What are some of the characteristics that virus classification is based on?
Nature of the host
Type of disease caused
Naked or enveloped
Type of nucleic acids and strandedness
What is the Baltimore classification scheme?
I- Double-stranded DNA genome
II- Single-stranded DNA genome
III- Double-stranded RNA genome
IV- Single-stranded RNA genome of plus configuration
V- Single-strande RNA genome of minus configuration
VI- Single-stranded RNA genome that replicated with DNA intermediate
VII- Double-stranded DNA genome that repliates wsih RNA intermediate
What is adsdorption?
The attachment of the virus to specific receptors on the surface of the cell. Plant viruses are usually introduced into the host by insect vectors, or following mechanical damage
What is penetration?
Virus genome enters the cell
In enveloped and naked viruses, the complete virion may enter the cell
In enveloped viruses the enveloped may be left at the cell surfact such that only the nucleocapside enter the cell
In naked viruses the nucleocapside may be left at the surface
What is the life cycle of a virus?
Synthesis of nucleic acid and protein
Assembly and packaging
What is uncoating of a virus?
Removal of the envelope and/or the nucleocapside by host enzymes, sometimes within lysosomes
What is replication?
Replication of the nucleic acid, transcription and protein synthesis
What is maturation?
Assembly of virus component, nucleic acid, nucleocapsides and accessory proteins to form new virions. Usually assembly is spontaneous
What is the release stage of the life cycle of viruses?
Mature virions exit the host cells by eans of budding or by causing lysis of the cell. Plant viruses exit and are transmitted by means of vectors
What is the latent period of virus replication?
Eclipse and maturation
What is the eclipse stage of virus replication>
Time necessary for the host cells to replicate the viral genome and to synthesize the viral component
What is the maturation stage of virus replication?
Time needed for the different component to be assembled
What is the release period of virus replication?
Lysis: virus-encoded protein damage the cytoplasmic membrane. In bacteria a virus- encoded protein destroy the peptidoglycan layer
What is burst size in virus replication?
Number of virions released (varies according to the virus, host cell, etc)
What are the best studied bacteriophages?
Best-studied bacteriophages infect E. Coli (gram-negative)
What genomes do most phages contain?
Most phages contain linear dsDNA genomes. Most are naked, but some possess lipid envelopes
What are virulent phages?
Replication results in the host cell lysis (T4)
What are temperate phages?
The genome becomes incorporated into the bacterial host genome (lysogenic pathway) (Lambda)
What happens during adsorption in bacteriophage T4?
T4 attaches to the core region of LPS by the tail fibers
What comes after attachment in bacteriophage T4?
Following attachment, the tail sheath contracts, forcing the central core through the outer membrane. Tail lysozyme, digest the peptidoglycan layer, forming a small pore.
How does the phage DNA enter the host cell?
The phage DNA is inected into the cytoplasm of the host cell
What is bacteriophaged lambda replication?
Infection by a temperate phage results in a prolonged, latent state of infection (lysogeny)
What is prophage in bacteriophage lambda replication?
Phage genome within the host cell chromosome
What is lysogen in bacteriophage lambda replication?
Bacterium that contain a prophage
What is excision in bacteriophage lambda replication?
Sometimes, the 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
What type of genome is involved in bacteriophage lambda replication?
Lambda genome is linear dsDNA with cohesive ends, a region of single-stranded complimentary DNA
The cohesive ends join, forming a circular molecule of dsDNA
What is the att(lambda) site?
The Lambda genome is integrated at a specific site in the bacterial chromosome: the att(lambda) site. The att region in the phage genome is homologous to the att(lambda) site. The enzyme lambda integrase catalysed integration og the phage genome at this site. The loambda integrase is encoded on the pahge genome
What is single-stranded DNA phage with reference to psiX174?
Phage psiX174 is a plus strand DNA phage (same strand as mRNA) the minus strand must be sunthetize firdst to produce a dsDNA genome, the replicative form
What is phage MS2?
Phage MS2 is a plus strand RNA phage (same strand as mRNA). The minus strand must be synthetize first
What is replicase with reference to single-stranded RNA phage?
The replicase synthesize a minus strand RNA that is used to produce additional mRNA and additional copies of the genome for the production of new virions
Where does DNA replication occur in eukaryotes?
In the nucleus
Where is the genome of DNA virus replicated?
The genome of DNA virus will usually be replicated in the nucleus of the cell
Where will the genome of RNA virus be replicated?
The genome of RNA virus will usually be replicated in the cytoplasm of the cell
Describe penetration of DNA genome
Fusion of cell cytoplasmic membrane with virus envelope
Nucleocapsid is transported to the nucleus where viral DNA is uncoated
Describe host transcription/translation apparatus synthetize in DNA genome
Immediate early proteins (transcription factors and in some case virus-specific RNA polymerase)
Delayed early proteins (viral-specific DNA polymerase, and other viral protein necessary for replication)
Late proteins: nucleocapsid
Describe assembly with respect to DNA genome
Assembly occur in the nucleus, envelope is added via a budding process through the inner membrane of the nucleus.
The complete virions are then secreted by the ER pathway outside the cell
What are some examples of RNA gehome: 1. plus-strand
Poliovirus, SARS, Hepatitis A virus
What happens in Poliovirus?
The plus-strand genome serves as a template for synthesis of a large polyprotein that is cleaved into proteins.
What happens in coronavirus?
Coronavirus (SARS), monocistronic mRNA (encoding for only one protein) are produced from the minus-strand
What are some examples of RNA genome 2. minus-strand
Rabies (Rhabadovirus), Influenza (orthomyxovirus)
What happens in RNA genome 2. minus-strand?
The RNA genome (minus-strand) is first transcribed into a plus-strand by a RNA dependent RNA polymerase carried in the virions
What is an example of RNA genome 3. dsRNA?
What happens to RNA genome 3. dsRNA?
The dsRNA genome can not be translated. First, a plus-strand RNA must be synthetize by a viral-encoded RNA-dependent RNA polymerase using the minus strand as the template
The plus strand is then translated into proteins and is used as a template to synthetize a minus strand to yield dsRNA genomes
What is an example of a retrovirus?
The virion carries two identical copies of the genome (plus-strand ssRNA), reverse transcriptase, integrase and proteases
The reverse transcriptase is a RNA-dependent DNA polymerase that reverse transcribe the RNA genome into DNA
What is LTR?
Long terminal repeats: contain promoter for transcription and participate in the integration process
What is provirus?
Intedgrated viral DNA. Contrary to lambda prophage, the provirus cannot excise from the host genome
What happens during cell fusion?
Enveloped virus that fuse with the host cell cytoplasm carries viral protein that fuse biological membrane
Cell fusion results in hbrids cells that have chromosomal aberration, and are usually short-lived
Describe the process of transformation with respect to oncoenic/tumor-producing viruses
Som virus infections are implicated in the conversion of a normal cell into a tumor cell, a cell that have acquired immortality, characterized by uncontrolled replication
What are the three different mechanisms of oncogenic viruses?
Insertion of a strong promoter
Inactivation of a tumor supressor gene
Expresion of a viral proteins that induce transformation (DNA virus)
Describe transduction, one of the mechanisms of oncogenic viruses
The virus carries one or many oncogene, called v-oncogene. The v-oncogene is acquired from the host. This process is called transduction
Describe insertion of a promoter, one of the mechanisms of oncogenic viruses
The virus does not encode an oncogene
Insertion of a strong promoter of the provirus next to a proto-oncogene. The proto-oncogene is then expressed at high level and results in misregulation of the pathway that prevent transformation. The site of integration of the provirus is very important and transformation wil occur only if a proto-oncogene is affected
Describe inactivation of a tmor supressor gene, one of the mechanisms of an oncogenic virus
The virus do not encode an oncogene
Insertion of the provirus in a gene involved in the regulation of proton-oncogene expression. Insertion cause inactivation of the gene and uncontrolled expression of the proton-oncogene, resulting in transformation
Describe the expression of a viral protein
The viral protein does not have a cell counterpart
Integration of the viral genome in the host genome may occur (such as in the case of adenovirus) or the viral genome may persist in the cell as extrachromosomal spisome