on 29.12.07 with 0 comments

Ambisense genome—these negative strand RNA viruses have genomes that can be negative strand and another part is positive strand. They have more complex forms of replication/transcription. They are all single strands of RNA.

  1. Arena virus

  2. Bunyaviruses

Double stranded RNA virus—Reovirus and Rotavirus—genome serves as a template for mRNA transcription. Virions carry the RNA dependent RNA polymerase in the core. They have two copies of the + DNA but it is not paired—not really double stranded.

Retroviruses—the genome is positive single-stranded but it is not translated. The RNA is reversed to DNA via reverse transcriptase that is in the core of the virion.

Single cycle growth curve for a virus—a single cell is infected. Generally, the infected cell is infected with a MOI of 1 (1 particle per cell). Once the virus binds to the host cell it is internalized to the cytoplasm or to the nucleus. They undergo transcription and translation—then they assemble and the resulting progeny are released.

As MOI goes up, the previously infected cell is more immune to prior infections.

The first part of the cycle is the macro molecule synthesis. The early phase is characterized by the synthesis of molecules.—the late phase ends with the assembly of the virus particles.

Some viruses can produce up to 100,000 viruses per infected cell. Slower viruses like the retroviruses only produce about 1000 per cell.

Steps if viral cycle:

    1. recognition and attachment to the host cell surface

    2. penetration and uncoating

    3. macromolecular synthesis

      1. transcription

      2. translation and modification of proteins

      3. replication

    4. assembly

    5. budding and release

Schematically—how does this happen?

At the stage of recognition/binding—viral attachment proteins bind to the receptors for the viral attachment proteins. This allows the virus to bind to the host cell. Those steps can be eliminated by antibodies that bind to the receptors or the viral proteins.

Once attached to the cell the virus must be internalized by one of three ways:

  1. receptor-mediated endocytosis

  2. Direct penetration

  3. Fusion with the plasma membrane—for enveloped viruses only that have their own lipid bi layer.

--once inside the cell, the nucleocapsid undergoes uncoating. This step can be inhibited by drugs like amantadine. Once the viral genome is uncoated, it undergoes transcription.

If transcription occurs, we can stop it with antisense RNA. Interferon can also stop this process. Once the transcription occurs, mRNA is translated to produce proteins of viruses. This step can be blocked by interferon as well.

Assembly of the capsid without the viral genome in it occurs.

Then, replication—of the genome occurs. You have lots of both polarities of the RNA. Once the RNA is incorporated into the capsid, the virion is complete. They can then leave by budding or by lysing the cell.

Category: Microbiology Notes



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