CLINICAL CASES
19
Helicase:
An enzyme that will catalyze the separation of the strands of the
DNA double helix during replication.
Nucleoside:
A nitrogenous base (purines such as adenine or guanine;
pyrimidines such as uracil, thymine, or cytosine) in an A-P-glycosidic
linkage to a pentose sugar (deoxyribose in DNA and ribose in RNA).
Nucleotide:
A nucleoside that has a phosphoester bond to one of the
hydroxyl groups of the pentose sugar.
Polymerase:
An enzyme that will add nucleotides to a growing nucleic
acid chain using a template strand to determine which nucleotide is
added. A nucleoside triphosphate condenses with the growing strand
releasing pyrophosphate.
Ribavirin:
1-P-D-ribofuranosyl-1A-1,2,4-triazole-3-carboxamide; a purine
nucleoside analog that exhibits antiviral activity against a broad spec-
trum of DNA and RNA viruses.
D ISC U SSIO N
DNA and RNA are both polymers of nucleosides joined by 3
'
,5
'
-phospho-
diester linkages.
Each
nucleoside
consists of a
heterocyclic nitrogenous
base in a glycosidic link with a pentose sugar.
The
backbone
of both DNA
and RNA is formed by the
phosphate bridges between the 3
'
-hydroxyl
group of one pentose and the 5
'
-hydroxyl group of another.
The nitroge-
nous bases form the “side chains.”
DNA contains the bases adenine (A),
guanine (G), cytosine (C),
and
thymine (T),
whereas
RNA contains A,
G, and C but has uracil (U) instead of T.
The
pentose sugar in DNA is
2
'
-deoxyribose,
whereas in
RNA it is ribose.
The
most stable DNA structure
is formed when
two polynucleotide
chains are joined by hydrogen bonding between the side chain bases.
The
base pairing is specific in that
adenine forms hydrogen bonds with thymine,
whereas
guanine forms hydrogen bonds with cytosine.
The result is an
antiparallel double helix in which one polynucleotide strand runs in the
5
'
to 3
' direction,
while the other runs in the 3
'
to 5
' direction. The
phosphate
groups are located on the outside of the double helix
with the
base pairs
forming the “stair steps” in the center of the spiral. RNA,
on the other hand,
is usually
single stranded
(the exception is certain RNA viruses), but the
strand can loop back on itself and form regions of base pairing
(A with U
and G with C). The presence of the
hydroxyl group at the 2
'
-position makes
RNA much more susceptible to hydrolysis and decreases its stability.
The
double helical structure of DNA must be disrupted
during almost all
biological processes
in which it participates, including
DNA replication and
repair, as well as transcription
of the DNA sequence information to RNA.
Experimentally, the
double helix can be separated, or denatured, by
increasing the temperature
to well above 50°C (122°F). If the temperature is
carefully decreased, renaturation occurs when the base pairs reform. Under
these conditions, hybridization can be induced by allowing the single strands
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