1. SBI 3U Ms.Zafar October 1st, 2012
Viruses
SBI 3U
Ms.Zafar
October 1st, 2012

2. What are Viruses? Virus  Latin for poison
Viruses do not fit within the 5-kingdom system
Often thought of as closely related to prokaryotes
A virus is a non-cellular particle made up of protein-covered genetic material that can invade living cells

3. What Makes Viruses Unique?
Lifeless chemical if outside living cell
Reproduces when it is inside living cell
Occupy a position between nonliving and living matter

4. Size & Shape
Viruses could not be identified until 1934  because of their small shape
5000 influenza viruses can be fit onto the head of a pin!
Shapes and sizes vary

5. Components of a Virus
Much less complex than the simplest living organism
Inner nucleic acid core
Outer protective protein coat called a capsid  makes up 95% of the total virus  gives shape
Genetic material can be either RNA or DNA

6. Viral Diversity Not every virus is disease-causing  some are benign
Viruses are selective
Specific viruses enter only specific host cells
Most bacteriophages have a very restricted host range
Bacteriophage: Infect bacteria
Host Range: The limited # of host species that a virus can infect
Ex: AIDS virus has a very limited host range

7. Viral Replication The cell and the computer analogy
Virus invades a living cell and then reproduces
Viruses can reproduce in 2 ways: Lytic Cycle or Lysogenic Cycle

8. Whole process can take between 25-45 minutes
Lytic Cycle
Attachment and entrance. Either whole virus, or only DNA or RNA
Synthesis of protein and nucleic acid
Assembly of the nucleic acids, enzymes and proteins into new virus particles
Release of new virus particles
Whole process can take between minutes

9. Lytic Cycle Lysis: The destruction or bursting open of a cell
After lysis, the new viruses infect other cells
A 100 or so clones can be released
Bacteriophage that causes lysis of the host cell is said to be a virulent phage

10. Restriction Enzymes
Restriction Enzymes are those within bacteria that recognize foreign DNA and cut up the foreign DNA

11. Lysogenic Cycle
Bacteriophage that does not cause lysis of the host is called a temperate phage
This kind of virus can coexist with the host cell
In a lysogenic cycle, the virus does not kill the host cell outright

12. Lysogenic Cycle Virus injects nucleic acid into host
Nucleic acid becomes integrated into host DNA
Replicated with the host DNA  virus in dormant stage  lysogeny

13. RNA Viruses RNA Viruses are those that contain RNA
Act as messenger RNA once inside host cell or can provide instructions to manufacture its own messenger RNA
All RNA viruses are lytic – some exceptions

14. Retroviruses
Retroviruses need to be reverse transcribed into DNA before they can be copied the right way
Example: human immunodeficiency virus (HIV) that causes acquired immunodeficiency syndrome (AIDS)
Lysogenic cycle
Virus enters a human white blood cell and directs host cell to make DNA from viral RNA