Noun
an
organic
molecule
in
which
a
nitrogenous
heterocyclic
base
(
or
nucleobase
),
which
can
be
either
a
double-ringed
purine
or
a
single-ringed
pyrimidine
,
is
covalently
attached
to
a
five-carbon
pentose
sugar
(
deoxyribose
in
DNA
or
ribose
in
RNA
).
When
the
phosphate
group
is
covalently
attached
to
the
pentose
sugar
,
it
forms
a
nucleotide
.