Lecture 8 Flashcards
What is the viral reproductive cycle
When a virus infects a host cell, expression of viral genes leads to steps in this cycle, which leads to the production of more viruses
How can viral reproductive cycle be different among types of viruses
- A virus may have alternative cycles
- Generally consists of 5-6 common steps
- Compare bacteriophage I and HIV cycles
What are the 6 steps of the viral reproductive cycle
- Attachment
- Entry
- Integration (sometimes does not happen)
- Synthesis of viral components
- Viral assembly
- Release
Explain step 1: attachment
Virus attaches to the surface of a host cell
Usually specific for one kind of cell due to binding to specific molecules on cell surface
Explain step 2: Entry
Viral genome enters the cell
Attachment of phage I stimulates a conformational change in its coat proteins; shaft contracts, and phage injects its DNA into the bacterial cytoplasm
One or several viral genes are expressed immediately
Virus may proceed to synthesis of viral components OR integrate into host chromosome
Explain (optional) step 3: Integration
Viral gene for integrase
Integrase cuts host chromosomal DNA and inserts viral genome
What is prophage in step 3
Phage in bacterial DNA
When the bacteria cell divides the prophage DNA is copied and transmitted to daughter cells along with bacterial chromosomal DNA
What is the lysogenic cycle
A phase of viral reproductive cycle when prophage is integrated into chromosome
No new phages made; host cell is not destroyed
Prophage can be excised from the bacterial chromosome to end the lysogenic cycle and proceed to the lytic cycle
What is the process of retroviruses
Uses viral reverse transcriptase to make complementary DNA strand that will be template for double stranded viral DNA
Double-stranded DNA enters the host cell nucleus and is inserted into a host chromosome via integrase
Once integrated, the viral DNA is a provirus
Explain step 4: synthesis of viral components
Replication of the viral genome and the synthesis of viral proteins
Prophage must be excised using excisionase before synthesis of new viral components can occur
Host cell enzymes make many copies of the phage DNA and transcribe the genes within these copies into mRNA; host cell ribosomes translate mRNA
Explain step 5: viral assembly
Some viruses self-assemble - spontaneously bind to each other to form a complete virus particle
Others do not self-assemble
Assembly of phage I requires the help of noncapsid proteins; some modify capsid proteins or serve as scaffolding
Assembly of HIV occurs in two stages:
1. Capsid proteins assemble around 2 molecules of viral RNA and molecules of reverse transcriptase and integrase
- Next, the newly formed capsid acquires its outer envelope in a budding process
Explain step 6: release
Phages must lyse their host cell to escape
After phage assembly a phage-encoded enzyme called lysozyme digests the bacterial cell wall and causes the cell to burst
Newly assembled virus particle associates with a portion of the plasma membrane containing HIV spike glycoproteins
Membrane enfolds the viral capsid and eventually buds from the surface of the cell
What is the latency in bacteriophage
Some viruses can integrate their genomes into a host chromosome
Prophage or provirus is inactive or latent - latency is also called lysogeny
Most viral genes silenced
When host cell replicates, also copies prophage
Can be replicated repeatedly in this way without killing host cell or producing new phage particles - lysogenic cycle
What is temperate phages
Bacteriophages that can follow lysogenic (wait) or lytic (active)
Environmental conditions influence integration and length of latency
Virulent phages only have lytic cycles
What are the two different mechanisms for latency in human viruses
Virus integrates into host genome and may remain dormant for long periods of time
- HIV
Other viruses can exist as episomes - genetic elements that replicate independently but occassionally integrate into host DNA
- Chicken pox/Shingles
