Lec 14: Bacterial Viruses Flashcards
Phage diversity is reflected in
both morphological and genetic characteristics.
Phage Genome:
DNA or RNA,
single- or double-stranded,
circular or linear,
and is generally present as a single copy.
Phage Morphology:
from simple, icosahedral and filamentous phages to more complex tailed phages with an icosahedral head.
The majority of phages are
Tailed
The host controlled modification and restriction systems of bacteria are against…
However, certain phages can…
…invading double-stranded phage DNA, while self-DNA is protected by the modification system (ex: methylation)
…avoid degradation of their DNA by the restriction systems. (Occasionally, some phage DNAs will undergo modification to avoid the degradation, and their progeny will also undergo the modification.)
Bac have –>
Restriction enzymes: cut DNA
Bac synth DNA and methylate –>
bac DNA protected against restriction enzymes
Non-methylated DNA –>
gets cut by restriction enzymes
Broadly, phages can be classified as:
Virulent OR Temperate.
Virulent phages typically
culminate in cell lysis (for obligately lytic phages) and release of progeny virions.
Temperate phages have
alternative replication cycles: a productive, lytic infection or a reductive infection, in which the phage remains latent in the host, establishing lysogeny.
Reductive infections generally occur when
environmental conditions are poor, allowing survival as a prophage in the host (which is referred to as a lysogen
Lysogen
a bacterial cell in which a phage exists as a prophage).
Prophage is either
integrated into bacterial chromosomes or exists as an extrachromosomal plasmid
During Lysogeny:
the phage genome is repressed for lytic functions and often integrates into the bacterial chromosome, as is the case for phage lambda (λ), but it can exist extrachromosomally (e.g. phage P1).
The prophage replicates along with the host and…
This occurs under conditions that…
…remains dormant until induction of the lytic cycle.
…result in damage to the host DNA. The phage repressor is inactivated and the lytic process starts.
Why re-start the lytic process?
To allow propagation of phages when host survival is compromised.
A resident prophage can
protect the host from superinfection by the same or similar strains of phages by repressing the incoming phage genome (a phenomenon known as Superinfection Immunity)
Superinfection Immunity =
repressing the incoming phage genome
Some temperate phages contribute
‘lysogenic conversion genes’ (some phage carry genes that encode toxins.)
- For example shiga toxin or cholera toxin genes, when the phages establish lysogeny, they convert the host to virulence.
Bacteriophages have had key roles in: (2)
- developments in molecular biology
- biotechnology
the first to be sequenced =
Phage genomes
What were discovered in phage studies?
What was its meaning?
Restriction enzymes
meaning = foundation for gene cloning
Certain phages are developed into
vectors for use in molecular biology and biotechnology
Depending on __ __ __ __ ____ ___ and ___ _____ __ ___ ________, many different types of cloning vectors can be used for…
the size of the insert DNA
the purpose of the experiment
…the generation of recombinant molecules.
Phages are utilized in
the typing of bacteria.
Phages have potential in…
due largely to…
…therapeutics
…the rapid emergence of antibiotic-resistant bacteria and of new infectious diseases.
