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


1.
The Lytic Cycle is
responsible for viral
reproduction.
The Lytic Cycle has
five steps:
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.
HIV
Herpes
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.