Viral Life Cycles & Replication

Key Terms:

Virus

Host cell

Viruses attack specific host cells.

They fit into specific surface receptors like a lock and key.

The Lytic Cycle

The Lytic Cycle is responsible for viral reproduction.

The Lytic Cycle has five steps:

1. Attachment—the bacteriophage attaches to a specific host cell.

The Lytic Cycle

2. Entry—injection of viral DNA or RNA into host cell.

The Lytic Cycle

3. Replication—copying viral proteins and nucleic acids.

The Lytic Cycle

4. Assembly—production of many new bacteriophages.

The Lytic Cycle

5. Lysis and Release—host cells burst and releases bacteriophages.

The Lytic Cycle

Latent Viruses

Examples:

HIV

Herpes

Some viruses have the ability to become dormant inside the cell

They are activated by external signals such as stress or illness.

Herpes

HIV

AIDS symptom: Kaposi's sarcoma

Lysogenic Cycle

Latent viruses “hide out” in the Lysogenic Cycle until they receive a “signal” to attack.

There are three steps in the Lysogenic Cycle:

Lysogenic Cycle

1. Attachment & Entry—phage nucleic acid is injected into the host cell.

Lysogenic Cycle

2. Prophage Formation—viral nucleic acid joins host cell DNA forming a provirus.

Lysogenic Cycle

3. Cell Division—bacteria cells replicate via binary fission producing many provirus cells.

Lytic and Lysogenic Cycles

Lytic & Lysogenic Cycles

Other Infectious Particles

Viroids are small, circular RNA molecules without a protein coat.

Viroids infect plants.

Examples of viroids:Potato Spindle Tuber

Viroid

Other Infectious Particles

Prions are infectious proteins. Normal proteins change into

prions when they come into contact with prion proteins.

Prions form insoluable deposits in the brain that cause neuron degeneration.

Examples of Prions

Mad Cow Disease (BSE)

Kuru—a rare nerve disease in New Guinea that occurred because of a funeral practice of eating the brains of the dead.