Virus life cycle Flashcards
(20 cards)
Virus life cycle
Attachment
Penetration
Uncoating
Replication and synthesis
Assembly
Release
Attachment to the host cell:
Viruses use host cell surface proteins and receptors for entry
Penetration into the host cell
Nonenveloped viruses (naked): via endocytosis or transmembrane transport
Envelope viruses, via endocytosis or fusion with the host cell’s membrane
Uncoating
of the nucleic acids
Replication
of the nucleic acids and the formation of virus proteins by transcription
Early mRNA is for the synthesis of
Protein to shut down the host cell defense mechanism
Proteins for genome replication (e.g., viral RNA polymerase
Late mRNA for the synthesis of viral structural proteins
Entry by fusing with the plasma membrane
Some enveloped viruses fuse directly with the plasma membrane. Thus, the internal components of the virion are immediately delivered to the cytoplasm of the cell.
Entry via endosomes at the cell surface
Some enveloped viruses require an acid pH for fusion to occur and are unable to fuse directly with the plasma membrane. These viruses are taken up by invagination of the membrane into endosomes. As the endosomes become acidified, the latent fusion activity of the virus proteins becomes activated by the fall in pH and the virion membrane fuses with the endosome membrane. This results in delivery of the internal components of the virus to the cytoplasm of the cell
Assembly/maturation
New virus particles are assembled. There may be a maturation step that follows the initial assembly process.
Release
Enveloped viruses are released via budding
Nonenveloped viruses are released via cell lysis
Life cycle of viruses with Prokaryote hosts
Biocteriophages are a good model of how viruses affect the cells they infect
Similar processes are observed in eukaryotic viruses which have caused the immediate death of a cell or established latent infection or chronic infection
Virulent phagestypically lead to the death of the cell through cell lysis.
Temperate phages, become part of a host chromosome and are replicated with the cell genome until they are induced to make newly assembled viruses orprogeny viruses.
Bacteriophage
Well known as phage
Reproduce inside microscopic organisms - cytoplasm
DNA or RNA genome encased in protein sheath/head
Omnipresent – ubiquitous viruses found wherever bacterial exits
Straight or round ss or ds DNA or RNA
5 to 110 proteins encoded in the genome
MS2 phage encodes for four proteins
more than 1031bacteriophages on the planet
Attachment or absorption
Bacteriophage attaches to the surface of the bacterial cell wall at a specific receptor site by their tail fibers. This process of its attachment is called absorption. This attachment is the courtesy of week chemical bonding between the receptor site and bacteriophage.
Penetration
The tail of the bacteriophage discharges lysozyme which is an antibacterial enzyme to digest the small part of the cell wall. After digestion, the tail sheath contracts, and a rigid tube is propelled out of the sheath to puncture a hole on the bacterial cell membrane.
Genetic material is then passed through the tail tube and injected into the bacterial cell. The protein coat and tail of the bacteriophage do not enter the cell and remain outside.
Lytic cycle
The virus destroys the host cell.
A rapid process where host cells undergo lysis
Lysogenic cycle
the viral genome becomes integrated into the host genome.
The means in the lytic cycle are as per the following:
Attachment/Adsorption is the course of a bacteriophage’s tail strings, mooring it to the bacterial cell divider.
Penetration involves infusing phage DNA into microscopic organisms.
Replication and combination are disturbed, and the viral genome assumes command over the bacterial apparatus.
The parts of the phage are assembled to frame new infection particles.
Bacterial cells are lysed, and new infection particles are delivered into the climate to contaminate more cells.
Lysogenic cycle
Infection will enter a latent period
Host cell not killed
Viral nucleic acids undergo genetic recombination
When bacterial DNA replicates, the prophages also replicate
Change in phenotype is called lysogenic conversion or phage conversion
Vibrio cholerae the toxin can cause severe diarrhea
Clostridium bortulinum the toxin that causes paralysis
Transduction
Bacteriophage transfers bacterial DNA from one bacterium to another during sequential infections.
Generalized transduction
a random piece of the bacterial chromosome
Specialized transduction
The prophage replicates part of the host genome
