DNA: THE GENETIC
MATERIAL
CHAPTER 11
DISCOVERING THE STRUCTURE OF
DNA
• DNA is comprised of
subunits called
nucleotides.
• Each DNA nucleotide
has three parts:
• A central
deoxyribose sugar.
• A phosphate
group.
• An organic base.
DISCOVERING THE STRUCTURE OF
DNA
• Nucleotides differ with regards to their bases
• Large bases (purines) with double-ring structure
• either adenine (A) or guanine (G)
• Small bases (pyrimidines) with single rings
• either cytosine (C) or thymine (T)
DISCOVERING THE STRUCTURE OF
DNA
• Edwin Chargaff noted that DNA molecules
always had equal amounts of purines and
pyrimidines.
• Chargaff’s rule suggested that DNA had a regular
structure.
• The amount of A always equaled the amount of
T
• The amount of C always equaled the amount of
G
NUCLEOTIDES
DISCOVERING THE STRUCTURE OF
DNA
• Rosalind Franklin’s
work in 1953 using Xray diffraction
showed that DNA
had a regular
structure that was
shaped like a
corkscrew, or helix.
DISCOVERING THE STRUCTURE OF
DNA
• Francis Crick and James Watson elaborated
on the discoveries of Franklin and Chargaff
and deduced that the structure of DNA was
a double helix.
• Two strands of DNA bound together by hydrogen
bonds between the bases.
• Because a purine of one strand binds to a
pyrimidine on the other strand to form a base
pair, the molecule keeps a constant thickness.
HOW THE DNA MOLECULE COPIES
ITSELF
• The two strands of DNA that form the double
helix DNA molecule are complementary to
each other.
• Each chain is essentially a mirror image of the
other.
• This complementarity makes it possible for DNA to
copy itself in preparation for cell division.
HOW THE DNA MOLECULE COPIES
ITSELF
• The process of DNA replication involves
several enzymes:
• DNA polymerase
• Adds the correct complementary nucleotide to
the growing daughter strand, but can only add
nucleotides to the 3´ end of an existing strand.
• Helicase
• Unwinds the DNA to expose the templates.
• This creates a replication fork.
• DNA ligase
• Seals fragments of DNA together.
HOW NUCLEOTIDES ARE ADDED IN DNA
REPLICATION
Template strand
New strand
HO 3’
Template strand
C
3′
HO
5’
5′
C
P
G
New strand
P
G
P
P
Sugarphosphate
backbone
T
A
P
P
T
A
A
T
P
P
T
A
P
P
DNA polymerase III
P
P
C
C
G
P
1′
2′
P
A
4′
G
P
5′
P
3′
3’ OH
A
T
P
T
P
A
P
1′
4′
2′ 3′
P
P
P
P
5′
A
OH
P
5′
3′ OH
5′
10
HOW THE DNA MOLECULE COPIES
ITSELF
• At the replication fork, a primer
must first be added to give a place
for DNA polymerase to start.
• Using one template, DNA polymerase adds
nucleotides in a continuous fashion; this new
daughter strand is called the leading strand.
2 Priming the
Leading Strand
5′
3′
1 Unwinding
Parental
DNA helix
DNA polymerase IIIPrimase
3′
5′
Helicase
3′
Primer
Leading strand
Template strands
Single-strand
binding proteins
3′
Helicase
5′
3′
5′
Replication fork
Single-strand
binding proteins
5′
HOW THE DNA MOLECULE COPIES
ITSELF
• Because the other template is a mirror image,
directionality becomes a problem because DNA
polymerase can build a new strand in one direction
only.
• This second daughter strand is assembled in
segments, each one beginning with a primer.
• The segments are joined together by DNA ligase to
form the lagging strand. 4 Priming and Building the
Lagging Strand
Single-strand
binding protein
Helicase
5’
3’
3 Building the
Leading Strand
DNA polymerase I
3’
DNA polymerase III
5’
Helicase
5’
3’
3’
Single-strand
binding proteins
Leading strand
5’
5’
3’
3’
Primase
Leading
strand
Okazaki
Primer fragment
DNA polymerase III
DNA polymerase I
5’
3’
Lagging
strand
3’
5’
DNA ligase
HOW THE DNA MOLECULE COPIES
ITSELF
• Before the newly formed DNA molecules
wind back into the double helix shape, the
primers must be removed and the DNA
fragments sealed together.
• DNA ligase joins the ends of the fragments of DNA
to form continuous strands.
http://www.youtube.com/watch?v=TEQMeP9GG6M
HOW THE DNA MOLECULE COPIES
ITSELF
• Because so much DNA is being replicated in
the many cells of the body, there is a
potential for errors to occur.
• DNA repair involves comparing the daughter
strand to the parent DNA template to check for
mistakes.
• The proofreading is not perfect because
mutations are still possible, although rare;
however, genetic variation is the raw material
of evolution.
MUTATION
• There are 2 main ways in which the genetic
message encoded in DNA can be altered.
• Mutation
• Results from errors in replication.
• Can involve changes, additions, or deletions
to nucleotides.
• Recombination
• Causes change in the position of all or part
of a gene.
MUTATION
• Mutations can alter the genetic
message and affect protein synthesis.
• The effect of a mutation depends on the
identity of the cell where it occurs.
• Mutations in germ-line cells - will be
passed to future generations
• Important for evolutionary change
• Mutations in somatic cells are not passed
to future generations but passed to all
other somatic cells derived from it.
MUTATION
• Some mutations
alter the sequence
of DNA nucleotides.
• Base substitution
changes the identity
of a base or bases.
• Insertion adds a base
or bases.
• Deletion removes a
base or bases.
DNA
A
G
A
G
T
A
C
T
A
G
G
A
T
C
T
C
A
T
G
A
T
C
C
T
Serine
Histidine
Aspartate
Proline
DNA replication
DNA
A
G
A
G
T
A
C
T
A
T
G
A
T
C
T
C
A
T
G
A
T
C
C
T
Serine
Histidine
Aspartate
Threonine
(a) Base substitution (red) in DNA: changes G to T in the DNA strand
and, as a result, proline to threonine in the protein.
Normal
protein
Mutated
protein
(b) The mutated protein with the amino acid substitute folds
differently than the normal protein and its function will most likely
be affected.
MUTATION
• If the insertion or deletion throws the reading
frame of the genetic message out of
register, a frame-shift mutation results.
• These are extremely detrimental because the final
protein intended by the message may be altered
or not made.
MUTATION
• Some mutations affect how a genetic
message is organized.
• Transposition occurs when individual genes
move from one place in the genome to
another.
• Sometimes entire regions of chromosomes
may change their relative location or
undergo duplication.
• This is called chromosomal rearrangement.
MUTATION
• All evolutionary change begins with
alterations in the genetic message.
• Mutation and recombination provide the raw
materials for evolution.
MUTATION
• Chemicals or radiation that cause mutation
are called mutagens.
• For example, chemicals in cigarette smoke and
UV light can cause cancer.