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DNA Discovery and Structure
Interest Grabber• Order! Order!• Genes are made of DNA, a large, complex molecule. DNA is
composed of individual units called nucleotides. Three of these units form a code. The order, or sequence, of a code and the type of code determine the meaning of the message.
1. On a sheet of paper, write the word cats. List the letters or units that make up the word cats.
2. Try rearranging the units to form other words. Remember that each new word can have only three units. Write each word on your paper, and then add a definition for each word.
3. Did any of the codes you formed have the same meaning?4. How do you think changing the order of the nucleotides in
the DNA codon changes the codon’s message?Go to Section:
DNA Discovery and Structure
Section Outline
• 12–1 DNAA. Griffith and Transformation
1. Griffith’s Experiments
2. Transformation
B. Avery and DNA
C. The Hershey-Chase Experiment
1. Bacteriophages
2. Radioactive Markers
D. The Structure of DNA
1. Chargaff’s Rules
2. X-Ray Evidence
3. The Double Helix
Section 12-1
Go to Section:
DNA Discovery and Structure
Discovery of Genetic Material
• Mendel’s work actually created more questions in the minds of scientists.
• What are genes made of?• Chromosomes consist of two materials DNA
wrapped around a protein.• Which of the two materials carried the genetic
code?• Transformation• A process where one strain of bacteria is
changed to another
DNA Discovery and Structure
KEY CONCEPT DNA was identified as the genetic material through a series of experiments.
DNA Discovery and Structure
DNA Discovery and StructureDNA
I.Deoxyribonucleic Acid
A. Chromatin/chromosomes
B. In nucleus
C. Functions:
i. Stores , copies & transmits genetic information (genes); genes code for proteins determining cell functions & traits
13.1 Ecologists Study Relationships
•Is DNA the genetic material?•3 experiments = evidence
1.1928 Frederick Griffith – discovers that a heritable factor in bacteria can transform harmless bacteria into deadly bacteria in mice
II. A Genetic MysteryII. A Genetic Mystery
DNA Discovery and Structure
1. Griffith’s Transformation
•An army medical officer•attempting to develop a vaccine against pneumonia, but unexpectedly advances molecular biology instead•isolated and cultured two different strains of the pneumonia bacteria
R = rough surface, not deadly
S = smooth surface (protein coat), deadly
Turn to p 339 in your book.
DNA Discovery and Structure
TWO types of Streptococcus pneumonia
a. Smooth edged colony (S type) - have a polysaccharide (sugar) capsule around the bacteria's cell membrane:capsule which protects the bacteria from the cell's defenses and causes PNEUMONIA
b. Rough edged colony (R type) - lack the sugar capsule and does not cause pneumonia
DNA Discovery and Structure
Griffith’s Transformation
CLUE: Live S cells found in mouse body injected with heat-killed S cells!!!!
DNA Discovery and Structure
What Happened?
Maybe the heat-killed “S” cells in the mixture weren’t really killed. If this was true, what would have happened to the mice in Experiment 3?
Maybe the harmless “R” cells in the mixture can mutate into a killer form. If this was true, what would have happened to the mice in Experiment #1?
DNA Discovery and Structure
DISCOVERY:
1) The heat killed = harmless R bacteria was changed (transformed) into the deadly S form of bacteria
2) Genetic material transferred between dead
bacteria and living bacteria.
WHY did transformation take place? What was transferred to the R bacteria?
We know now that some bacteria can take up DNA released from other cells and use it.
DNA Discovery and Structure
The Importance of Griffith’s Work
Griffith called this “transformation.”
He had shown that something had been transferred from the heat-killed disease-causing bacteria to the live harmless bacteria that could be inherited.
.
Investigators began searching for this unknown material.
13.1 Ecologists Study Relationships
2.1944 Oswald Avery – shows that DNA is the factor in Griffith’s experiment by selectively destroying different types molecules & checking whether transformation still occurred
II. A Genetic MysteryII. A Genetic Mystery
DNA Discovery and Structure
Oswald Avery's Experiment (1944)
Project: Repeat Griffith's experiment with a TWIST
R bacteria + "Heat killed" S bacteria mixture: Used an
ENZYME that destroyed RNA and sugars
(polysaccharides)
RESULT: Transformation still took place and the mice died.
R bacteria + "Heat killed" S bacteria mixture: Used an
ENZYME that destroyed DNA.
RESULT: Transformation did not take place, and mice
lived!!!
DNA Discovery and Structure
Avery’s Experiments
CLUE: Only culture not transformed is treated with DNAse (enzyme cuts up DNA).
DNA Discovery and Structure
DISCOVERY:
1) DNA-destroying enzymes prevented harmless bacteria from transforming into deadly bacteria
2) DNA was the genetic material NOT protein.
13.1 Ecologists Study Relationships
3.1952 Alfred Hershey & Martha Chase - use radioactive markers to trace where DNA and proteins are located during a bacteriophage infection; supports Avery.
DNA is the genetic material!
II. A Genetic MysteryII. A Genetic Mystery
DNA Discovery and Structure
Alfred Hershey and Martha Chase Experiment (1952)
Background of Experiment:
Studied the effect of a bacteriophage on bacteria
BACTERIOPHAGE = a virus that infects bacteria only
Bacteriophage attaches to bacteria and injects its DNA
into the bacteria - protein capsule remains outside the
Bacteria.
DNA becomes part of the bacteria's DNA - instructs the
bacteria to make hundreds of new phages.
Bacteria becomes full of new virus and finally bursts -
releasing new bacteriophages
DNA Discovery and Structure
Hershey and Chase confirm that DNA is the genetic material.
• Hershey and Chase studied viruses that infect bacteria, or bacteriophages.
• Tagged DNA was found inside the bacteria; tagged proteins were not.
– They tagged viral DNA with radioactive phosphorus.– They tagged viral proteins with radioactive sulfur.
DNA Discovery and Structure
Hershey & Chase
DNA Discovery and Structure
Hershey & Chase
Clue: Radioactivity inside cell only when attached to DNA
DNA Discovery and Structure
Mystery Solved: Mystery Solved:
All evidence points to DNA as All evidence points to DNA as the genetic material. the genetic material.
No evidence points to protein.No evidence points to protein.
II. A Genetic MysteryII. A Genetic Mystery
DNA Discovery and Structure
Checkpoint
1. What is the function of DNA?
2. What is the molecular function of genes?
3. Describe the three experiments that demonstrated that DNA must be the genetic material?
24ANY QUESTIONS?
DNA Discovery and Structure
III. A Race to DNA Structure
Once known that DNA is the molecule of heredity – new question:
HOW DOES IT WORK?
Like all biological molecules and their chemistry, the key to function lies in their structure!
Probably the most important biological discovery of the 20th century.
DNA Discovery and Structure
• Competition, personality clashes and secret collaborations describe the race to be the first to find the actual molecular structure of DNA.
• What did we know in 1920?• – DNA is a nucleic acid.
III. A Race to DNA Structure III. A Race to DNA Structure
DNA Discovery and Structure
KEY CONCEPT DNA structure is the same in all organisms.
DNA Discovery and Structure
A. DNA is a Nucleic Acid
1. nucleic acid - chains of nucleotides linked by covalent bonds.
2. DNA Nucleotide
a) phosphate,b) deoxyribose
(sugar) c) Nitrogen base
DNA Discovery and Structure
2. DNA Nucleotide 2. DNA Nucleotide
O
O=P-O
O
PhosphateGroup
NNitrogenous baseNitrogenous base
(A, G, C, or T)(A, G, C, or T)
CH2
O
C1C4
C3 C2
5
Deoxyribose Deoxyribose (sugar)(sugar)
Draw & label in your notes
DNA Discovery and Structure
• The nitrogen containing bases are the only difference in the four nucleotides.
DNA Discovery and Structure
Purines PyrimidinesAdenine Guanine Cytosine Thymine
Phosphate groupDeoxyribose
Figure 12–5 DNA NucleotidesSection 12-1
Go to Section:
DNA Discovery and Structure
TAC
G
Nucleotides always pair in the same way.
• The base-pairing rules show how nucleotides always pair up in DNA.
• Because a pyrimidine (single ring) pairs with a purine (double ring), the helix has a uniform width.
– A pairs with T– C pairs with G
DNA Discovery and Structure
Percentage of Bases in Four Organisms
Section 12-1
Source of DNA A T G CSource of DNA A T G C
Streptococcus 29.8 31.6 20.5 18.0
Yeast 31.3 32.9 18.7 17.1
Herring 27.8 27.5 22.2 22.6
Human 30.9 29.4 19.9 19.8
Streptococcus 29.8 31.6 20.5 18.0
Yeast 31.3 32.9 18.7 17.1
Herring 27.8 27.5 22.2 22.6
Human 30.9 29.4 19.9 19.8
Go to Section:
DNA Discovery and Structure
B. Chargaff’s Rules – 1949 Erwin Chargaff - amount of Adenine always equal to amount of Thymine, Cytosine always equal to Guanine and amount of Purines = pyrimidines in a DNA Purines = pyrimidines in a DNA sample sample
III. A Race to DNA Structure III. A Race to DNA Structure
Due to Due to Base Pair Rules: Base Pair Rules: AA bonds with bonds with TT
& & GG bonds with bonds with CC
DNA Discovery and Structure
Checkpoint Quiz
1. What kind of macromolecule is DNA?
2. What are the three parts of a nucleotide?
3. How do you tell purines and pyrimidines apart?
4. Which bases are paired together?
5. According to Chargaff’s rules, if 30% of DNA sample is Adenine, how much is Thymine?
6. How much is Cytosine?
35 3535
ANY QUESTIONS?
DNA Discovery and Structure
1950 - How do these 4 building blocks fit together to form the molecule responsible for all of the similarity and diversity of LIFE??
C. 1952 Linus Pauling – Nobel American chemist uses modeling (toys!) to propose first structure.
– he suggests a
triple helix DNA
III. A Race to DNA Structure III. A Race to DNA Structure
DNA Discovery and Structure
D. 1952 Rosalind Franklin & Maurice Wilkins capture X-ray pattern of DNA – key clue to structure; not a triple helix
Hated each other but worked in same lab
III. A Race to DNA Structure III. A Race to DNA Structure
DNA Discovery and Structure
DNA Discovery and Structure
• Watson and Crick’s discovery built on the work of Rosalind Franklin and Erwin Chargaff.
– Franklin’s x-ray images suggested that DNA was a double helix of even width.
– Chargaff’s rules stated that A=T and C=G.
DNA Discovery and Structure
At the college down the road, two students, using Pauling’s models, see Franklin’s photo from Wilkins and publish their own idea – WITHOUT crediting Franklin!
III. A Race to DNA Structure III. A Race to DNA Structure
DNA Discovery and Structure
E. 1953 Watson & Crick reveal 3-D model of DNA molecule: DOUBLE HELIX with 2 complementary antiparallel strands joined by hydrogen bonds between base pairs
III. A Race to DNA Structure III. A Race to DNA Structure
- used the information from the above scientists and established the structure of DNA (with the unknowing help of Rosalind Franklin)
DNA Discovery and Structure
Watson and Crick determined the three-dimensional structure of DNA by building models.
• They realized that DNA is a double helix that is made up of a sugar-phosphate backbone on the outside with bases on the inside.
DNA Discovery and Structure
Hydrogen bonds
Nucleotide
Sugar-phosphate backbone
Key
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
Figure 12–7 Structure of DNASection 12-1
Go to Section:
DNA Discovery and Structure
III. A Race to DNA Structure III. A Race to DNA Structure 1.Double Helix – 1.Double Helix – 2 strands in a twisted ladder 2 strands in a twisted ladder
with covalent bonded sugar-phosphate rails with covalent bonded sugar-phosphate rails and H-bonded base pair rungsand H-bonded base pair rungs
2.Complementary – 2.Complementary – strands strands match according to base match according to base pairing rules (A-T, C-G)pairing rules (A-T, C-G)
• If know one strand, then can If know one strand, then can code for the other.code for the other.
• Important for copying!Important for copying!
DNA Discovery and Structure
DNA Model3.Antiparallel = 2 strands of DNA run in opposite directions (5’ carbon v 3’carbon at end)
Draw & label in your notes
Sugar-
phosphate rail
3’
5’
DNA Discovery and Structure
E.1958- Franklin dies of cancer at 37.
F. 1962 Watson & Crick (& Wilkins)
Win Nobel Prize for double helix DNA
III. A Race to DNA Structure III. A Race to DNA Structure
DNA Discovery and Structure
Checkpoint
1. Who is credited with the double helix model?
2. What is a double helix?
3. Which nucleotide parts make up the rails of a DNA molecule?
4. Which nucleotide parts make up the rungs?
5. If a DNA strand reads ATCGTAG, what does the complementary strand read?
47ANY QUESTIONS?