Viral Replication Cycle Quiz

Virus Replication

• Viruses need a host cell for replication.
• Viruses are obligate intracellular parasites.
• Viruses are host-specific.
• Viruses have a two-phase lifecycle, either residing in a dormant state within the host genome (lysogenic cycle) or hijacking the host cellular machinery for their own replication (lytic cycle).

Lytic Cycle

• A virus introduces its genome into a host cell to initiate replication by hijacking the host's cellular machinery to make copies of the virus.
• Once the infection is complete, the newly replicated and assembled virus particles are released through lysis of the host cell into the surrounding environment.
• The lytic cycle has steps which include: attachment, entry, replication, self assembly, and release.

Steps of Lytic Cycle

• Attachment: The phage attaches to the host cell surface to administer the DNA within the cell.
• Entry: The DNA is injected by the phage into the host cell by penetrating the membrane of the cell.
• Transcription: The DNA of the host cell is destroyed and the metabolism of the cell is modified to begin the biosynthesis of the phage.
• Self Assembly: The replication of the phage DNA takes place within the cell, producing novel phage proteins and DNA.
• Release: The newly produced phages are discharged from the infected cell looking for novel host cells to infect. Release during the Lytic Life Cycle of a Lytic Bacteriophage. A bacteriophage-coded enzyme breaks down the peptidoglycan in the bacterial cell wall causing osmotic lysis.

Lysogenic Cycle

• A viral genome infects the bacterial or archaeal cell and reproduces along with it, rather than taking control of the host cell and producing new virions.
• Lysogeny is characterized by the integration of the bacteriophage nucleic acid into the host bacterium genome or formation of a circular replicon in the bacterial cytoplasm.
• In this condition, bacteria continue to live and reproduce.
• The genetic material of the bacteriophage is called a prophage.
• It has steps like attachment, penetration, integration, replication, induction, synthesis, assembly and release.

Steps of Lysogenic cycle

• Adsorption: Virus attached to the host cell.
• Penetration: Entry genetic material is injected into the host cell.
• Integration: The phage DNA integrates into the host chromosome forming a prophage.
• Replication: The prophage replicate, viral DNA is copied along with the host cell DNA, each new daughter cell is infected with the virus.
• Induction: When the daughter cell is induced, the viral DNA is excised.
• Synthesis: The phage DNA and protein are synthesized using the host cell machinery.
• Assembly: The phage DNA and protein are assembled to form new viruses.
• Release: The virions are released on lysis of the host cell.

Viral Replication Cycle

• Host cell requirement: Viruses require a host cell to provide energy and synthetic machinery.
• Viral Replication Cycle in Host Cell: The viral replication cycle causes dramatic biochemical and structural changes in the host cell, leading to cell damage.
• Symptoms of Viral Diseases: The symptoms of viral diseases result from the immune response to the virus, attempting to control and eliminate the virus from the body.
• Viral Release: Many animal viruses, such as HIV, leave infected cells of the immune system through a process called budding.

Viral Replication Cycle Steps

• Attachment, Penetration, Uncoating, Replication, Assembly, Release.

Initial Attachment

• Virus particles attach to specific receptors on the host cell surface.
• Receptors may be proteins or carbohydrate residues on glycoproteins or glycolipids.
• Expression of receptors on host cells determines viral tropism.
• Attachment phase influences viral pathogenesis and infection course.
• Initial attachment may be facilitated by cations.
• Firm binding requires specific receptors on the plasma membrane.
• Viral surface molecules attach to cellular receptors.

Penetration

• Enveloped viruses fuse lipid membranes with host cell membrane.
• Allows viral genome to enter host cell.
• Viruses are taken into the cell through host’s own membrane.
• Includes bulk-phase endocytosis and phagocytosis.
• Non-enveloped viruses inject genetic material directly into host cell.

Uncoating

• Removal of virion's protein coat.
• Release of genetic material.
• Location of uncoating occurs in the same area as viral transcription, either in the cytoplasm or nucleus.

Replication

• Rapid production of viral genome.
• Relies on the type of genetic material the virus possesses.
• DNA viruses use host cell nucleus replication enzymes.
• Double-stranded DNA (dsDNA) viruses.
• RNA viruses replicate in cytosol.
• Directly access host cell's ribosome for viral protein manufacture.

Additional

• Virus types: dsDNA, ssDNA, dsRNA, +sense RNA, -sense RNA, RNA & DNA that reverse transcribe.
• Diagram of a virus lifecycle and Lytic/Lysogenic Lifecycle diagrams are included.
• Translation and protein synthesis: viruses employ a diverse array of host-directed strategies for efficient expression of viral proteins.
• Lacking essential machinery of protein synthesis, viruses take advantage of the host apparatus.
• Some viruses have co-opted eukaryotic sequence or structural elements for translation initiation.
• Assembly: Formation of an immature virus requires assembly of the capsid, envelopment, and packaging of the nucleic acid genome within a host cell.
• Assembly depends on the type of virus.
• Can occur in plasma membrane, cytosol, nucleus, Golgi apparatus, or other locations.
• Some viruses only insert their genome after the capsid is completed.
• In other viruses, the capsid wraps around the genome as it's being copied.
• Maturation: Final changes in an immature virion for an infectious virus particle.
• Structural changes include proteolytic cleavage, disulfide bond formation, envelope maturation, envelope protein incorporation, lipid bilayer formation, genome condensation, genome packaging, and nucleocapsid formation.
• Release: The final stage when newly assembled viruses leave the host cell.
• Release methods include lysis (cytolytic) and budding (cytopathic).