1. Structure, Function, and Reproduction
VIRUSES
Structure, Function, and Reproduction

2. WHAT IS A VIRUS?
A non-cellular particle made up of genetic material and a protein coat that can invade living cells.
Viral structure is very simple.
Nucleic acid (DNA or RNA)
Surrounded by a protein coat (capsid)
Viruses are much smaller than even the smallest bacteria
micrometers

3. VIRUS SHAPES Complex Cylindrical Polyhedral Tobacco Mosaic
T4 Bacteriophage
Influenza Virus
RNA
Capsid
proteins
Head
Tail sheath
DNA
Tail fiber
RNA
Capsid
Surface
proteins
Membrane
envelope

4. VIRAL REPRODUCTION Viruses cannot reproduce on their own
They must invade, or infect, a living host cell
There are two different reproductive cycles:
Lytic cycle
Lysogenic cycle

5. LYTIC CYCLE Attachment to the host cell Penetration
Injects DNA/RNA into host cell
Take over of the host cell activity
Construction of new virus parts (What are the new parts?)
Assembly of new viruses
Release of new viruses
Viruses break out of (lyse) the host cell
Host cell dies
Ie. bacteriophage

6. THE LYTIC CYCLE Lytic Cycle Bacteriophage protein coat
Bacteriophage DNA
Bacterial
chromosome
Bacteriophage attaches to bacterium’s cell wall
Bacteriophage enzyme lyses the
bacterium’s cell wall, releasing
new bacteriophage particles that
can attack other cells.
Lytic Cycle
Bacteriophage injects DNA into bacterium
Bacteriophage proteins and nucleic acids assemble into complete bacteriophage particles
Bacteriophage
Bacteriophage DNA
Bacteriophage protein
Bacteriophage takes over bacterium’s metabolism, causing synthesis of new bacteriophage proteins and nucleic acids

7. LYSOGENIC CYCLE
The virus does not reproduce and lyse its host cell… at least not right away.
Attachment to the host cell
Penetration
Viral DNA is inserted into the host cell’s DNA
Once inserted into the host cell’s DNA, the viral DNA is known as a prophage
Every time the host cell divides, it makes more viral nucleic acid
Does not harm the host cell
May live in the host cell for many generations
Eventually, the DNA of the prophage will become active
Prophage removes itself from host DNA
Enters into the lytic cycle

8. THE LYSOGENIC CYCLE Lytic Cycle Lysogenic Cycle Bacterial chromosome
Bacteriophage DNA
Bacteriophage injects DNA into bacterium
Bacteriophage DNA (prophage) can exit the bacterial chromosome
Lytic Cycle
Lysogenic Cycle
Bacteriophage enzyme lyses the bacterium’s cell wall, releasing new bacteriophage particles that can attack other cells
Bacteriophage DNA forms a circle
Bacteriophage DNA (prophage) may replicate with bacterium for many generations
Prophage
Bacteriophage proteins and nucleic acids assemble into complete bacteriophage particles
Bacteriophage DNA inserts itself into bacterial chromosome

9. Beneficial and Harmful Aspects
VIRUSES
Beneficial and Harmful Aspects

10. BENEFICIAL ASPECTS
The presence of a prophage can sometimes be beneficial
Can block the entry of other viruses into the cell
Can add useful DNA to the host cells DNA
Viruses are extremely valuable in genetic research
Gene therapy
Introduce corrected genes into a victim of a genetic defect (ie. Cystic fibrosis)
It’s possible that modified viruses may one day be used as medical tools
Genetic engineering
put gene that codes for big, red tomato plant in inactive virus and inject virus into plant

11. BENEFICIAL ASPECTS continued
Viruses can be used to make vaccines
Substances that contain the weakened or inactive disease-causing virus
When injected into the body, vaccines provide immunity to the disease
They can only provide protection if they are used before an infection begins

12. HARMFUL ASPECTS Viruses are parasites Causes disease
Depend entirely on the host cell for reproduction and energy supply
They harm the host cell
Causes disease
Only a small number of viruses are capable of causing diseases in humans
However, these pathogens (disease producing agents) are responsible for a large amount of human suffering
Can infect:
Plants (causing destruction of crops)
Animals (including humans; influenza viruses mutate rapidly and can spread from animals to people)
Bacteria (even the very smallest of single-celled organisms are susceptible to viral infection)

13. COMMON DISEASES CAUSED BY VIRUSES
Type of Virus
Nucleic Acid
Disease
Oncogenic viruses
Retroviruses
Adenoviruses
Herpesviruses
Poxviruses
DNA
RNA
cancer
cancer, AIDS
respiratory infections
chickenpox
smallpox

14. HOW ARE VIRUSES TRANSMITTED?
Air
Chicken pox, measles, flu
Contaminated food or water
Hepatitis
Mosquitoes
Yellow fever, West Nile
Sexual contact
Herpes, AIDS

15. VIRAL SPECIFICITY Viruses have very specific host requirements
This is seen at the attachment step during viral reproduction
How does a virus recognize and attach to a host cell?
Viral capsid or envelope fits into a receptor on the surface of the host cell like a lock and key
If the lock and key don’t fit, then the virus cannot enter the cell

16. ARE VIRUSES ALIVE? NO!!! They do not have cellular characteristics
They are not made of cells
They cannot reproduce on their own
When did viruses evolve?
Probably evolved from living cells since they cannot “survive” without a living cell