Micro 14

end in "viridae"

end in "virus"

• Nature of the host
• Type of disease caused
• Life cycle
• Naked or enveloped
• Type of nucleic acids and strandedness

• 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

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

• 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

• Attachment (adsorption)
• Penetration (injection)
• Synthesis of nucleic acid and protein
• Assembly and packaging
• Release (lysis)

Removal of the envelope and/or the nucleocapside by host enzymes, sometimes within lysosomes

Replication of the nucleic acid, transcription and protein synthesis

Assembly of virus component, nucleic acid, nucleocapsides and accessory proteins to form new virions. Usually assembly is spontaneous

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

Eclipse and maturation

Time necessary for the host cells to replicate the viral genome and to synthesize the viral component

Time needed for the different component to be assembled

• Lysis: virus-encoded protein damage the cytoplasmic membrane. In bacteria a virus- encoded protein destroy the peptidoglycan layer
• Budding

Number of virions released (varies according to the virus, host cell, etc)

Best-studied bacteriophages infect E. Coli (gram-negative)

Most phages contain linear dsDNA genomes. Most are naked, but some possess lipid envelopes

Replication results in the host cell lysis (T4)

The genome becomes incorporated into the bacterial host genome (lysogenic pathway) (Lambda)

T4 attaches to the core region of LPS by the tail fibers

Following attachment, the tail sheath contracts, forcing the central core through the outer membrane. Tail lysozyme, digest the peptidoglycan layer, forming a small pore.

The phage DNA is inected into the cytoplasm of the host cell

Infection by a temperate phage results in a prolonged, latent state of infection (lysogeny)

Phage genome within the host cell chromosome

Bacterium that contain a prophage

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

• 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

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

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

Phage MS2 is a plus strand RNA phage (same strand as mRNA). The minus strand must be synthetize first

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

In the nucleus

The genome of DNA virus will usually be replicated in the nucleus of the cell

The genome of RNA virus will usually be replicated in the cytoplasm of the cell

• Fusion of cell cytoplasmic membrane with virus envelope
• Nucleocapsid is transported to the nucleus where viral DNA is uncoated

• 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

• 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

Poliovirus, SARS, Hepatitis A virus

The plus-strand genome serves as a template for synthesis of a large polyprotein that is cleaved into proteins.

Coronavirus (SARS), monocistronic mRNA (encoding for only one protein) are produced from the minus-strand

Rabies (Rhabadovirus), Influenza (orthomyxovirus)

The RNA genome (minus-strand) is first transcribed into a plus-strand by a RNA dependent RNA polymerase carried in the virions

Rotavirus

• 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

• 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

Long terminal repeats: contain promoter for transcription and participate in the integration process

Intedgrated viral DNA. Contrary to lambda prophage, the provirus cannot excise from the host genome

• 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

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

• Transduction
• Insertion of a strong promoter
• Inactivation of a tumor supressor gene
• Expresion of a viral proteins that induce transformation (DNA virus)

The virus carries one or many oncogene, called v-oncogene. The v-oncogene is acquired from the host. This process is called transduction

• 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

• 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

• 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