cases of HIV
worldwide every day
2012 Q8 (Higher Level)
Solution to 2012 Q8
2012 Section C Q14C
2012 Section C Q14C Solutions
2011 Paper Section C Question 15 Part c
2011 Paper Section C Question 15 Part c
Solutions