Medical Microbiology & Immunology
Lecture 16 Intoduction to Virology Structure, Classification, Replication, Genetics and
Pathogenesis of Viruses
1) To understand the difference between viruses and other microbes. Viruses are the
smallest of all microorganisms with a replicating set of genes, DNA or RNA,
packaged into a protein coat, which require intracellular growth because they depend
on host cell machinery for replication.
2) To describe the structure of human or animal viruses. Virus structures vary but
the two basic kinds of viruses are: naked capsid viruses, which are not enveloped but
do have a defined external capsid and a genome protein core, and enveloped viruses,
which have an envelope containing a nucleic-acid protein complex with a
surrounding matrix.
a) Genome structure can be DNA (double stranded linear/circular/incomplete, or
single stranded) or RNA (single stranded linear/segmented/circular or double
stranded; with +, –, or mixed +/– polarity).
b) Capsid structure can be cylindrical (nucleocapsid), where protein subunits
associate with the nucleic acid to form a helix, or spherical having 20 triangular
faces, 12 vertices, and 30 sides. Other forms include: Capsomeres, Bullet, Brick,
and Filamentous.
c) Envelope structure is derived from cellular or nuclear membranes forming a
bilayer lipid membrane with proteins, glycoproteins, and peplomers that bind
receptors, aid in fusion, and elicit immune responses.
3) To understand the patterns of viral infections. Five patterns of viral infection exist.
a) Acute viral infection followed by clearance.
b) Acute viral infection followed by latent infection and periodic reactivation.
c) Acute infection followed by chronic infection.
d) Slow chronic infections.
e) Acute followed by persistent infection and finally overproduction (AIDS).
4) To understand the criteria used for viral classification. Properties of Virions,
Properties of Genome, Properties of Proteins, Properties of Envelope, Replication,
Physical properties, and Biological properties.
Virus Family: Designated with suffix – viridae, ex: Herpesviridae
Virus Subfamilies: with suffix – virinae, ex: Herpesvirinae
Virus Genera: - with suffix virus, ex: Herpesvirus
Virus species: - with virus, ex: Herpes simplex virus 2
Properties include: DNA or RNA, cubic or helical, enveloped or naked, doublestranded or single-stranded, positive, negative or both, and linear, circular or
segmented genome.
5) To describe the six discrete phases of a typical virus replication cycle.
a) Adsorption to the host cell – binding to host cell receptors.
b) Penetration/entry – fusion (enveloped) or receptor-mediated endocytosis (naked).
c) Uncoating – removal of viral proteins thus releasing genome into the cytoplasm.
d) Synthetic phase – transcription, translation, genome replication.
e) Virus Assembly – proteins aggregate then nucleic acids incorporate
f) Virus Release – virion either buds off (enveloped) or host cell lyses.
6) To know the one step growth curve.
Latent phase: no virions outside the cells
Eclipse phase: no virions outside or inside the cells
Early phase: synthetic phase, viral gene expression
for genome replication
Late phase: begins with synthesis of proteins
necessary for construction of the new particles.
Burst size: number of infectious virions released
per average cell.
7) To know how viruses are quantified.
Plaque formation (most common) – viruses plated on cells, plaques are lysed cells.
Pock formation (embryonated eggs)
Focus formation (retroviruses)
Animal inoculation
Hemagglutination assay
Optical density, Electron microscopy
Antigen assay, Enzymatic assay
Immunological assays:
Radioimmunoassay (RIA)
Enzyme–linked immunosorbent assay (ELISA)
Molecular assays: PCR, RNA assays
8) To understand viral-host cell interactions and pathogenesis.
a) Permissive cell: permits production of progeny virus particles and/or viral
transformation.
b) Non–permissive cell: virus cannot replicate, but it may be able to transform the
cell.
c) Abortive infection: no progeny virus particles are produced; however, the cell
may die because early viral functions.
d) Persistent infection: small number of virus particles are produced; little or no
CPE.
e) Interference: Infection by virus A will inhibit second infection with virus B.
f) Transformation: induces unregulated cellular growth; cells form tumors in
susceptible animals.
g) Latency: viral genetic material remains in host cell without production of virus;
may be activated at a later time to produce virus and/or transform the host cell.
h) Cytopathic effects (CPE)
1. Nucleus: inclusion bodies, nucleus thickening, swelling, nucleolar changes.
2. Cytoplasm: inclusion bodies, vacuoles,
9) To recognize the role of interferon in viral infections. Interferons are small
proteins produced by cells in response to virus infection, mitogens or immune
stimulus. Interferon is activated by double stranded RNA upon viral infection. Both
RNA and DNA viruses induce interferon production.
Three major types of interferon:
a) INF- α (Leukocyte) virus induced
b) INF-β (Fibroblasts) virus induced
c) INF-γ (T-cells) antigen or mitogen induced
Interferon activates oligoadenylate synthetase, which activates ribonuclease that
degrades mRNA. It also activates protein kinase that phosphorylates and inactivates
one of the subunits of initiation factor eIF-2 necessary for protein synthesis, both
enzymes require double stranded RNA for activation. All protein synthesis is
inhibited thus preventing the virus from replicating and destroying the cell.
Uninfected cells will be activated by the double stranded RNA if the virus enters the
cell thereby only killing the infected cells.
10) To recognize the mechanisms of genetic change. Genetic change occurs through
mutation and recombination. Mutation of only a few nucleotides can cause subtle
changes in a viral antigen thus allowing the virus to escape pre-existing antibody or
primed cytotoxic T-lymphocytes (Ex. Influenza and HIV). Recombination is the
exchange of genetic information (RNA or DNA) with related, co-infected viruses.
11) To distinguish between antigenic drift and shift. Antigenic drift occurs due to
mutations that accumulate resulting in changes in the antigenic nature of the virus.
This occurs with influenza A envelope proteins, hemagglutinin and neuraminidase,
causing antigens that escape recognition by previously formed antibodies. Antigenic
shift occurs due to genetic recombination with other viruses resulting in new viral
strains. This has occurred with influenza and HIV causing pandemics.
12) To understand viral transformation and oncogenicity of viruses. Viral
transformation is the conversion of normal cells to abnormal tumor cells (malignant
or benign). This occurs when viral genes are incorporated into the chromosome and
expressed continually. Oncogenic or cancer causing viruses are usually the
retroviruses but all known DNA animal viruses (except parvoviruses) are capable of
causing aberrant cell proliferation under certain circumstances.