Viruses: General Properties and Structure

Study Notes

Smallest infectious agents, 20-300 nm in diameter
Contain either DNA or RNA
Lack active protein synthesizing apparatus, nucleus, cytoplasm, mitochondria or ribosomes
No metabolic activity outside a host cell, obligate intracellular parasites
Can only replicate inside living cells, not on inanimate media

Genome: DNA or RNA, single-stranded (ss) or double-stranded (ds), circular or linear
Capsid: Protein coat surrounding the genome
- Provides structural symmetry
- Facilitates attachment to host cells
- Protects the viral genome from nucleases in the bloodstream
Capsomeres: Structural units making up the capsid, composed of one or several proteins

Lipoprotein surrounding the capsid
May contain host cell material as well as viral origin
Virus-encoded glycoproteins are exposed on the surface of the envelope
Most human helical viruses are enveloped, while icosahedral can be either enveloped or non-enveloped
Nucleocapsid: The protein-nucleic acid complex
Virion: The complete infective virus particle

More unstable, sensitive to heat, drying, detergents, and alcohols
Often transmitted by direct contact (blood, sexual contact), such as HIV, HBV, HCV, rabies virus, measles, mumps, rubella viruses

Cubic Symmetry (Icosahedral): Have 60 subunits on the surface of an icosahedron, with fivefold, threefold, and twofold rotational symmetry
Helical Symmetry: Elongated nucleocapsid, capsomeres arranged around a spiral of nucleic acid, most helical viruses are enveloped
Complex Symmetry: Does not conform to cubic or helical symmetry

Defective Viruses: Viral nucleic acid and proteins but cannot replicate without a helper virus
Pseudovirions: Contain host cell DNA instead of viral DNA within the capsid, can infect cells but do not replicate
Viroids: Solely composed of a single molecule of circular RNA without a protein coat or envelope, cause plant diseases.
Prions: Infectious particles composed solely of protein without detectable nucleic acid, cause slow diseases like Creutzfeldt-Jacob Disease (CJD) and scrapie.

Attachment: Reactive sites on the virus surface interact with specific receptors on susceptible host cells.
Penetration: The coat of enveloped viruses fuses with the host cell membrane and releases the nucleocapsid into the host cytoplasm. Other viruses enter via endocytosis.
Uncoating: Viral capsid is broken down by viral or cellular enzymes, releasing the viral nucleic acid into the host cell.
Biosynthesis: Viral nucleic acid and protein synthesis occurs, utilizing the host cell's machinery.
- Capsid/Envelope: Structural proteins that build the virion.
- Enzymes: DNA/RNA polymerase, other replication enzymes.
Assembly: Viral nucleocapsids are assembled, in either the nucleus (e.g., Herpes virus, Adenovirus), cytoplasm (e.g., Poliovirus), or at the cell surface (e.g., Influenza virus).
- Inclusion bodies: Accumulation of virions at sites of assembly, visible under a light microscope.
Release: New virions are released from the infected cell.
- Budding: Enveloped viruses.
- Lysis: Non-enveloped viruses, causing cell death.