Virus Reproduction Overview Cycles for reproduction Retrovirus Lytic Lysogenic Retrovirus HIV Prevention

Two Cycles of Reproduction Viruses must be inside a living cell (host) to reproduce. Viruses attack host cells by attaching to their cell membrane and injecting their nucleic acid into the cell.

Two Cycles of Reproduction Viral reproduction is very complex in vertebrates such as humans. Much of what we understand about viral life-cycles we have learned by studying bacteriophages. Bacteriophages follow one of two general paths: Lytic cycle - Lysogenic cycle

Two Cycles for Reproduction Lytic cycle- Characterized by viral infection, replication, and cell lysing (bursting) In humans it causes active infections that occur within 1-4 days like a cold or influenza (the flu) Lysogenic cycle- The viral genome replicates without destroying the host cell Viral genome becomes permanent part of the host cell’s chromosome In humans it can cause infections that can remain inactive for months or years like herpes, shingles or AIDS

Cycles for Reproduction: The Lytic Cycle Viral DNA/RNA injected into the cell Viral DNA/RNA replicates and is expressed (protein synthesis) to produce new viruses inside the host cell Soon the cell is full of new viruses that burst from the cell, often destroying it and spreading the disease

It’s not just bacteriophages that do this…

Review: The Lytic Cycle (you don’t need to write this down) Virus attaches to host Virus enters cell (nucleic acid is injected) Replicates IMMEDIATELY Produces many new viruses Viruses burst from the cell to invade other cells (This is what harms/kills the cell)

Cycles for Reproduction: The Lysogenic Cycle Virus inserts its DNA into its host DNA and it “hides” there Viral DNA is now called a provirus/prophage The DNA divides/replicated with the “help” of the host cell The host cell is not harmed Later it will begin the lytic cycle to spread the virus

Cycles for Reproduction: The Lysogenic Cycle Virus invades cell Does not immediately kill host cell Becomes part of host’s DNA Host cell reproduces as usual (making more viral DNA without knowing it) Eventually goes into the lytic cycle to spread the virus

Replication of HIV HIV can result in the disease AIDS Can hide for years in the nucleus of T-cells (white blood cells that help fight infection) in the Lysogenic cycle People can have no symptoms, yet be spreading the disease to many partners.

HIV replication continued HIV then begins the Lytic cycle and T-cells begin to die. The patient eventually dies from another disease (opportunistic infection) There are not enough T-cells to help fight infection. This can lead to AIDS (when T cell count is below 200 cells/ml).

Prevention Antibiotics won’t cure viral diseases! (The Doctor can’t cure your cold! Only your symptoms are treated.) Some viruses can be prevented by vaccines which help your own immune system fight the disease. Edward Jenner developed the first vaccine from cowpox.

Prevention 4. Vaccines for smallpox, a deadly virus, helped wipe it out. 5. Other diseases that can be prevented by vaccines are: Polio Measles Mumps Influenza