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).
