Virus and Bacteriophage Replication

Virus Replication

• Viruses must enter a host cell to replicate, allowing for the virus to reproduce and survive
• Most bacteriophages have double-stranded DNA.
• Viruses generate copies of their genome and package them into new virions, allowing infection of new hosts.

Bacteriophage Replication

• The replication cycle of bacteriophages is well-understood and researched,
• Bacteriophages are viruses that infect bacteria and are used in medical and pharmaceutical applications.
• Some bacteriophages can be used to create cloning and expression vectors.
• Bacteriophages are also used for industrial production:
  • They can be used to express heterologous proteins, which are essential for the creation of pharmaceuticals, vaccines, and antibodies.

Lytic Cycle

• In the lytic cycle, a phage hijacks a host cell, using it to create new phage particles.
• The host cell is then lysed, bursting and releasing new virions.

Lysogenic Cycle

• The phage DNA integrates with the host chromosome to form a prophage, which replicates along with the host's own DNA.
• Prophages can remain dormant and be transmitted to daughter cells during cell division.
• Lysogenic cycles are activated via triggers such as UV radiation or presence of specific chemicals, resulting in new phage production.
• Prophages can sometimes benefit the host bacterium by providing new functions to the bacterial genome.

Animal Virus Replication

• Animal viruses are often species- and cell-specific.
• There are six basic stages in the replication cycle of animal viruses.
• Virus replication depends on the interaction between the virus and the host cell.
  • Permissive cells allow for viral replication, while nonpermissive cells do not but may allow for viral transformation, a change in the host cell's phenotype.

Stages of Animal Virus Replication

• Adsorption or attachment: the virus binds to the host cell's surface via a random collision. Virion attachment proteins bind to specific receptors on the host cell membrane.
• Penetration or entry: the virus enters the cell using different mechanisms like fusion, endocytosis, or direct penetration.
• Un-coating: the virus releases its nucleic acid into the cell's cytoplasm.
• Replication of virus nucleic acids and protein synthesis: the virus uses the cell's machinery to produce more nucleic acids and proteins.
• Assembly of virion components: the virus uses the synthesized proteins to assemble new virion components.
• Maturation: the newly assembled virions mature and acquire functional components.
• Release of mature viruses: the new virions are released from the cell, usually by budding or lysis.

Animal Virus Lysogenic Cycle

• The lysogenic cycle occurs when the viral genome integrates into the host cell's genome.
• The viral genome is replicated along with the host cell's DNA.
• The viral genome may remain integrated indefinitely, or it can be triggered to enter the lytic cycle.

Herpes Virus Replication

• These viruses have large DNA genomes and infect non-dividing cells.
• Their replication cycle is complex and includes early and late transcription phases.
• The late transcription phase produces mRNAs encoding structural proteins, such as capsid proteins, tegument proteins, and envelope glycoproteins.
• They assemble in the nucleus, where they acquire their envelope.
• They are released by exocytosis.

Poxvirus Replication

• Poxviruses replicate in the cytoplasm of the host cell.
• Their replication cycle is complex and involves synthesis of early, intermediate, and late mRNAs.
• They are released by budding or lysis.

Parvovirus B19 Replication

• Parvoviruses are small, single-stranded DNA viruses.
• They bind to P antigen (globoside) on the host cell surface and enter the cell via endocytosis.
• They release their DNA into the nucleus, where it replicates.
• They are released by lysis when the host cell dies.

Budding

• Budding is a process where a virus acquires its envelope from the host cell membrane.
• The envelope is coated with specific viral proteins and the nucleocapsid binds to the membrane, enabling the virus to bud out of the cell.

Release

• Some viruses are released through exocytosis.
• Others are released through lysis, which destroys the host cell.
• The release of too many virions can lead to cell death.
• Retroviruses, except HIV, replicate without cell death.