1. Viruses

2. What is a Virus? Particles of nucleic acid and protein
reproduce by infecting living cells

3. What do all viruses have in common?
They enter living cells and, once inside, use the machinery of the infected cell to produce more viruses.

4. Comparing the size of a virus, a bacterium, and an animal cell Most viruses are so small they can only be seen with the aid of a powerful electron microscope
A virus is smaller than a bacterium
0.25 m
Virus
Animal cell
Bacterium
Animal cell nucleus

5. Viral Structure A typical virus is composed of a core of either DNA or RNA surrounded by a protein coat
Capsid – made of protein
DNA or RNA
sheath
T4 Bacteriophage

6. (a) Tobacco mosaic virus
Viral structure
Virus consists of a protein shell called a Capsid
The capsid may have many shapes
18  250 mm
70–90 nm (diameter)
80–200 nm (diameter)
80  225 nm
20 nm
50 nm
(a) Tobacco mosaic virus
(b) Adenoviruses
(c) Influenza viruses
(d) Bacteriophage T4
RNA
Capsomere of capsid
DNA
Capsomere
Glycoprotein
Membranous envelope
Capsid
Head
Tail fiber
Tail sheath
Respiratory infections

7. How does a virus get inside a cell
The capsid proteins bind to receptors on the host cell
It “tricks” the cell into allowing it inside.

8. Once inside the viral genes are expressed.
The host cell makes more viruses and the host cell is destroyed.

9. Viruses are very specific to the cells they infect
a particular virus can only infect a specific cell.
Bacteriophage – infects bacteria

10. Reproductive Cycles of Phages
Two types
Lytic cycle or Virulent
Host cell is lysed (bursts) and immediately destroyed
Lysogenic cycle or Temperate
The virus embeds its DNA into the host’s DNA
The virus’s DNA is replicated along with the host cell’s DNA
A host cell makes copies of the virus indefinitely.

11. The lytic cycle of phage T4, a virulent phage
Bacteriophage attaches to
The bacterium’s cell wall
Bacteriophage injects its
DNA into the cell
The cell makes mRNA from the
Bacteriophage’s genes
New viruses are assembled. The
Virus wrecks the cell causing it to
burst
The bursting of the cell
Releases new bacteriophage
particles 1 2 4 3 5
Phage assembly
Head
Tails
Tail fibers

12. Lysogenic Infection The host cell is not immediately destroyed
Viral DNA inserted into the host cell as a prophage

13. A comparison of the lytic and lysogenic cycles of a bacteriophage
Many cell divisions produce a large population of bacteria infected with the prophage.
The bacterium reproduces
normally, copying the prophage
and transmitting it to daughter cells.
The viral DNA is embedded in the
Host’s DNA
New phage DNA and proteins are synthesized and assembled into phages.
Occasionally, a prophage exits the bacterial chromosome,
initiating a lytic cycle.
Certain factors
determine whether
The phage attaches to a
host cell and injects its DNA.
Phage DNA
circularizes
The cell lyses, releasing phages.
Lytic cycle
is induced
Lysogenic cycle or
Prophage
Bacterial
chromosome
Phage
DNA

14. Retroviruses Contain RNA as their genetic information
They produce a DNA copy of their RNA
RNA produces DNA
(this is the reverse of what normally happens in healthy cells)
This DNA is inserted into the DNA of the host cell
Example….AIDS

15. Is a Virus a living thing?
Some biologists do not consider viruses to be alive because they can’t reproduce independently.
Viruses do have some characteristics of a living organism once it infects a host cell, so they are at the borderline of living and non-living things.

16. Viruses and Cells Virus Cell DNA or RNA, core, capsid
Structure
DNA or RNA, core, capsid
Reproduction
only within a host cell
Genetic Code – DNA or RNA
Growth and Development – NO
Obtain and Use Energy – NO
Response to the Environment NO
Change Over Time - YES
Structure
cell membrane, cytoplams;eukaryotes also contain nucleus and organelles
Reproduction
Independent cell division either asexually or sexually
Genetic Code – DNA
Growth and Development – YES
Obtain and Use Energy - YES
Response to the Environment - YES
Change Over Time - YES

17. Viral Diseases rubella AIDS HPV influenza Hand, foot and mouth mumps
Ebola
Cold sore
Chicken pox
rubella
AIDS
Small pox

18. Viral Disease Diseases in Humans
Polio, measles, AIDS, mumps, influenza, yellow fever, smallpox, the common cold, and many others

19. Viral Diseases in Humans Disrupt the bodies normal equilibrium
Symptoms caused by
Toxins produced by infected cells
Toxic components of the viruses themselves
Cells killed or damaged by the virus
Body defense mechanisms fighting the infection

20. Viral Damage
Depends on the ability of infected tissue to repair itself
Epithelium of respiratory tract heals fast so we get over colds
Polio –damage to nerve cells is permanent

21. Cancer causing viruses
Oncogenic viruses
Examples:
Human papilloma virus
Human herpes virus
Hepatitis B

22. Viral Diseases in Animals
Rabies
Parvo
Hoof and mouth disease

23. Viral Diseases in Plants
Have a difficult time invading plants due to the cell wall
Plant injuries make a plant susceptible to viral disease
Insects act as carriers

24. The simplest infectious agents
Viroids – small infectious molecules of single stranded RNA that have no surrounding capsids.
Affect plant cells
Cause stunted plant growth

25. Prions – protein infectious particles
Linked to several degenerative brain diseases
Ex. Mad cow, Creutzfeldt-Jacob
Transferred in food
Prions are nearly indestructible

26. Ways to protect against viruses
Vaccines
Variants or derivatives of pathogens
Stimulate the immune system to react against the actual disease
Few drugs treat or cure viral diseases
Antiviral drugs that are effective interfere with viral DNA or RNA synthesis
Example polio, smallpox, rubella, mumps, Hepatitis B

27. Most viral infections cannot be cured once they occur
Antibiotics are powerless against viruses
Viruses do not contain the enzymes which antibiotics work against.
How the body attacks a virus