DNA Structure - hrsbstaff.ednet.ns.ca

Historical Events
DNA Structure & Replication
1869
Bio 12
A. Allen
C.P. Allen High School
Friedrich Miescher identified DNA, which he called nuclein, from pus
cells
1889
Richard Altman renamed nuclein nucleic acid
1931
Griffith discovered that genetic information could be passed from one
bacterium to another; known as the transforming principle
1944
Avery showed that the transforming material was pure DNA not protein,
lipid or carbohydrate.
1952
Hershey and Chase used bacteriophage (virus) and E. coli to show that
only viral DNA entered the host
1953
Watson and Crick discovered the structure of DNA was a double helix
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Walter Sutton-1902
Friedrich Miescher-1869
• observed that chromosomes obey
Mendel's rules of inheritance
• Suggested that “genes” are located on
chromosomes.
• Noticed that “genes” are inherited in
the same fashion as chromosomes.
• Swiss chemist
• Isolated the nuclei from pus cells.
• Discovered a chemical that didn’t act like
protein. He named it “nuclein”
• Many chemists continued his work
• Found that nuclein was rich in
phosphorus and contained no sulfur
(unlike proteins)
• Found that nuclein contained an acidic
substance they termed “nucleic acid”
• Found 2 types of nucleic acids: DNA &
RNA
Thomas Hunt Morgan-1910
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• Stated that each “gene” had a locus on a
particular chromosome.
• Used Drosophila melanogaster (fruit fly) for
his studies.
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BIG questions remained…
Hit the PAUSE button…
• At this point, the idea of “genes” was an
accepted notion. It was a very abstract idea:
gene = trait that can be passed on.
• DNA had been discovered. Chromosomes had
been discovered.
• Nobody had yet made the connection between
genes & DNA.
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• What IS the genetic material? What molecular
substance is a “gene” made of?
• Scientists agreed that—no matter what substance
genes were made of—this substance must be:
1. able to store information
2. stable so that it can be copied and passed on
3. able to undergo rare changes called mutations in
order for evolution to occur
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Frederick Griffith-1931
Joachim Hammerling 1930's.
• Giffith was trying to make a vaccine to prevent pneumonia infections in the
"Spanish flu" influenza pandemic by using two strains of the Streptococcus
pneumoniae bacterium.
• The smooth strain (S strain) had a polysaccharide capsule and was virulent .
• The rough strain (R strain) had a no polysaccharide capsule and was avirulent
• Hammerling’s
experiments with
Acetabularia suggested
that hereditary
information was
localized to nuclei.
The capsule is a
slimy layer on the
cells' surface that
allows the bacteria
to resist the human
immune system.
The rough strain
(R strain) did not
cause pneumonia
when injected into
mice (it was
avirulent), since it
lacked a capsule.
Somehow the type rough strain had been transformed into smooth strain, a process
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Grififth christened the transforming principle
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•
So Avery had proven that the genetic
material was DNA, right?
Oswald Avery & associates-1944
• Griffith’s experiment led Avery to design
his own experiments to identify the
“transforming agent.”
• Avery’s evidence showed:
1. DNA from S strain bacteria caused
R strain bacteria to be transformed.
2. Enzymes that degrade proteins and
RNA did not prevent transformation.
3. Enzymes that digest DNA did
prevent transformation.
4. The DNA segment that transformed
the bacteria contained about 1600
nucleotides—enough for genetic
variability to be possible.
• Nope. Some folks still didn’t buy it.
• The skeptics replied that maybe DNA was just used to activate proteinbased genes.
• In the 1950s, bacteriophages were beginning to be used as scientific tools.
•
Scientists of the day were still asking: What does the virus inject into
the bacteria? That’s where we’ll find the answer to our question:
• Is protein or DNA the genetic material??
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Alfred Hershey/Martha Chase-1952
Hershey & Chase experiment
Hershey & Chase’s conclusion?
Note: DNA contains phosphorus but no sulphur
Protein contains sulphur but no phosphorus
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DNA—not protein—is the genetic
material! DNA transmits all the genetic
information needed to produce new
viruses.
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Now Put it all together!
Case Closed! Well, not quite…
• After almost 100 years, scientists had finally
proven that DNA is the material that genes are
made of.
• But they only had a foggy idea of what DNA
really was.
• What was its structure?
Scientist/Date
Key Discovery.
(if applicable, what previous works were used?)
Miescher-1869
Found that DNA was rich in phosphorus and contained no sulfur
(unlike proteins)
Hammerling 1930s
Genetic information is in the nucleus
Griffith 1928
Genetic material could be passed from bacterium to another
(transforming principle)
Avery 1944
showed that the transforming material was pure DNA not protein.
(Used Griffith’s work on the transforming principle)
Hershey & Chase
1952
DNA—not protein—is the genetic material!
(Used Meischer’s work –Phosphorous in DNA, not Sufur)
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Since Friedrich Mieschner discovered “nuclein” in 1869, chemists
had been working to figure out its chemical composition
Chemists knew DNA
was a polymer made of
nucleotide monomers.
Nucleotides are classified into 2 groups
based on structure:
1. Purines
contain these bases…
– Adenine
– Guanine
They also knew that
each nucleotide
consisted of:
1.a phosphate group
2. a 5-carbon sugar
3.One of 4 nitrogencontaining bases
2. Pyrimidines
= 5 carbon
sugar
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Purine Nucleotides
contain these bases…
– Cytosine
– Thymine (DNA only)
– Uracil (RNA only)
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Pyrimidines Nucleotides
Study tip: small name = BIG BASE STRUCTURE (2 RINGS)
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Study tip: BIG NAME = small base structure (one ring)
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Erwin Chargaff-1940s
•
•
•
Stop & Think
Remember: each nucleotide
has one of four possible
bases, A, T, C & G
Chargaff analyzed DNA from
various species.
Conclusions of his
experiments came to be
known as “Chargaff’s Rules”
Suppose you were Chargaff who observed the ratios of A, T, C & G
we just discussed. Use deductive reasoning to determine how the bases
pair up.
1. What can you say about the relative amounts of A, T, C
& G in different species
2. What can you say about the relative amounts of A, T, C
& G in a given species
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Rosalind Franklin-1953
Chargaff’s Rules
• DNA from any cell of all organisms should
have a 1:1 ratio of pyrimidine and purine
bases.
• The amount of guanine is equal to cytosine &
the amount of adenine is equal to thymine.
• An expert in X-ray crystallography. Used this technique to
discover the physical shape of DNA!
• This was the first indication that DNA was composed of a
double helix structure which had a constant diameter of 2nm.
• Died at age of 37 cancer
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Watson & Crick’s model of DNA
structure
James Watson & Francis Crick-1953
• The rules of complementary base pairing
(or nucleotide pairing) are:
• Constructed a model for DNA
structure
• Awarded the Nobel Prize in 1962
• Rosalind Franklin’s work was
essential to the Watson and Crick
model. She died of cancer due to
overexposure to x-rays. Her lab
partner, Maurice Wilkins was also
awarded the Nobel Prize along
with Watson & Crick because of
his contributions to Ms.
Franklin’s work.
•
Purines are always paired with
pyrimidines but only in certain
combinations…
• A with T: the purine adenine (A)
always pairs with the pyrimidine
thymine (T)
• C with G: the purine guanine (G)
always pairs with the pyrimidine
cytosine (C)
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Watson & Crick’s model of DNA
structure
Stop & Think
So why can`t…
• The 2 strands of DNA are held
together by the hydrogen bonds
between the complimentary bases.
A (purine) pair with C (pyrimidine)
or
G (purine) pair with T (pyrimidine)?
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Stop & Think
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What`s this?
Why can`t …
A (purine) pair with G (purine)
or
C (pyrimidine) pair with T (pyrimidine)?
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DNA Packaging
DNA Structure
• DNA
– Double Helix
• Histones
– Proteins
• Nucleosome
– DNA coils around
histones.
• Coils
• Supercoils
• Chromosome
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Strands of DNA are Antiparallel
Linked Nucleotides in DNA
•
•
•
•
Nucleotides are linked
by phosphodiester
bonds.
Hydroxyl groups from
the 3rd carbon and a
phosphate group (off
5th carbon) of two
nucleotides react to form
water.
A phosphodiester bond5
is a type of
condensation reaction
•
That wraps up one mystery!
The two strands in a
DNA molecule run
antiparallel to each
other (the two strands
have opposite
orientations; the 5' end
of one strand aligns
with the 3' end of the
other strand.
3’ and 5’ pertain to the
3rd and 5th carbons in
the deoxyribose
molecules.
Now we know “what”… but how?
• What is the genetic material?
–DNA!
• How is it passed on to the next
generation? How is it copied?
But in science, new information almost
always raises new questions…
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