Block D Lecture 2: Viruses of Archaea and Bacteria Flashcards
What does the latent period of a virus life cycle encompass?
The eclipse and maturation periods
(Slide 7)
When does the eclipse period start?
As soon as the virus enters the host cell
(Slide 7)
What does “burst size” refer to?
The number of virions released from an infected host cell
(Slide 7)
What is a bacteriophage (sometimes known as a phage)?
A type of virus which infects (and replicates) within bacteria
(Slide 9)
What genomes do most bacteriophages use?
dsDNA genomes
(Slide 9)
What components do bacteriophages contain?
Heads
Tails
Other components
(Slide 9)
What do viruses which infect archaea resemble?
Those which infect enteric bacteria
(Slide 12)
What type of viruses are the only type which have been identified to infect archaea?
dsDNA viruses
(Slide 12)
What is the oldest known predator-prey interaction?
Bacteria and bacteriophages
(Slide 14)
Why is attachment of a virion to a host cell highly specific?
As it requires complementary receptors on the surface of the host and virus cells
(Slide 15)
What does the attachment of a virus to its host cell result in?
Changes in both the virus and the host cell’s cell surface that facilitate penetration
(Slide 16)
What bacteria does the bacteriophage T4 infect?
E.coli
(Slide 16)
How does the T4 bacteriophage attach to E.coli?
Via its tail fibres which interact with polysaccharides on the E.coli cell envelope
(Slide 16)
What happens after T4 bacteriophage tail fibres attach to polysaccharides on the E.coli cell surface?
The tail fibres retract, and the tail core makes contact with the E.coli cell wall
(Slide 16)
What happens after the T4 bacteriophage tail core makes contact with the E.coli cell wall?
A Lysozyme-like enzyme forms a small pore in the peptidoglycan layer
(Slide 16)
What happens after the bacteriophage T4 uses a lysozyme-like enzyme to form a pore in the peptidoglycan layer?
The tail sheath contracts, and viral DNA passes into the cytoplasm by travelling down the central tail tube
(Slide 16)
What 2 features does the doubled stranded DNA genome of the T4 bacteriophage have?
It is circularly permuted and terminally redundant
(Slide 17)
What does the T4 bacteriophage having a dsDNA genome that is circularly permuted and terminally redundant affect?
Genome packaging
(Slide 17)
What does circularly permuted mean?
Circular permutation involves breaking and re-joining the circular molecule at different positions along its backbone, resulting in a new circular molecule with a different starting point for the sequence
(Slide 17)
What does terminally redundant mean?
That it contains identical or nearly identical sequences at both ends of the genome
(Slide 17)
Many eukaryotes can utilise RNA interference to diminish virial infections. What is RNA interference?
It is a natural cellular process that regulates gene expression by degrading or inhibiting the translation of specific messenger RNA (mRNA) molecules
(Slide 17)
What are 3 examples of mechanisms which prokaryotes can use to combat virial infections?
CRISPR (similar to RNA interference)
Restriction modification system
Restriction enzymes
(Slide 17)
What is the restriction modification system?
A DNA destruction system which is only effective against dsDNA viruses
(Slide 17)
How can bacteria prevent restriction enzymes they use as defence against bacteria from damaging their own DNA?
By modifying their own DNA at restriction enzyme recognition sites
(Slide 17)
What are 3 virial mechanisms that T4 ( and other viruses) can use to evade bacterial restriction systems?
Chemical modification of virial DNA (via glycosylation or methylation)
Production of proteins which inhibit host cell restriction system
T4 specifically contains a modified base, called 5-hydroxymethylcytosine which is resistant to almost all known restriction enzymes
(Slide 18)
What 3 things do T4 bacteriophage early proteins code for?
Enzyme for the synthesis and glucosylation of the T4 base 5-hydroxymethylcytosine
Enzymes that function in T4 replisome
Proteins which modify host RNA polyremase
(Slide 20)
What does a replisome refer to?
The molecular machinery responsible for DNA replication
(Slide 20)
What are T4 middle proteins?
Additional proteins which modify host RNA polymerase
(Slide 20)
What do T4 late proteins code for?
Proteins of the virus coat
Structural components
(Slide 21)
How is the T4 genome packaged?
Precursor of head is assembled
Packaging motor is assembled
dsDNA is pumped into head under pressure using ATP
(Slide 21)
What occurs after the dsDNA of the T4 genome is pumped into the head?
The T4 tail, tail fibres and other components are added
(Slide 21)
What 2 modes can a virus life cycle be?
Virulent and temperate modes
(Slide 22)
What is the virulent mode?
Viruses lyse host cells after infection
(Slide 22)
What is the temperate mode?
Viruses replicate their genomes in tandem with the host genome and do so without killing the host cell
(Slide 22)
What relationship can temperate viruses form with the host cell?
A stable genetic relationship
(Slide 22)
What is lysogeny?
A state where most virus genes are not expressed and the virial genome (known as a prophage) is replicated with the host chromosome
(Slide 22)
What is a lysogen?
A bacterium containing a prophage
(Slide 22)
What may lysogenic viruses do?
They may revert back to the lytic pathway and begin producing virions under certain conditions
(Slide 22)
What kind of genome does the bacteriophage lambda have?
A linear, dsDNA genome
(Slide 23)
How can the bacteriophage lambda genome from a double-stranded circle?
It contains single-stranded regions at the 5’ terminus of each strand and upon penetration of a cell, these DNA ends base-pair, which forms the cos site, and then the DNA ligates and forms a double-stranded circle
(Slide 23)
What can bacteriophage lambda do when it is lysogenic?
Integrate its gene into E.coli chromosome at the lambda attachment site
(Slide 23)
What does bacteriophage lambda do once it enters the lytic pathway?
It synthesises long, linear concatemers of DNA using rolling circle replication
(Slide 25)
What is a concatemer?
A long DNA molecule that consists of multiple copies of the same DNA sequence linked end to end
(Slide 25)
How are lysogenic and lytic events regulated in bacteriophage lambda?
By a complex genetic switch containing 2 repressor proteins called the cl protein and the Cro repressor
(Slide 26)
What does the cl protein do?
It is also known as the lambda repressor and causes repression of lambda lytic events resulting in the activation of lysogeny
(Slide 26)
What does the cro repressor do?
It controls activation of lytic events by repressing lysogenic genes
(Slide 26)
What is key to the balance of cl protein and cro repressor in the cell of the bacteriophage lambda?
The physiology of the bacterial host
(Slide 26)
What is transduction?
The transfer of DNA from one cell to another due to host cell DNA being mispackaged into a phage, which then “infects”another cell with the previous host cell’s DNA
(Slide 27)
What does transduction drive?
Bacterial evolution
(Slide 27)
What are the 2 modes of transduction?
Generalised and Specialised transduction
(Slide 27)
What is the difference between generalised and specialised transduction?
Generalised transduction is when DNA from any portion of the host genome is packed inside the virion
Specialised transduction is when DNA from a specific region of the host chromosome is integrated directly into the virus genome
(Slide 27)
