Taxonomy of viruses Flashcards
Virus are given 3 different names…
- Into FAMILIES: end in viridae
- Viruses are given a genus name : end in virus
- Viruses are given a species name: Usually in english, descriptive
Classification of viruses is based on different characteristics. Name them…
- Nature of the host (ex: bacteriophage infect bacteria)
- Type of disease caused (ex: murine leukemia virus)
- Life cycle (pathways of nucleic acid replication and transcription)
- Naked or enveloped
- Type of nucleic acids and strandedness (+ strand = same as mRNA)
What is the baltimore classification scheme?
-Scheme where classification is based on the type of genome. It is useful because THE KIND OF GENOME WILL DICTATE THE REPLICATION MECHANISM.
What are RNA genomes?
+ configuration: Same strand than mRNA (can be translated directly)
- configuration: complementary to mRNA (needs to be transcribed into + strand before it can be translated)
What are the 5 steps of the life cycle of viruses?
- Attachment ( absorption)
- Penetration
- Synthesis of nucleic acid and prots
- Assembly and packaging
- Release (lysis)
OR
- Absorption
- Penetration
- Uncoating
- Replication
- Maturation
- Release
What happens during the penetration step of a viruses life cycle?
The virus genome enters the cell:
- In ENVELOPED AND NAKED VIRUSES: the complete virion ma enter the cell
- In ENVELOPED viruses , the envelope may be left at the surface of the cell, such that only the nucleocapsid enters the cell
- In NAKED viruses, the nucleocapsid may be left at the surface
What is uncoating in the life cycle of the virus?
removal of the envelope and/or the nucleocapsid by host enzymes, sometimes within lysosomes (in eukaryotes)
What are the 3 penetration method used by viruses?
- Direct penetration
- Membrane fusion
- Endocytosis
What penetration method is most commonly used by ENVELOPED VIRUSES OF EUKARYOTES?
Most enveloped viruses of eukaryotes use endocytosis (VIROPEXIS). Such viruses are then delivered to lysosomes which degrade the nucleocapsid and the nucleic acid is released into the cytoplasm.
What happens during the maturation phase of lice cycle of viruses?
- assembly of virus components, nucleic acid, nuceloapsid and accessory prots to form new virions
- Usually, the assembly is spontaneous
What happens during the release phase of life cycle of viruses?
- Mature virions exit the host cell by means of BUDDING or by causing lysis of the cell. Plant viruses exit and are transmitted by means of vectors.
What are the main phases ( on the curve) of virus replication ? What subphase does it include?
Latent Phase: eclipse + maturation
- ECLIPSE : time necessary for the host cells to replicate the viral genome and to synthesize the viral components - MATURATION : time needed for the different components to be assembled.
Release phase (RISE period)
- Virions are detected OUTSIDE the cell - LYSIS : damage of cytoplasm (in bacteria, destroy peptidoglycan layer) - BUDDING: developed virus
What is the definition of Burst size?
Number of virions released
What type of DNA do most of the phages contain?
Most of the phages contain LINEAR dsDNA genomes. Most are NAKED, but some possess lipid envelope.
What are the 2 pathways of a viral life cycle?
LYTIC PATHWAY: in virulent phages, replication results in host cell lysis–> T4
LYSOGENIC PATHWAY: in temperate phages, the genome becomes incorporated into the bacterial host genome –> Lambda
What type of phage is T4 bacteriophage?
Virulent phage
How does bacteriophage T4 invade a host?
- Absorption: T4 attaches to core region of LPS by the tail fibers
- Tail sheath contracts, force central core through outer membrane. LYSOSYMES of the tail digest peptidoglycan layer, forming a pore
- The phage DNA is then injected into the cytoplasm of the host cell.
How do we call a prolonged, latent state of infection?
lysogeny
What kind of phage is Lambda?
temperate phage
What is a prophage?
Phage genome within the host cell chromosome
What is Lysogen?
bacterium that contains a prophage
What are the 2 pathways possible in the infection of Lambda bacteriophage?
Lytic pathway and lysogenic pathway (see slides…)
True or false
Prophages can never exit the chromosome of the host.
False. Prophage can exit the chromosome (a process called EXCISION) and continue along the lytic pathway, resulting in the production of NEW VIRUS PARTICLES, and HOST CELL LYSIS.
Describe the genome of Lambda.
- linear dsDNA with cohesive ends, a region of ss complementary DNA
aller voir la slide qui parle de labda replication, tout le process
GO
What kind of phage is MS2 phage?
it is a + strand RNA phage
What is the process of replication of MS2 genome?
- genome is used as mRNA
- direct synthesis of and RNA REPLICASE (RNA-dependant RNA polymerase, fct not performed by the host) and other viral prots
- RNA replicase synthesize a minus strand RNA is used to produce additional mRNA and additional copies of genome (+ strand) for the production of new virions)
Where does DNa replication occur in eukaryotic cells?
Nucleus
Where does the genome of DNA viruses be replicated in animal viruses?
nucleus
Where does the genome of RNA viruses be usually replicated in animal viruses?
in the cytoplasm
Give an exemple of DNA virus
herpesvirus
Give and exemple of RNA genome viruses + strand RNA
Poliovirus and hepatisis A
Go check RNA genome 1- poliovirus
Go
Give an exemple of RNA genome -minus strand RNA
Measles, Influenza, Rhabdovirus
how is a - strand RNA transcribed?
- First transcribed into + strand RNA by RNA-dependant RNA polymerase carried inside the virions
Give an exemple of RNA genome dsRNA of a virus
Rotavirus
Explain the transcription of dsRNA genome of a virus
- the dsRNA genome cannot be transcribed
- First, the + strand RNA must be transcribed using RNA-DEPENDANT RNA POLYMERASE using the MINUS STRAND as the template
+ strand is then translated into prots and is used as template to produce - strand to yield dsRNA genomes
Give an exemple of retrovirus RNA genome
HIV
Causing cancer= Rous sarcoma virus
What is reverse transcriptase?
Reverse transcriptase is a RNA-dependant DNA polymerase that reverse transcribes the RNA genome into DNA
What does retrovirus virions carry
- 2 identical copies of the genome (+ strand ssRNA )
- Reverse transcriptase
- Integrases
- Proteases
Where does the DNA genome transcribed from retrovirus (+strand RNA) go?
DNA genome travels to the nucleus and is intergrated into the host cell DNA
What is LTR?
Long terminal repeats
It contains the promoter for transcription and participate n the integration process
What is a Provirus?
It is an integrated viral DNA . Contrary to the lambda prophage, the provirus cannot be excise from the host genome.
What are the consequences of viral infection in animals?
Can lead to :
Attachment and penetration
- Cell fusion
Virus multiplication:
- Virus present but not replicating —> LATENT INFECTION
- Slow release of virus without cell death —> PERSISTANT INFECTION
- Death of cell and release of virus—> LYSIS
Transformation into tumor cell:
- Tumor cell division–> TRANSFORMATION
What is cell fusion
When virus causes 2 cells to fuse. Results in hybrid that has chromosomal aberrations. Are usually short-lived.
What are 4 different mechanism of tumor producing viruses?
- Transduction
- Insertion of a strong promoter
- Inactivation of a tumor suppressor gene
- Expression of a viral protein that induces transformation (DNA virus)
Explain transduction in tumor producing mechanism
- RNA virus
- Rous Sarcoma virus
- Virus carries many oncogenes ( tumor-causing genes) called v-oncogens
- v-oncogen is acquired from the host
- High expression of the oncgene affects the regulation of replication and/or cell death and causes transformation
Explain insertion of a promoter in tumor producing mechanism
- RETROVIRUS
- virus doesn’t encode oncogene
- voir la slide
Explain inactivation of a tumor suppressor gene in tumor producing mechanism
- RETROVIRUS
- the virus does’nt encode oncogene
- Insertion of a provirus in a gene involved in the regulation of the proto-oncogene
- Inactivation of proto-oncogene
- uncontrolled expression of the proto-oncogene
Explain expression of a viral protein in tumor producing mechanism
- DNA virus
- Viral protein doesn’t have a cell counterpart
- Integration of the viral genome into the host genome may occur, or the viral genome may persist in the cell as an extrachromosomal episome
- VOIR LA SLIDE
What is C-oncogen
- also called proto-oncogen
- involved in the regulation of the cell cycle or in safety mechanisms that prevent transformation
What is transformation? (oncogenic/tumor providing viruses)
Conversion of a normal cell into a tumor cell–> cell that has acquire immortality, characterized by UNCONTROLLED REPLICATION
What are the steps of replication of a single DNA phages genome?
THE STRAND IS POSITIVE
- Copy a negative strand using the positive strand
- dsDNA being the replicative form - The positive strand is cut
- Derolling of the olad postivite strand and new positive strand produced
products:
dsDNA strand and postive DNA strand
Can ssRNA of MS2 be replicated by the host cell?
NO!
Because the cell doens’t have the function to replicate RNA strands… RNA replicase (RNA-dependant RNA polymerase is provided by the phage!)
What are the steps of replication of a ssRNA phage?
- synthesis of minus strand by RNA replicase
2. Use of minus strand to produce more positive strand
Can the ssRNA phage of MS2 be translated right away?
Yes, because it is a plus-strand, so has the same structure as mRNA
An exemple of DNA virus?
Human papilloma virus
