VIRUS MORPHOLOGY

• Capsid: Composed of numerous repeating subunits. Viral genome is thus conserved in that a single protein can be transcribed many times to make the capsid.

• Capsomere: Clusters of polypeptides which, when completely assembled, form the capsid.

• Nucleocapsid: Genome together with capsid.

• Virion: Free viral particle, consisting of nucleocapsid, or nucleocapsid plus envelope.

• Envelope: Lipid bilayer, usually obtained from host cell.

• Symmetry: Three types

  • Cubic Symmetry: Icosahedral symmetry. 20 equilateral triangles.

  • Helical symmetry

  • Complex: No regular symmetry, as in poxvirus.

• Viral Conditions:

  • Alkaline kills all viruses.

  • Ether: Enveloped viruses are susceptible to ether. Poxviruses vary in susceptibility.

  • Enveloped viruses are generally less stabile than naked viruses.

• Host-cell Virus Interactions:

  • Productive Infection: Virus successfully completes replication cycle and produces progeny virions.

  • Abortive Infection: Virus enters the host-cell but cannot successfully complete replication.

    • Can result from a non-permissive host-cell, or because the virus is defective.

    • Abortive infections can become persistent infections.

  • Restrictive Infection: Cells are only transiently permissive, after which they become non-permissive. This results in transformation of the cell.

  • Latent Infection: Virus remains in a cell and does not replicate, as in HSV, VZV, CMV, EBV, and HIV.

• Replication Cycle:

  • Divided according to presence of infectious virions:

    • Eclipse Phase: Virus has entered cell, but no progeny virions are created yet.

    • Maturation Phase: Viral progeny are produced and infectious virions are again present.

  • According to biosynthesis sequence

    • Early Phase: Biosynthesis of prerequisite proteins, required before replication can take place, such as replicase enzymes or enzymes that inhibit host-cell synthesis.

    • Late Phase: Synthesis of structural capsid proteins, and actual replication of the genome.

Category: Microbiology Notes