Lecture 9
DNA Structure & Replication
What is a Gene?
 Mendel’s work left a key question unanswered:
 What is a gene?
 The work of Sutton and Morgan established that
genes reside on chromosomes
 But chromosomes contain proteins and DNA
 So which one is the hereditary material
 Several experiments ultimately revealed the
nature of the genetic material
The Griffith Experiment
 In 1928, Frederick Griffith discovered transformation
while working on Streptococcus pneumoniae
 The bacterium exists in two strains
 S
 Forms smooth colonies in a culture dish
 Cells produce a polysaccharide coat and can cause disease
 R
 Forms rough colonies in a culture dish
 Cells do not produce a polysaccharide coat and are therefore
harmless
How Griffith Discovered Transformation
Thus, the dead S bacteria
somehow “transformed” the live R
bacteria into live S bacteria
The Avery and Hershey-Chase Experiments
 Two key experiments that demonstrated conclusively
that DNA, and not protein, is the hereditary material
 Oswald Avery and his coworkers Colin MacLeod and
Maclyn McCarty published their results in 1944
 Alfred Hershey and Martha Chase published their
results in 1952
The Avery Experiments

Avery and his colleagues prepared the same mixture of dead
S and live R bacteria as Griffith did


They then subjected it to various experiments
All of the experiments revealed that the properties of the
transforming principle resembled those of DNA
1.
2.
3.
4.
Same chemistry and physical properties as DNA
Not affected by lipid and protein extraction
Not destroyed by protein- or RNA-digesting enzymes
Destroyed by DNA-digesting enzymes
The Hershey-Chase Experiment
 Viruses that infect
bacteria have a simple
structure
 DNA core surrounded
by a protein coat
 Hershey and Chase used
two different radioactive
isotopes to label the protein
and DNA
 Incubation of the labeled
viruses with host bacteria
revealed that only the DNA
entered the cell
 Therefore, DNA is the
genetic material
Thus, viral DNA
directs the
production of new
viruses
Discovering the Structure of DNA
 Chemically, DNA is made up of nucleotides
 Each nucleotide has a central sugar, a phosphate group and an
organic base
 The bases are of two
main types:
Nitrogenous
base
 Purines
Large bases:
 Adenine (A)
 Guanine (G)
 Pyrimidines
Small bases:
 Cytosine (C)
 Thymine (T)
5-C
sugar
More Key Observations About DNA
 Erwin Chargaff made key DNA observations that became known as
Chargaff’s rule
Purines = Pyrimidines
A=T
C=G
Rosalind Franklin’s X-ray
diffraction experiments revealed
that DNA had the shape of a
coiled spring or helix
Rosalind
Franklin (19201958)
The Structure of DNA Revealed
 In 1953, James Watson and Francis Crick deduced that DNA was a
double helix
They came to their conclusion using Tinker toy models and the research of
Chargaff and Franklin
James Watson
(1928)
Francis Crick
(1916-2004)
The DNA Double Helix
 The two DNA strands are held
together by weak hydrogen bonds
between complementary base pairs
 A and T
 C and G
 If the sequence on one strand is
ATACGCAT
TATGCGTA
 The other’s sequence must be
The two
possible
base pairs
 Each chain is a complementary
mirror image of the other
 So either can be used as template
to reconstruct the other
Dimensions
suggested by
X-ray diffraction
3 possible methods for DNA replication
Daughter DNAs
contain one old
and one new
strand
Original DNA
molecule is
preserved
Old and new
DNA are
dispersed in
daughter
molecules
Evidence for Semi-Conservative Replication

These three mechanisms were tested
in 1958 by Matthew Meselson and
Franklin Stahl
Thus, DNA replication
is semi-conservative
Overview of How DNA Copies Itself
 DNA helices begin unwinding
from the nucleosomes
 Helicase untwists the double
helix and exposes
complementary strands
 The enzyme primase puts
down a short piece of RNA
termed the primer
 DNA polymerase reads along
each naked single strand
adding the complementary
nucleotide
 The site of replication is the
replication bubble
 Each nucleotide strand serves
as a template for building a
new complementary strand
PLAY
DNA Replication Overview
How nucleotides are added in DNA replication
Template strand
HO 3’
Sugarphosphate
backbone
New strand
Template strand
New strand
HO 3’
5’
5’
C
C
P
G
O
O
O
O
P
P
T
A
T
P
O
O
A
P
O
O
P
P
T
A
T
A
P
O
DNA polymerase
C
G
C
P
G
P
O
O
P
P
A
3’
OH
A
O
T
O
O
P
P
Pyrophosphate
P
A
O
T
A
P P P
O
O
P
5’
O
P
O
O
P
O
O
P
P
G
P
OH
5’
3’
OH
Summary of How DNA Copies Itself
 The process of DNA
replication can be
summarized as such
 The enzyme helicase first
unwinds the double helix
 The enzyme primase puts
down a short piece of RNA
termed the primer
 DNA polymerase reads
along each naked single
strand adding the
complementary nucleotide
DNA polymerase can only build a strand of DNA in one direction
The leading strand is made continuously from one primer
The lagging strand is assembled in segments created from many primers
DNA Replication
 Since DNA polymerase
only works in one direction:
 A continuous leading
strand is synthesized
 A discontinuous lagging
strand is synthesized
 DNA ligase splices
together the short
segments of the
discontinuous strand
 Two new telomeres are
also synthesized
 This process is called
semiconservative
replication
DNA Replication (cont.)
RNA primers are removed and replaced with DNA
Ligase joins the ends of newly-synthesized DNA
Mechanisms exist for DNA proofreading and repair
PLAY
DNA Replication
Transcription
 The path of genetic information is often called the central
dogma
DNA
RNA
A cell uses three kinds of RNA to make proteins
Messenger RNA (mRNA)
Transfer RNA (tRNA)
Ribosomal RNA (rRNA)
Protein
Transcription
 Gene expression is the use of information in DNA to direct
the production of proteins
 It occurs in two stages
Transcription
 The transcriber is
RNA polymerase
It binds to one DNA strand at a site
called the promoter
It then moves along the DNA
pairing complementary nucleotides
It disengages at a stop signal
Working with DNA
 Key techniques used by today’s genetic engineers include
 PCR amplification
 Used to increase the amounts of DNA
 cDNA formation
 Used to build genes from their mRNA
 DNA fingerprinting
 Used to identify particular individuals