Lesson 3: How do viruses carry out their next generation? (Overview)

• Viral replication is the formation of biological viruses during the infection process in the target host cells.
• Replication is the total of all the events whereby a single particle attaches to a cell and, in a relatively short time, the cell releases many virions  ‐ Produces multiple copies of the genome‐ Pack the genomes into particles‐ One particle gives rise to hundreds or thousands of particles that can infect again• All viral infections begin with events in a single cell
• The virus replication follows the following steps;
  1. Attachment and Entry:
     • Viruses recognize specific receptors on the surface of host cells. This recognition is typically mediated by viral surface proteins or glycoproteins.
     • Once attached, viruses enter the host cell through fusion with the host cell membrane or endocytosis, depending on the virus type.
  2. Release of Viral Genetic Material:
     • Once inside the host cell, the virus releases its genetic material (either RNA or DNA) into the host cell’s cytoplasm.
  3. Replication and Transcription:
     • The viral genetic material serves as a template for the synthesis of new viral RNA or DNA. This step is often facilitated by viral enzymes, such as RNA-dependent RNA polymerase or reverse transcriptase.
     • Viral genes may also encode proteins that manipulate the host cell’s machinery to favor the replication and transcription of the viral genome.
  4. Translation and Protein Synthesis:
     • The host cell’s ribosomes are recruited to translate viral mRNA, producing viral proteins. These proteins play essential roles in various stages of the viral life cycle, including replication, assembly, and evasion of host defenses.
  5. Assembly:
     • Newly synthesized viral genetic material and proteins are assembled into complete virions within the host cell. This process may occur in specific cellular compartments or at the cell membrane.
  6. Maturation:
     • The assembled virions undergo maturation, acquiring their final structure. This may involve post-translational modifications of viral proteins.
  7. Release:
     • Once the assembly and maturation are complete, new virions are released from the host cell. This can occur through various mechanisms, such as cell lysis (cellular rupture), exocytosis, or budding.
     • Some enveloped viruses acquire a portion of the host cell membrane as an envelope during the release process.
  8. Transmission to New Hosts:
     • The released virions can then infect new host cells, starting the infection cycle anew. The transmission between hosts can occur through various means, such as respiratory droplets, bodily fluids, or vectors like insects.

The overall process of virus replication and the production of the next generation of virions is highly dependent on the host cell’s machinery. Viruses exploit the cellular processes of the host to replicate and propagate, and the successful completion of the viral life cycle often involves complex interactions between viral and host cell.

Some important definitions

• A susceptible cell has a functional receptor for a given virus ‐ the cell may or may not be able to support viral replication
• A resistant cell has no receptor ‐ it may or may not be competent to support viral replication
• A permissive cell has the capacity to replicate a virus ‐ it may or may not be susceptible
• A susceptible AND permissive cell is the only cell that can take up a virus particle and replicate it