d1: bacteriophages + animal viruses

what is a bacteriophage?

viruses that infect bacteria

where are bacteriophages found?

found anywhere that bacteria live

• present in drinking water and many of our foods

• can number up to 1×10⁹ virions per mL in seawater

do bacteriophages ever cause us harm? could they be used to help us?

bacteriophages may alter the genes of the bacteria they infect → causing new strains to arise

bacteriophages may be useful as antibiotics

list the 5 steps of a lytic (virulent) infection

1. absorption (attachment)

2. penetration (entry)

3. synthesis

4. assembly

5. release

what happens in absorption (attachment)?

virus finds a protein on the surface of the cell it’s going to infect

attaches in a very specific way and then becomes irreversibly anchored to the outside of the host cell

what happens in penetration (entry)?

viral genome enters the cytoplasm of the cell

• contract their tail and deliver their DNA into the cytoplasm by physically stabbing through the cell wall and membrane

• soften the cell by predigesting it with enzymes

what happens in synthesis?

tricks host enzymes into transcribing and translating viral genes

you end up making viral proteins

what is one of the first things viral proteins will do after being synthesized?

shut down and shut off all host protein creation

what happens in assembly?

take the viral genomes, viral capsid components. and begin assembling

end up with complete virions in the cytoplasm of the cell

what happens in release?

break the cell open

produce enzymes:

• lysozyme = chews away at the peptidoglycan

• holin = pokes holes in the membrane

what stage is particularly important for determining host range?

ABSORPTION (ATTACHMENT)

• without the right receptor = infection isn’t possible

• determines host range → what things can be infected

list the steps for a lysogenic (temperate) infection

1. absorption

2. penetration

3. integration of prophage

4. growth

5. viral induction

6. synthesis

7. assembly

8. release

what happens during integration of prophage?

viral DNA integrates itself into the chromosome

prohage = inserted virus

what happens during growth?

lysogenic phase will remain integrated for a long time

prophage expresses viral repressors

cell divides and replicates

every time the chromosome gets replicated, the prophage gets replicated also

what do the viral repressors do?

turns off the lytic genes

cell becomes immune to superinfection

how is the cell immune to superinfection?

if a cousin virus tries to infect the cell → viral repressors will stop it from happening

what happens during viral induction?

stress: starve, cell suffers DNA damage

prophage gets a signal to loop its way back out of the chromosome

turn on its lytic pathway

how does the viral infection of animal cells differ from infection of bacteria? what cell structure accounts for most of this difference?

we have no cell walls → viral infection

penetration and release are different

how is penetration different for animal viruses?

if a virus sticks to a receptor → cell with just bring it in without any further work

how is release different for animal viruses?

2 methods:

lysis = cell breaks open, releasing all the virions at once

budding = virions bud off the infected cell

how are animal virus infections the same as bacterial infections?

same rules for anatomy:

• genome

• capsid

• sometimes an envelope

same basic life cycle:

• absorption

• penetration

  • temperate viruses: integration into genome (provirus)

• synthesis

• assembly

• release