CLINICAL CASES
57
Denaturation: The process by which the secondary and tertiary structure
of proteins and nucleic acids is broken down to form random chains. In
the case of DNA, it specifically refers to the separation of double-
stranded DNA into single strands by the breaking of the hydrogen bonds
that form the complementary base pairing, usually by increasing the
temperature.
PCR (polymerase chain reaction): PCR is a method by which DNA or
DNA fragments can be amplified through several steps of denaturation,
annealing, and elongation. Through this process, the amount of target
DNA can be increased by a factor of 106 to 109.
Restriction enzymes: Endonucleases isolated from bacteria that will selec-
tively cleave DNA having specific nucleotide sequences. The enzyme rec-
ognizes particular palindromic nucleotide sequences and will hydrolyze
both strands within that sequence. Molecular biologists use these enzymes
in various recombinant DNA techniques.
Taq DNA polymerase: A DNA polymerase isolated from the thermophilic
bacteria
Thermus aquaticus.
It has the advantage that it remains active dur-
ing the denaturation steps of PCR in which the temperature is increased to
separate the DNA strands.
Palindrome: having the same sequence in the complimentary strand as the
original strand of DNA or RNA when read from the 5' to the 3' direction.
Acyclovir (Acycloguanosine): Acycloguanosine, a prodrug that is activated
to the active acyclo-GTP, which inhibits viral DNA polymerase by acting
as a DNA chain terminator.
D ISC U SSIO N
Herpes simplex viruses (HSV) are members of Herpesviridae which also
includes Epstein-Barr virus (infectious mononucleosis), varicella zoster virus
(chicken pox), and cytomegalovirus. Herpesviruses are large (150 nm diameter)
enveloped linear double-stranded DNA viruses. Because of the phospholipid
envelope, herpesviruses are sensitive to acids, detergents, solvents, and drying.
In general, herpesvirus replication begins with the attachment of the viral par-
ticle to the host cell surface, followed by the fusion of the viral envelope and
cell membrane and release of the viral nucleocapsid into the cell cytoplasm.
The nucleocapsid attaches to the nuclear envelope and then the viral genome
is released into the cellular nucleus. The herpesvirus genome is transcribed
and translated by host factors but it is replicated by a viral encoded DNA poly-
merase. This HSV DNA polymerase is an important target for drug therapy.
The viral nucleocapsids are formed in the nucleus and the envelope is acquired
from the nuclear or Golgi membrane. Mature viral particles are released from
the host cell by exocytosis or cell lysis. Depending on the herpesvirus and type
of host cell that is infected, a lytic, persistent (macrophages and lymphocytes),
or latent (nerve cells) infection can be established.
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