Chapter 9 (3) Flashcards
Bacterial Viruses (Bacteriophages)
- Bacteriophages are very diverse.
- dsDNA genomes.
- Most are naked, but some have lipid envelopes.
- Structurally complex, containing heads, tails and other components.
Bacterial Viral Life Cycles
Virulent mode: viruses lyse host cells after infection.
Temperate mode: viruses replicate their genomes in tandem with host genome and without killing host.
Virulent bacteriophages and T4
Virulent Bacteriophages
- First viruses studied in detail contained linear, dsDNA genomes that infect gut bacteria.
- –T1, T2 and T7.
T4
T4 has a dsDNA genome that is circularly permuted and terminally redundant.
-both factors affect genome packaging.
T4 modified base
T4 contains modified base 5-hydroxymethylcytosine.
–DNA is resistant to virtually all known restriction enzymes.
T4 Genome
Can be divided into three parts: early, middle and late proteins.
- Early and middle proteins: enzymes needed for DNA replication and transcription.
- Late proteins: head and tail proteins require enzymes to liberate mature phage particles.
Temperate Bacteriophages, lambda and P1
Temperate viruses: can undergo a stable genetic relationship within host.
Lysogeny: state where most virus genes not expressed and virus genome (prophage) replicated with host.
Lysogen: a bacterium containing a prophage.
–Under certain conditions lysogenic viruses revert to lytic pathway.
Corynebacterium Diphtheriae
Exotoxin inhibits protein synthesis, exotoxin gene encoded by lysogenic phage.
Bacteriophage Lambda
Linear, dsDNA genome.
Upon penetration, DNA ends base-pair, DNA ligates and forms double-stranded circle.
When lambda is lysogenic, its DNA integrates into E. coli chromosome at lambda attachment site.
Bacteriophage Lambda: Entering lytic pathway
When it enters lytic pathway, lambda synthesizes long, linear concatamers of DNA by rolling circle replication.
Bacteriophage Lambda: Regulation
Regulation of lytic vs. lysogenic events in lambda controlled by a complex genetic switch.
- Key elements are two repressor proteins
- –cl protein (lambda repressor): causes repression of lambda lytic events.
- –Cro repressor: controls activation of lytic events.
Animal viruses
Entire virion enters animal cell.
Eukaryotic cells contain a nucleus, site of replication for many animal viruses.
Animal viruses contain all known modes of viral genome replication.
Many more enveloped animal viruses than enveloped bacterial viruses.
–As animal viruses leave host cell they can remove part of host’s lipid bilayer.
Consequences of Virus Infection Animal Cells
Persistent infections: release of virions from host cell does not result in cell lysis.
–Infected cell remains alive and continues to produce virus.
Latent infections: delay between infection by the virus and lytic events.
Transformation: conversion of normal cell into tumor cell.
Cell fusion: two or more cells become one cell with many nuclei.
Teratogenesis
Teratogen- agent that induces defects during embryonic development.
Viruses can act as teratogens by crossing the placenta and infecting embryonic cells.
–Earlier the pregnancy, more defects.
Ex.- rubella, cytomegaloviruses, hepaviruses.
Viruses and cancer
Cancer results from a cell losing control of its cell cycle:
-divides uncontrollably
-transformed
-cell can’t perform normal functions because it is busy replicating.
Some animal viruses can transform their host cells.
-transformed cells may divide independently of growth factors.
