Each
nucleotide
is
composed
of
(1)
a
phosphate
group,
(2)a
five-carbon
sugar,
and
(3)
a
cyclic
nitrogen
are
commonly
found
in
DNA:
adenine,
guanine,
thymine,
and
cytosine.
RNA
also
usually
contains
adenine,
guanine,
and
cytosine
but
has
a
different
base,
uracil,
in
place
of
thymine.
Adenine
and
guanine
are
double-ring
bases
called
purines;
cytocine,
thymine,
and
uracil
are
single
ring
bases
called
pyrimidines.
Hence
both
DNA
and
RNA
contain
four
different
subunits
or
nucleotides
two
purine
nucleotides
and
two
pyrimidine
nucleotides.
RNA
usually
exists
as
a
single
stranded
polymer
composed
of
a
long
sequence
of
nucleotides.
DNA,
however,
has
one
very
important
additional
level
of
organization;
it
is
usually
a
double
stranded
molecule.
The
correct
structure
of
DNA
was
first
deduced
by
Watson
and
Crick
in
1953
,as
a
double
helix
in
which
the
two
polynucleotide
chains
are
coiled
about
one
another
in
a
spiral.
Each
polynucleotide
chain
consists
of
a
sequence
of
nucleotides
linked
together
by
phosphodiester
bonds,
joining
adjacent
deoxyribose
moieties
