DNA: The Molecular
Basis of Inheritance
Hbio
Ms. Pagodin
Do Now:
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
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Happy Pi Day!
Grab your clickers
Talk to your classmates and find out who
read the same article as you!
In your article group, discuss the experiment
and conclusion!
Nuclear Composition?

1868- Johann Miescher

Collected pus & fish sperm

Isolated and identified acidic compound with
nitrogen and phosphorus…. Today we know it as
Deoxyribonucleic acid
Molecule of Heredity

Is it Proteins or Nucleic Acid??
What makes up proteins?
33%
33%
ch
a
on
os
ac
M
Am
in
o
ac
id
r id
es
s
es
C.
33%
ot
id
B.
Nucleotides
Amino acids
Monosaccharides
Nu
c le
A.
What makes up nucleic acids?
33%
33%
ch
a
on
os
ac
M
Am
in
o
ac
id
r id
es
s
es
C.
33%
ot
id
B.
Nucleotides
Amino acids
Monosaccharides
Nu
c le
A.
How many amino acids are
there?
25%
25%
60
D.
25%
20
C.
25%
16
B.
4
16
20
60
4
A.
How many different DNA
nucleotides are there?
25%
25%
60
D.
25%
20
C.
25%
16
B.
4
16
20
60
4
A.
Identifying the Genetic
Material

1928 Fredrick Griffith (English Bacteriologist)


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Trying to find a vaccine for pneumonia
Vaccine: prepared from killed/weakened microorganisms
introduced into the body to produce immunity
Griffith worked with 2 strains of Streptococcus pneumoniae
bacteria
 S strain




Polysaccharide Capsule
“Smooth” edged colonies
Virulent – able to cause disease
R strain



No Capsule
“Rough” edged colonies
Nonvirulent - does not cause disease
Griffith’s Experiment


Griffith’s Conclusion: Something had passed from heat killed
bacteria to the nonvirulent R strain making them virulent… he called
this the “transforming principal”
Griffith did not know what it was, but many scientists thought it was
proteins
Today we know…



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Transformation – cells take up foreign genetic
material, changing their own genes (used for genetic
engineering)
Heat killed S bacteria – enzymes were denatured
therefore the DNA could not be copied
Proteins are denatured at 600C and DNA is
denatured at 900C
DNA of heat killed S bacteria survived and
transformed DNA of R bacteria
Virulent strains
D
C&
A&
B
di
se
as
e
e
tc
au
no
Do
no
th
av
ea
se
di
se
ca
ps
ul
as
e
e
se
Do
F.
su
l
E.
Ca
u
D.
ca
p
C.
17% 17% 17% 17% 17% 17%
ea
B.
Have a capsule
Cause disease
Do not have a
capsule
Do not cause
disease
A&B
C&D
Ha
v
A.
se
Fa
l
Tr
ue
Transformation is the addition of
genes to another organisms genome
A. True
50%
50%
B. False
The Search for what caused
the Transformation…
1944 – Oswald Avery, MacLeod, & McCarty
(American Bacteriologists)
Experiment:


1.
2.
Added protease to “R and heat-killed S” mixture
Result  Mice died
Added DNAase to “R and heat-killed S” mixture
Result  Mice Lived
Conclusion:
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DNA, not protein, is the transforming factor in
Griffith’s experiment
More Evidence that DNA is the
Genetic Material…
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1952 – Alfred Hershey & Martha Chase (NY)
Used T2 bacteriophages (phage) – virus
that infects bacteria

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Composed of nucleic acid surrounded by a
protein coat
Viruses infect specific host
Viruses are not living

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Not composed of cells
Cannot reproduce on their own
Do not grow and develop
Background Info on Viruses
Which type of virus is chicken
pox?
ic
50%
so
ge
n
50%
ly
B.
Lytic
lysogenic
Ly
t ic
A.
Which type of virus is the flu?
ic
50%
so
ge
n
50%
ly
B.
Lytic
lysogenic
Ly
t ic
A.
Hershey & Chase Experiment
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
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Experiment:
1.
Grew T2 w/radioactive Sulfur 35S (protein coat takes in 35S)
2.
Grew another group of T2 w/ radioactive Phosphorus 32P (DNA
takes in the 32P)
35S-labeled and 32P–labeled phages were used to infect E.Coli
3.
bacteria
4.
Separated phages from bacteria using a blender and a
centrifuge… the bacterial cells at bottom and viral parts at the
top
Results:
35S-labels still in viral parts

32P-labels mostly in the bacterial cells, and new phages also

contained 32P DNA
Conclusion:
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Viral DNA (not protein) enters bacteria and carries instructions
on how to make more phages
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Without a doubt, DNA is the hereditary material!
Hershey & Chase Experiment
A bacteriophage
33%
xp
...
r ia
33%
be
no
t
Ca
n
Is
a
vir
u
us
ed
le
nt
b
fo
re
ac
te
...
ba
c
ts
nf
ec
ti
st
ha
C.
33%
vir
u
B.
Is a virus that
infects bacteria
Is a virulent
bacteria
Cannot be used
for experiments
Is
a
A.
Structure of DNA?

Linus Pauling


Nobel prize for deducing structure of protein
Collagen
If protein structure could be determined and
modeled, why not DNA?
Structure of DNA


By 1950’s most scientists were convinced that
 Chromosomes carry genetic material
 Genes are on chromosomes
 Genes are made of DNA
Basic Structure of DNA
 Composed of nucleotides
 Nucleotides made of 3 parts deoxyribose, phosphate, N base
 2 types nitrogen bases:

Purines – double ring of C and N



Adenine
Guanine
Pyrimidines – single ring of C and N


Cytosine
Thymine
Discovering DNA’s Structure

Erwin Chargaff (NYC)


1947 – DNA composition varies among different
species
1949 -Chargaff’s Rules- Discovered regularity of
ratios:

# Adenines = # Thymines


# Guanines = # Cytosines

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(ie. Humans A =30%, T=30%)
(ie. Humans G = 20%, C = 20%)
1952 Rosalind Franklin & Maurice Wilkins
(England)


Developed X-ray crystallography photographs of
DNA
Suggested “helix” shape of 2-3 chains of
nucleotides
April 25th, 1953

James Watson & Francis Crick
(England)
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Built the 1st accurate 3D (tin
and wire) model of DNA
“Double Helix” – spiral
staircase
Purine is always linked by hbond to a pyrimidine
2 strands of DNA are
complimentary to each other
2 strands are anti-parallel
5’(phosphate end)
3’(deoxyribose end)
1962 Awarded the Nobel Prize
More on DNA


Ex. If the sequence of bases on one strand is AATGCGCAT,
than the complimentary strand will be: ________________
Human DNA has 3 billion base pairs.. Less than 1% of our
DNA makes us different from one another!
Which seems most likely?
Models of DNA Synthesis

Semiconservative
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Conservative
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ea/ daughter molecule
will have 1 new strand
and 1old strand
Parent molecule reforms
Dispersive

All 4 strands have a
combination of old and
new strands
Assignment:
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Propose an experiment to determine how
DNA replication occurs
1950’s Meselson & Stahl
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Cultured Ecoli on medium
labeled w/ 15N nt
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Transferred EColi to medium
labeled w/ 14N nt
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Centrifuge after each
replication and analyze
Origin of Replication
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Prokaryotic Cell – single origin of replication where proteins
separate the 2 strands and create a replication bubble,
replication proceeds in both directions from the replication fork
Eukaryotic Cells – hundreds or thousands of replication
bubbles form to speed up the copying process, replication
proceeds in both directions from the replication fork
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http://sites.fas.harvard.edu/~biotext/animation
s/replication1.swf
DNA Replication
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Watson and Crick proposed that the complimentary strand of DNA serves
as a template for which the other strand is built…experiments confirmed
this 5 years later

DNA Replication: Process of Synthesizing new molecules of DNA
Helicases catalyze the breaking of H-bonds (driven by ATP) and
opens up the double helix forming replication forks (point at which
DNA separates into single strands)
Topoisomerase temporarily bind to relieve strain ahead of
replication fork
Single-strand binding protein – binds to unpaired DNA strands until
they serve as templates for new complimentary strand
1.
2.
3.
Elongation
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
1.
2.
3.
DNA pol adds 50 nt/sec in Euk cells
Each nt is a nucleoside triphosphate
At the replication fork, DNA Polymerase III continuously adds
complimentary nucleotides to exposed bases on 3’ end of new
strand, this is called the leading strand
DNA polymerase III must work away from the replication fork on
the other strand, the lagging strand, to follow the 5’-3’ direction
creating short segments of DNA called Okazaki fragments.
DNA Ligase joins the Okazaki fragments together.
Process continues until all DNA has been copied, end result is 2
new molecules of DNA each identical to the original and
composed of one new and one old strand
Priming DNA Synthesis



DNA pol can not initiate – only add nt to 3’ end of
existing chain
Primer – short chain (5-10nt) of RNA
Primase – enzyme starts RNA chain from scratch



Leading strand – 1 primer needed
Lagging strand – 1 primer needed for ea/Okazaki
fragment
DNA pol I replaces RNA nt of primers w/DNA
versions
DNA Synthesis
http://www.dnai.org/a/index.html
Proofreading
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


DNA polymerase only moves to the next
nucleotide if the previous nucleotide was a
correct match
If mismatched, DNA Polymerase backs up,
removes the mismatched nucleotide(s) and
replaces it with the correct one(s).
Repair enzymes can recognize and repair
damaged sites too
Only 1 error per 1 billion nucleotides!
DNA Replication & Aging
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
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Every time DNA is copied,
DNA polymerase cannot
complete replication on the
ends
Eukaryotic DNA has a noncoding, repeating nucleotide
sequence on the ends called
telomeres that protects
genes from being eroded
over successive replications
It is believed that telomeres
are directly related to the
aging process