The Genetic
Material
 DNA can be found in the nucleus of
eukaryotic
(animals, plants, some
single celled organisms) cells, and in the
cytoplasm of prokaryotes
(bacteria).
The Structure of DNA
 DNA (DEOXYRIBONUCLEIC ACID) is a
polymer composed of monomers
called NUCLEOTIDES.
Each nucleotide consists of
three subunits:
1. A five-carbon sugar
2. A phosphate group
3. A nitrogen containing base
DNA
(Polymer)
Made up of
NUCLEOTIDES
(monomer)
contain
PHOSPHATE
Group
5 CARBON
SUGAR
NITROGEN
CONTAINING
BASE
A nucleotide looks like this
Phosphate
group
Nitrogen
base
5 Carbon sugar
 The sugar and phosphate molecules
do not change in each nucleotide but the
nitrogen base may be one of four
different kinds.
 The 4 bases in DNA are:
 ADENINE (A)
 GUANINE (G)
 THYMINE (T)
 CYTOSINE (C)
 Two scientists, Watson and Crick,
determined that the DNA molecule
consists of two strands twisted around
each other into a double helix
resembling a spiral staircase.
If DNA were to be flattened out it
would look like a ladder.
The 2 strands
are held
together by
hydrogen bonds
between the
pairs of bases.
 The sugar-phosphate molecules in
the nucleotide are like the 2 side rails
of the ladder while the nitrogencontaining bases are similar to the
steps or rungs of the ladder.
 In DNA these nitrogen bases
always pair so that :
 T pairs with A and
 G pairs with C
 thymine - adenine pair interacts
through
two hydrogen bonds
T=A
 cytosine-guanine pair interacts
through
three hydrogen bonds
C
G
 The base pairs have a “specific fit” for each
other- like Cinderella’s foot and shoe!
Base-Pairing Principle
 The base pairing is called
complementary because 2 specific
bases bond together to make a
complete unit.
 hydrogen bonds connect the 2
bases.
 The specificity of this bonding
results in the 2 DNA strands running
in opposite directions.
 A completed double
strand of DNA looks like
the picture to the left.
Note that the sugars
point in opposite
directions. Each strand is
antiparallel to the other
strand. This orientation
is important in DNA
replication.