DNA
The Molecular Basis of
Inheritance
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Identifying the Genetic
Material
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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
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Polysaccharide Capsule
“Smooth” edged colonies
Virulent – able to cause disease
R strain
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No Capsule
“Rough” edged colonies
Nonvirulent - does not cause disease
Griffith’s Experiment
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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
The Search for what caused
the Transformation…
1944 – Oswald Avery, MacLeod, & McCarty
(American Bacteriologists)
Experiment:
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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
Hershey & Chase Experiment
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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
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32P-labels mostly in the bacterial cells, and new phages also
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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
Structure of DNA
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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:
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Purines – double ring of C and N
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Adenine
Guanine
Pyrimidines – single ring of C and N
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Cytosine
Thymine
Discovering DNA’s Structure
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Erwin Chargaff (NYC)
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1947 – DNA composition varies among different
species
1949 -Chargaff’s Rules- Discovered regularity of
ratios:
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# Adenines = # Thymines
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# Guanines = # Cytosines
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(ie. Humans A =30%, T=30%)
(ie. Humans G = 20%, C = 20%)
1952 Rosalind Franklin & Maurice Wilkins
(England)
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Developed X-ray crystallography photographs of
DNA
Suggested “helix” shape of 2-3 chains of
nucleotides
April 25th, 1953
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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
h-bond to a pyrimidine
2 strands of DNA are
complimentary to each
other
2 strands are anti-parallel
1962 Awarded the Nobel
Prize
More on DNA
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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!
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
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
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DNA Replication: Process of Synthesizing new molecules of DNA
Helicase breaks H-bonds and opens up the double helix forming
replication forks (point at which DNA separates)
At the replication fork, DNA Polymerases continuously adds
complimentary nucleotides to exposed bases
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
1.
2.
3.
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http://www.youtube.com/watch?v=L9RjNNfgaEQ
DNA Synthesis
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 nucleotides and
replaces it with the correct one(s).
Only 1 error per 1 billion nucleotides!
http://www.youtube.com/watch?v=oJZH4lV3h
6I&mode=related&search=
DNA Replication & Aging
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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