Virology Notes For USMLE Step 1 (Microbiology)

A. Virus particles are called virions.

1. Virions are composed of either RNA or DNA that is encased in a protein coat called a capsid.

2. They are either naked or enveloped, depending on whether the capsid is surrounded by a lipoprotein envelope.

3. Virions replicate only in living cells and therefore are obligate intracellular parasites.

4. They cannot be observed with a light microscope.

B. Viral genome.

1. Viral genome may be single stranded or double stranded, linear or circular, and segmented or nonsegmented.

2. Viral-specific enzymes, other proteins within the virion, or both may be associated with the genome.


C. Viral capsid >> capsomers >> (helical, icosahedral (a 20-sided polygon), or complex)

Viral nucleocapsid = capsid and enclosed viral genome = naked viruse

D. Viral envelope >> surrounds the nucleocapsid of enveloped viruses and is composed of viral-specific glycoproteins and host-cell-derived lipids and lipoproteins.

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