Outline the classification of viruses giving examples of each class (60% marks). Describe the mechanism of action of acyclovir and oseltamivir (40% marks).
Viruses are classified according to
a) genetic material
b) mode replication
c) structural proteins (capsids)
d) presence of an envelope
Thus DNA viruses, double or single stranded DNA usually replicate in the nucleus of
the host cell via polymerase, not incorporated into the host genetic material.
Examples being double stranded, herpes, adenovirus, poxvirus and single stranded,
parvovirus. In comparison, RNA viruses, single strand and have 2 different
reproduction strategies, being RNA sense(positive) and RNA antisense(negative), an
example is paramyxovirus. For retroviruses, the single stranded RNA can’t act as
mRNA and is transcribed into DNA by a reverse transcriptase. This DNA is
incorporated into the host DNA, so the host makes the viral RNA, for example HIV.
Candidates were also expected to briefly mention capsids and viral envelopes.
In relation to the second part of the question, candidates were expected to mention
that acyclovir inhibits DNA polymerase in the terminal nucleic acid chain and that
oseltamivir is a neuraminidase inhibitor which prevents the budding of new viruses
from the infected cells. Most candidates had very little knowledge of this area.
Reference: Medical Microbiology and Infection at a Glance, Gillespie &, Bamford pgs
58,59, Basic and Clinical Pharmacology, Katzung pgs791,815