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DNA is one of the strangest molecules known to science.
For one thing, the DNA molecule is incredibly complex.
atoms
Also, DNA is incredibly long – the longest molecule we have
ever discovered.
Lebron James – 6’8”
Lebron James’ DNA – 9’8”
How the (@#$^* does that all fit inside the nucleus of a cell?
Even when DNA is not condensed into chromosomes, it is
coiled to allow it to fit into the nucleus of a cell.
Look at it this way: you have 23 feet of intestines, coiled
inside your abdominal cavity.
Also, DNA is the only molecule we know of that contains
instructions for making other molecules.
R
group
RR
group
R
group
R
group
group
DNA Molecule
Amino Acids
Protein
Most diagrams of DNA look like this:
However, we can see the molecule more clearly if we un-twist
it:
Looking at it this way, we can see that the DNA molecule is
actually double-stranded.
Strand #1
Strand #2
Furthermore, we can see that the DNA molecule is made
from smaller, repeating molecules which are labeled A, T, C,
and G.
DNA
Nucleic acid
R
group
R
group
R
group
R
group
L
D
P
Long,
double-stranded,
polymer
molecule that tells
R
A
A
P
ribosomes
how to arrange amino
acids
into protein.
The DNA polymer is made from nucleotides.
Nucleotides
DNA
Nucleotide
New page
M
D and another nucleic acid, RNA.
R
Monomer
of DNA
A
T
C
There are 4 nucleotides in DNA: adenine,
thymine,
cytosine,
G
and guanine.
J
T
O
Nucleotides can join
together
in any order.
O
P
The order
of nucleotides in DNA determines the protein
that
C
is made (the “genetic code.”)
ATTACGACA =
AGCGGATAT=
GCCATAAGC=
Most diagrams of DNA are highly simplified.
If this is a molecule,
where are the bonds?
We can see that DNA is
made from nucleotides…
but what are the
nucleotides made from?
In reality, DNA looks more like this:
3 parts
nucleotide
Strand #1
Strand #2
Bond
Bond
DNA = deoxyribose
Diagram of a Nucleotide
Nitrogenous base
(contains nitrogen)
Phosphate group
(contains phosphorus)
Deoxyribose
(a 5-carbon
sugar)
Nucleotides join together to form a strand.
DNA, column 3:
S
Each DNA strand
is held
B
together by bonds
between
D
the deoxyribose
and
P
phosphate
groups of each
N
nucleotide.
“sugar-phosphate backbone”
Sugar-phosphate backbone
Sugar-phosphate backbone
A
T
A
T
C
G
T
A
Genetic code
What holds the 2 strands together?
B
The two strands are held together by bonds
between the
N
B
nitrogenous
bases.
“Base pair”
Sugar-phosphate backbone
Sugar-phosphate backbone
T
A
Base pair
A
C
T
T
Base pair
G
A
In DNA, A always bonds with T, and C always bonds with G;
B
this is called the rule of base-pairing.
There are two types of bonds in the DNA molecule.
C
B
S
The covalent
bonds in the backbone
are very strong,
to
C
prevent the genetic code from changing.
Covalent bond
(strong)
Hydrogen bond
(weak)
H
B
P
The hydrogen
bonds holding the base
pairs
together are
W
relatively weak…
Covalent bond
(strong)
Hydrogen bond
(weak)
S
S
R
…which allows the two strands
to separate
during replication.
N
B
The 4 nucleotides have different nitrogenous
bases.
Adenine
Cytosine
Guanine
Thymine
P
A and G are purines
(base has 2 rings). C and T are
P
pyrimidines
(base has 1 ring).
Adenine
Guanine
Purines
Cytosine
Thymine
Looks more like
a pyramid
Pyrimidines
B
P
Purines always form base
pairs
with pyramidines.
Purine
Pyrimidine
Purine
Pyrimidine
Pyrimidine
Purine
Pyrimidine
Purine
I
The two strands in DNA are not identical.
Instead, they are
O
complementary to each other (opposite).
Purine
Pyrimidine
Purine
Pyrimidine
Pyrimidine
Purine
Pyrimidine
Purine
Complimentary:
Complementary:
If you know one DNA strand, you can figure out the
complementary strand using base-pairing rules.
T A C C G A T A C
A T G G C T A T G
Now you know the structure of DNA: two long,
complementary strands of nucleotides, held together in the
middle by weak hydrogen bonds.
There’s one last thing you need to know about
DNA structure…
The two strands of DNA form a double helix.
Double helix
M
T parallel strands
S
Shape of the DNA molecule;
formed by two
T
that twist
around each other.
Single helix
Double helix
N
A
C
Nucleic
acids
are twisted molecules because the covalent
B
B
bonds in their backbones
are bent.
Bent bonds = spiral shape