Taxonomy of Viruses Flashcards
General characteristics of viruses
Ecological interactions? Hosts? Consists of?
- Obligate intracellular parasites
- Host cells include all groups of organisms
- Piece of nucleic acid enclosed within a protein coat
- 1 type of nucleic acid found in the virion of a given virus
Virions
- Outside of the host
- inert infectious particles
Capsid
Protein coat around the nucleic acid
Nucleocapsid
Nucleic acid + protein coat
Capsomeres
Protein subunit that makes up the capsid
Envelope
Lipid-containing layer with embedded proteins
* Proteins in the envelope are virus specific
Shapes of Virions
- Helical virus: Typical of many plant viruses
- Polyhedral virus: most common is icosahedron
- Complex virus: Composed of several parts
Viroids
- Closed circles of single-stranded RNA containing 240-380 nucleotides
- Replication is dependent on host machinery
Prions
- Consists solely of 1 protein
- Commonly found in neurons
- Induces misfolding of normal prion proteins who’s accumulation kills cells
- Cause of many neurological degenerative disorders
Taxonomy
Virus
- Grouped into families (end in “-viridae”)
- Given a genus name (end in “-virus”)
- Given a species name (usually English)
Classificationof viruses
Based on characteristics:
* Nature of the host
* Type of disease caused
* Life cycle
* Naked or enveloped
* Type of nucleic acids and strandedness
Baltimore classification scheme
Classification is based on the type of genome
Pathway of viruses
- Absorption
- Penetration
- Uncoating
- Replication
- Maturation
- Release
Absorption
Life cycle
Attachment of the virus to
specific receptors on the surface of the cell
Penetration
Life cycle
Virus genome enters the cell
* The complete virion may enter the cell
* In enveloped viruses: Envelope may be left at the cell surface
* In naked viruses: Capsid may be left at the surface
Uncoating
Life cycle
Removal of the envelope and\or
the capsid by host enzymes
Replication
Replication of the nucleic acid,
transcription and protein synthesis
Maturation
Assembly of virus components, nucleic acid, nucleocapsid and accessory proteins to form new virions
Release
Mature virions exit the host cell by means of budding or by causing lysis of the cell.
Latent period
Eclipse (replication) + maturaiton
* Eclipse (replication): 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
Rise period (release)
Virions are detected outside the cell
* Lysis: virus-encoded proteins damage the CM
* Budding
Burst size
Number of virions released
Bacteriophages
- Viruses of bacteria
- Most phages contain linear dsDNA genomes
- Most are naked, but some possess lipid envelopes
Bacteriophage Types
- Virulent phages
- Temperate phages
Lytic pathway
Infection of host cells always leads to replication resulting in host cell lysis
* Virulent and temperate phages
Lysogenic pathway
Genome becomes incorporated into the bacterial host genome
* Temperate phages
Virulent Phage: T4
- T4 attaches to the core region of LPS by the tail fibers
- Tail lysozymes digest the peptidoglycan layer, forming a small pore
- Phage DNA is injected into the cytoplasm of the host cell
- Replication of T4 dsDNA
- T4 lysosome production leads to cell lysis
Temperate Phage: Lambda
- Genome = linear dsDNA with cohesive ends that join to form circular molecule
- Genome integrated in the bacterial chromosome at the attƛ site (homologous to the att site in the phage)
- Infection by temperate phages results in a prolonged, latent state of infection
- Sometimes, the prophage can exit the chromosome and continue along the lytic pathway
Prophage
phage genome within the
host cell chromosome
Lysogen
bacterium that contains a
prophage
Location of replication in animals viruses
- DNA genome: in the nucleus
- RNA genome: in the cytoplasm
Animals viruses types
DNA genome
RNA genome:
* plus-strand RNA
* minus strand RNA
* dsRNA
* Retrovirus RNA
DNA genome
- Fusion of cell CM with virus envelope
- Nucleocapsid transported to nucleus where viral DNA is uncoated
- Host apparatus synthetizes immediate early proteins, delayed early proteins, & late proteins
- Assembly occurs in the nucleus
- envelope is added via a budding process through the inner membrane of the nucleus
- Ex: Herpes virus
RNA genome: + strand RNA
The genome can be translated directly
* Ex: Poliovirus, Hepatitis A virus
RNA genome: - strand RNA
The minus-strand is first transcribed into a plus-
strand RNA by an RNA-dependent RNA polymerase carried inside the virions
* Ex: Measles, Rabies, Influenza
RNA genome: dsRNA
- A plus-strand RNA must be synthetized 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 synthesize a minus strand to yield dsRNA genomes
- Ex: Rotavirus
RNA genome: Retrovirus
- The virion carries two identical copies of the genome (plus-strand ssRNA), reverse transcriptase, integrase and proteases.
- The reverse transcriptase reverse transcribes the RNA genome into DNA
- The DNA genome travels to the nucleus and is integrated into the host DNA
- Ex: HIV
Long terminal repeats (LTR)
Contain promoters for transcription and participate in the integration process of viral DNA
Provirus
- Integrated viral DNA
- Cannot excise from the host genome
Consequences of viral infections in animal cells
- Cell fusion
- Oncogenic viruses
Cell fusion
Enveloped viruses that fuse with the host cell cytoplasmic membrane
* results in hybrid cells that have chromosomal aberrations
Transformation
Conversion of a normal cell into a tumor cell
Oncogenic Viruses
Viral infection is implicated in transformation
* Both DNA and RNA viruses are know to cause tumours
Mechanisms of oncogenic viruses
- Transduction
- Insertion of a strong promoter
- Inactivation of a tumor suppressor gene.
- Expression of viral protein that induces transformation
Expression of viral protein that induces transformation
- DNA virus
- The viral protein does not 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
- Ex: HPV
Nonpermissive host
A host in which a virus cannot complete its infection cycle
* The infected undergoes uncontrolled replication (tranformation)
* Because the virus cannot complete its replication, the cells will never die
Nucleoside analogs
Block reverse transcriptase & production of viral DNA
Protease inhibitors
Inhibit processing of large viral proteins into individual components
Fusion inhibitors
Prevent viruses from successfully fusing with the host cell
