[W2] Bacteriophages Flashcards
What are bacteriophages?
Viruses that infect prokaryotes (bacteria and archaea), typically with high species/strain specificity.
Why are bacteriophages significant?
1) Models for animal viruses, 2) Facilitate horizontal gene transfer (transduction), 3) Used in medical applications like phage typing, therapy, and display.
What are the four main stages of the phage replication cycle?
1) Adsorption & Penetration,
2) Transcription & Translation,
3) Replication,
4) Assembly & Release.
How do bacteriophages adsorb to bacterial cells?
Tail fibres bind to specific receptors on the host cell wall such as LamB, LPS, pili, O/V antigens, flagellar proteins, or porins.
What determines phage host specificity?
The interaction between phage tail fibres and specific bacterial receptors.
How can bacteria modify receptors to resist phage infection?
By altering cell surface receptors to prevent adsorption or by producing proteins that mask them (e.g. protein A in Staphylococcus aureus).
How does EPS affect phage adsorption?
EPS can hide receptors, but phages may produce enzymes like polysaccharide lyase to degrade EPS.
What is the role of O and K antigens in phage infection?
Phages have evolved to specifically recognize these polysaccharides as receptors.
How do phages with sheaths (e.g. T4) penetrate bacterial cells?
Binding triggers sheath contraction, pushing the tail through the envelope to inject DNA.
What about phages without contractile sheaths?
They use alternative mechanisms to penetrate the bacterial envelope.
How many genes does the T7 genome encode?
Approximately 56 genes.
What is the function of T7 Class I genes?
Set the cell up for infection (e.g., gene 0.3 inactivates host restriction systems).
What do Class II and Class III T7 genes do?
Class II: genome replication; Class III: structural components and lytic enzymes.
What is unique about phage T7’s gene expression?
Temporal regulation through distinct classes (I, II, III) with specific timing and function.
What is the replication problem for linear DNA?
Difficulty in replicating the end sequences.
What are three strategies to avoid end replication issues?
1) Circular DNA with rolling circle replication,
2) Terminal redundancy in linear DNA,
3) Lysogeny.
What is rolling circle replication used for?
Rapid production of large amounts of a single DNA strand in circular DNA.
How does terminal redundancy help in DNA replication?
Overlapping end sequences in linear genomes (e.g. T4) allow genome concatenation and complete replication.
How is phage assembly carried out?
Through intermediate sub-structures like the pro-head; may involve scaffold proteins.
What happens during phage release?
Holins disrupt membranes and lysozymes degrade cell walls, releasing progeny.
What error can occur during DNA packaging?
Bacterial DNA may be packaged, resulting in a transducing phage.
What do lytic enzymes do?
Destroy bacterial cell walls, allowing phage progeny to escape (e.g. lysins punching holes).
What is a lytic bacteriophage?
A phage that replicates only via the lytic cycle.
What is a temperate phage?
A phage that can either integrate into the host genome or enter the lytic cycle.
