Virus Specificity

Life Cycles of a Virus

Virus Specificity:

- Plant viruses can only infect plants

- Bacteriophages can only infect bacteria

- Most viruses only infect one host (Ex – Measles only infect humans)

- Some viruses can infect more than 1 host (Ex – Rabies can infect mammals and birds

Life Cycles of a Virus :

- In order for a virus to reproduce it must invade, or infect a living host cell.

- There are two ways in which a virus can infect a cell: 1. Lytic Infection: The process by which a host cell is invaded, lysed and destroyed by a virus

Lytic Life Cycle (Ex- Bacteriophage)

1. Adsorption Virus comes in contact with host cell by chance. Macromolecules on tail fibers of T4 phage attach to receptors on cell wall of the bacterium.

2. Penetration  A phage injects its DNA into the host’s cytoplasm

3. Replication The virus takes over and uses material of the host cell to make copies of its own protein coat and DNA 4. Assembly  Assembly of phage parts around the new copies of DNA.

5. Lysis  The infected cell lyses (bursts) and releases hundreds of virus particles which may now go and infect other cell

2. Lysogenic Infection Process in which viral DNA is inserted into the DNA of a host cell by where it can remain for many generations before becoming active

Lysogenic Life Cycle (Ex –lambda, which infects E.coli)

1. Adsorption – By chance virus come in contact with host. Lysogenic phage uses molecules on tail fibers attach to receptors located on cell wall of bacterium

2. Penetration  Bacteriophage injects viral DNA into the cytoplasm of the host cell.

3. Integration  Bacteriophage’s genetic material is incorporated into bacterium’s DNA

**bacteriophage DNA may replicate within bacterium for several generations -Once viral DNA has been inserted into host cell’s DNA the viral DNA is referred to as a Prophage.

4. Removal Conditions change (sudden change in temp and availability of nutrients cause the DNA of prophage to become active. DNA removes itself from DNA of host cell.

5. Synthesis  Viral DNA directs the synthesis of new viral particles (DNA and protein parts)

6. Assembly  Assembly of phage parts around the new copies of DNA.

7. Lyses  Bacteriophage enzyme lyses the bacterium’s cell wall and the new viruses are released.