What is a Virus?
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Viruses are strands of DNA or RNA inside a protective coat.
They cannot live on their own, but invade cells in your body and use them as factories to
make more viruses.
This eventually kills the cell.
Diseases caused by the viruses includes colds, flu and AIDS.
Properties of a Virus
Viruses have 5 properties that separate them from living cells:
• 1. They have DNA or RNA - never both.
• 2. Replication is controlled by the viral DNA in the host cell.
• 3. They do not divide by binary fission or mitosis.
• 4. Not capable of producing their own energy because they lack the required genes.
• 5. They depend upon the ribosomes and nutrients of infected cells for protein
production.
Virus Structure
Bacteriophages
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These are viruses that invade bacterial cells.
Infective cycle is the same as previous description.
In some cases the viral and bacterial cell DNA join to form a complex.
The subsequent daughter cells will contain both DNA types.
Bacteriophages Structure
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The Capsid- basically the “brain”. Contains a outer protein coat which is wrapped
around the nucleic acid.
– X-rays show that the capsid has 30 sides
The Body- rod like structure that consists of a retractable sheath
The Tail- 6 slender tail fibres which help anchor the virus to the host
Invasion of Host Cell (Lytic Cycle)
This is a cyclic process.
1. Virus contacts host cell.
2. Virus or DNA enters host cell.
3. Host cell DNA is broken up and new viral DNA or RNA is produced.
4. Viruses use protein from host cell membranes to make new protein coats.
5. Viral enzymes are produced that lyse the host cells.
6. Host cell membrane breaks.
7. New viruses are released.
Lytic Cycle Animation:
http://highered.mcgrawhill.com/sites/0072556781/student_view0/chapter17/animation_quiz_2.htm
Lysongenic Cycle
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Virus does not kill host cell right away
Virus appears to be in a dormant state called lysogeny
Dormant virus may be activated triggering lytic cycle
Examples: cancer-causing viruses, HPV, Herpes Simplex, HIV
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Steps of Lysogenic Cycle
Virus injects it’s own DNA/RNA into host cell
Viral DNA combines with host DNA
• New combined DNA called prophage
Prophage is passed along to daughter cells by dividing naturally
When activated, original viral DNA exits host cell and lytic cycle begins