Microbiology Chapter 6: Viral Multiplication

Five basic steps: attachment, entry, synthesis, assembly, and virion release.
Attachment involves specific interactions with host cell receptors and microdomains.
Entry occurs when the nucleocapsid penetrates the host cytoplasm; includes uncoating and membrane fusion.
During synthesis, there is strict regulation of viral gene expression and protein production.
Assembly involves multiple subassembly lines that later join to form the complete nucleocapsid structure.
Virion release occurs through lysis or budding, facilitated by viral proteins.

Both viruses and cellular organisms possess genetic material that can replicate and be inherited.
Viral genetic material is enclosed within a protein shell, unlike cellular organisms which have a membrane.
Viruses encode both DNA and RNA but require a host for replication due to missing essential proteins.
This dependency on a host leads to viruses not being classified as living organisms.

Nucleocapsid consists of nucleic acid (DNA or RNA) and forms the simplest viral structure.
Capsid is the protective protein coat surrounding the nucleocapsid.

Three types of capsid symmetry: helical, icosahedral, and complex.
Helical capsids resemble hollow tubes; icosahedral capsids are formed from ring or knob-shaped protomers.
Complex capsids do not fit into helical or icosahedral categories, seen in poxviruses and large bacteriophages.

Cellular genomes typically contain double-stranded (ds) DNA.
Viral genomes can be dsDNA, ssDNA, dsRNA, ssRNA; variations are common across plant and bacterial viruses.
Small genomes (~4,000 nucleotides) can code for 3-4 proteins; large genomes, like mimiviruses, can exceed 1.2 million nucleotides.

Infections vary:
- Lytic infections result in rapid multiplication and acute diseases.
- Latent infections persist with the viral genome present.
- Chronic infections release virions slowly without cell death.
- Some infections can transform host cells, activating proto-oncogenes leading to cancer.

Lysogeny is the integration of a bacterial virus's DNA into the host genome, beneficial for the virus.
Temperate phages can either lyse the host or remain within without destruction.

Viral receptors evolve from essential host surface proteins, highlighting their importance to host cell functions.
Enveloped viruses are rare in plants and bacteria due to cell walls differing from animal cell membranes, complicating envelope formation.