Chapter 6 Flashcards
• Virion (virus particle)
has nucleic acid genome (DNA or RNA, single or double stranded) and capsid
viriods
– RNA genome is itself the infectious particle
• Infect plants
prions
– protein only (no nucleic acid)
• Abnormal protein structure
• Cause of “mad cow” disease
Filamentous viruses
symmetrical virus with helical symmetry
•helical tube around genome
tailed viruses
multipart structure
• Icosahedral capsid has genome attached to a helical “neck” that channels nucleic acid into host
• Example: T4 bacteriophage
asymmetrical viruses
lack capsid symmetry
Viruses may have
lipid envelope
• Allows fusion to host cell membrane
• Occurs if host cell is not covered by cell wall
International Committee on Taxonomy of Viruses (ICTV)
- Genome composition –the Baltimore method
- Capsid symmetry (helical, icosahedral)
- Envelope
- Size
How do we compare virus
genomes? No rRNA
based on orthologs–genes of common ancestry in two genomes that share same function
Proteomic classification is useful for
viruses b/c their small genomes encode few proteins
all viruses require
- Host recognition and attachment
- Genome entry
- Replicate genome
- Make viral proteins
- Assemble capsids
- Exit
Cell-surface receptors
proteins on host that are specific to host species and bind to a specific component
Lytic cycle
virus injects its genome into cell and reproduces as many virus’ as possible
Lytic cycle phage genes
Early genes-
late gene-
- to produce new phage capsids etc
- after capsids are assembled; kills host and releasing new virus’
burst-
burst size-
- lysis is also referred to as burst.
- number of particles released
transduction
During exit from lysogeny the virus can get host genes, passes on to another cell
Bacteriophage Life Cycle
Slow-release cycle
• Phage particles reproduce without killing host cell
• Phage M13
bacterial host defenses
- Altered host receptor proteins (phage can’t bind)
- Restriction endonucleases
- CRISPR
CRISPR
copies piece of phage and remembers it. for next infection, it recognizes and cleaves it.
tropism
infect specific tissue type within a host
broad tropism
infect many kinds of host tissues
rna to rna
RNA-dependent RNA polymerase
(+) ssRNA viruses
RNA-dependent RNA polymerase, generates - rna strand.
(-) ssRNA viruses
used a template to create mRNA and progeny genomes
in the beginning of eclipse period (when it drops)
viruses binds to host
Eclipse period/Latent period
Replicating genomes and assembly, usually undetectable.
Rise period
- Cells begin to lyse and progeny viruses are released
* Burst of phage
