Lecture 12: Taxonomy of Viruses Flashcards
How are viruses classified?
Based on characteristics
Ie Nature of host, type of disease caused, life cycle, type of nucleic acid and strandedness
What is the baltimore classification scheme?
Classification based on type of genome
What are class I viruses?
dsDNA (lambda, T4, herpes, pox)
What are class II viruses?
ssDNA (phiX174)
What are class III viruses?
dsRNA
What are class IV viruses?
ssRNA (+)
What are class V viruses?
ssRNA (-)
What are class VI viruses?
ssRNA (Replicates with DNA intermediate)
What are class VII viruses?
dsDNA (replicated with RNA intermediate)
What does + configuration mean? (RNA)
Same strand as mRNA, translated directly
What does - configuration mean? (RNA)
Complementary to mRNA, needs to be transcribed into + strand before translation
What are the 5 steps of the viral life cycle?
- Attachment
- Penetration
- Replication of nucleic acid, transcription and protein synthesis
- Maturation
- Release
What occurs in the penetration stage of the viral life cycle?
Virus genome enters cell
Naked viruses: complete virion may enter cell
Enveloped viruses: may leave envelope @ cell surface, only nucleocapsid enters cell, or whole virus enters cell
What occurs in the maturation stage of the viral life cycle?
Assembly of virus components.
What is the latent period in the viral life cycle?
Replicating, making new proteins (eclipse)
Maturation-time needed for component assembly
What is the rise period of the viral life cycle?
Virions detected outside of cell, lysis
can also bud out if virus is enveloped
What is viropexis?
process whereby certain viruses become attached to a cell wall and are incorporated into the cell by endocytosis
What is burst size?
of virions released through lysis
What are the 2 types of bacteriophages?
Virulent and temperate
How does T4 infect bacteria?
- T4 attaches to core region of LPS by tail fibres
- Tail sheath contracts, forces central core through outer membrane
- Tail lysozymes digest peptidoglycan
- DNA enters cytoplasm
How does T4 replicate?
Nucleases and DNA pol and new sigma factors=new phage DNA
Self assembly=mature phage
What kind of phage is lambda?
Temperate phage
What kind of phage is lambda?
Temperate phage, phage carried on chromosome. Contains linear dsDNA with cohesive ends.
What is a prophage?
Phage genome within host cell membrane
What is a lysogen?
Bacterium that contains prophage
What are cohesive ends?
Single stranded dsDNA, allows it to form into circular DNA (complementary bps)
Where is the lamdba genome integrated on the bacterial chromosome?
attlambda site. Lambda integrase catalyzes integration of phage genome at this site.
What kind of phage is phix174?
+ strand phage , ssDNA
How does phix174 replicate?
- replicative form (- strand formed to make dsDNA) made, replication via rolling cycle
- replicative form of DNA nicked by Gene A protein
- New + strand begins synthesis
What kind of phage is MS2?
ssRNA, + strand
How does MS2 infect bacteria?
Genome is first used as mRNA, directs synthesis of RNA replicase (RNA dependant RNA polymerase:host doesn’t have)
RNA replicase synthesizes - strand RNA->used to provide additional mRNA and genome
Where are the genome of DNA viruses replicated?
Nucleus
Where are the genome of RNA viruses replicated?
Cytoplasm
What kind of viruses are Herpes and HPV?
DNA viruses
How do herpes and HPV infect cells?
Viropexis, nucleocapsid transported to nucleus=viral DNA uncoated
Assembly occurs in nucleus, envelope added via budding through inner membrane of nucleus
What kind of viruses are polio and hep A?
+ strand ssRNA
What kind of viruses are measles, rabies and influenza?
- strand ssRNA
What kind of viruses use dsRNA?
Rotavirous (Reovirus)
What is a retrovirus?
Virion carries 2 identical copies of genome, reverse transcriptase, integrase, and proteases
DNA Genome travels to nucleus and is integrated into host DNA
What are LTR’s?
Long terminal repeats: Contain promoters for transcription & participate in integration.
What is a provirus?
Integrated Viral DNA. cannot be excised from host
What does reverse transcriptase do?
Transcribes RNA to DNA
Which viruses use RNA dependent RNA polymerase?
dsRNA and -ssRNA
What is a heterokaryon?
multinucleate cell that contains genetically different nuclei
What is cell fusion?
Two cells are brought together by a virus and fuse.
What are the 4 ways a viral infection can turn normal cells into tumor cells?
- Transduction
- Insertion of strong promoter
- Inactivation of tumor supressor gene
- Expression of a viral protein that induces transformation
What is transduction? (RNA virus)
- Virus take oncogene from host and places oncogene into its own genome
- Virus places this oncogene into healthy cell=tumor
Describe the mechanism of inserting a promotor. (Retrovirus)
(No oncogenes encoded)
- Insertion of a strong promoter of provirus next to protooncogene
- oncogene is expressed at high levels
