Chapter 9 DNA: The Genetic Material Mrs. Cook Biology.
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Transcript of Chapter 9 DNA: The Genetic Material Mrs. Cook Biology.
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Chapter 9DNA: The Genetic Material
Mrs. Cook
Biology
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Identifying the Genetic Material
• Griffith’s Experiments- 1928 Frederick Griffith- Worked with Streptococcus pneumoniae- was trying to prepare a vaccine
A substance that is prepared from killed or weakened disease-causing agents, and is introduced into the body to protect the body against future infections by the disease-causing agent.
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Identifying the Genetic Material
-Griffith worked with two forms of the S.pneumoniae1st strain:
•has a capsule, which protects the cell from the body’s defenses•it virulent or able to cause disease. • because of the capsule it grows with smooth (S) edged colonies.
2nd Strain: •lacks a capsule•does not cause disease• forms rough (R) colonies
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Identifying the Genetic Material
- Griffith knew:• Mice infected with the S bacteria grew sick
and died.• Mice infected with the R bacteria were not
harmed
- Griffith Experiment:• Injected mice with dead S bacteria and
mice remained healthy• Prepared a vaccine by “heat killing”-
raised temperature of bacteria to the point where they were alive but could no longer reproduce.
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Identifying the Genetic Material
• Then injected mice with “heat killed” S bacteria, the mice lived.
• Then he mixed the harmless live R bacteria with the harmless “heat killed” S bacteria
- The mice died- The live R bacteria had acquired capsules- The harmless R bacteria had become
virulent.
- We now call what happened TRANSFORMATION- a change in genes when cells take in a foreign genetic material
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Identifying the Genetic Material
• Avery’s Experiments
-1944- Avery and his co-worker’s at the Rockefeller Institute in NYC, demonstrated that DNA is the material responsible for transformation.
- Not all scientists accepted this.
- Most thought proteins were the genetic material
- little was known about DNA at this time
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Identifying the Genetic Material• Alfred Hershey and Martha Chase Experiment
- 1952- worked at Cold Spring Harbor Laboratory in NY.
- worked with viruses, which are much simpler than cells, are composed of DNA or RNA surround by a protective protein coat.
-A bacteriophage, which is a virus that, infects bacteria and produce more viruses when the bacterial cell ruptures.
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Identifying the Genetic Material
- The experiment:• The 1st bacteriophage’s outer protein coat was
labeled with radioactive Sulfur.
• The 2nd bacteriophage’s DNA was labeled with radioactive Phosphorous.
• When they each infected a E.coli bacteria, Hershey and Chase discovered that
- the protein coat of the bacteriophage did not affect the E.coli bacteria
- the DNA of the 2nd bacteriophage had been injected into the E.coli.
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Identifying the Genetic Material
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Identifying the Genetic Material
• Chargaff’s Observations
- 1949, a biochemist at Columbia University, NY city.
- observed that adenine paired with thymine, cytosine pairs with guanine in the chemical make up of DNA.
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Identifying the Genetic Material
• Rosalind Franklin’s Photographs
- developed X-ray diffraction
- photographed DNA
- 1st to theorize that DNA was a helix
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Identifying the Genetic Material
• Watson and Crick
- 1953- used finding’s from Chargarff and Franklin
- used their knowledge of chemical bonding
- discovered that DNA is a double helix, a “spiral staircase” of two strands of nucleotides twisting around a central axis
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The Structure of DNA
• DNA is a Double Helix• It is a Nucleic Acid made of two
long chains (also called strands) of repeating subunits called nucleotides
• Each Nucleotide is made of three parts:
1. a Five Carbon Sugar2. a Phosphate Group3. a Nitrogenous Base
Phosphate
5 Carbon Sugar
Nitrogen Base
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The Structure of DNA
• In DNA, the sugar is Deoxyribose
• The phosphate group consists of a Phosphorous atom bonded to four oxygen atoms
• The Nitrogenous Base contains nitrogen atoms, and carbon atoms and is a BASE- accepts hydrogen ions.
• The alternating sugar and phosphate groups are held together by strong covalent bonds and make up the sides of the staircase/ladder.
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Covalent Bonds
Phosphate
5 Carbon Sugar
Nitrogen Base
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The Structure of DNA
• Each full turn of Helix has 10 Nucleotide pairs
• The Nitrogenous Bases face toward the center of the DNA Molecule and Bonds to the Bases on the other side with weak hydrogen bonds.
• 4 different Nitrogenous Bases
- Thymine (T)
- Cytosine (C)
- Adenine (A)
- Guanine (G)
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The Structure of DNA• A and G have a double ring
of carbon and nitrogen atoms and called Purines.
• C and T have a single ring of carbon and nitrogen atoms and are called Pyrimidines
Chargaff’s Rule: A Purine must always pair with a Pyrimidine!
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The Structure of DNA
• Complementary Bases:
- Base Pairing Rules state that
C always bonds with G
A always bonds with T
- These pairs of bases are called Complementary Base Pairs
- The order of the nitrogenous bases on a chain of DNA is called Base Sequence.
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The DNA Code
• The sequence of nucleotide pairs controls how an organism will produce its proteins, therefore DNA controls the structure & function of the organism…it is not random!
• All organisms share the same chemical DNA. The difference is in the number, kind and order of the nucleotides!
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DNA Replication
• DNA Replication is the process by which DNA is copied in a cell before a cell divides by Mitosis or Meiosis. Also called Duplication.
• Three Major Steps:
1. Enzyme called Helicase separates the DNA strands by breaking the hydrogen bonds between the base pairs.
- The Y-shaped region that is formed is called the Replication Fork.
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DNA Replication
2. Enzymes called DNA Polymerases then add complementary nucleotides (which are found floating freely in the nucleus) to the original strands
- at a rate of about 50 nucleotides per second!
3. Bases are added in opposite directions on the strands.
- Synthesis moving away from the replication fork leaves gaps which can later be filled in by the enzyme DNA Ligase.
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DNA Replication
Replication Fork- Hydrogen bonds broken by Helicase.
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DNA Replication
• While the new bases are being added, new covalent bonds are formed on the outside between the new phosphate and sugar molecules
• New hydrogen bonds are formed between the new complementary base pairs on the original and new strands.
• DNA Polymerase falls off, resulting in two separate and identical DNA molecules that are ready to move to new cells in cell division.
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DNA Replication
• After DNA Replication is complete, the end result is the original DNA molecule and an identical new DNA strand.
• This type of replication is called
Semi-Conservative Replication, because each of the new DNA molecules has kept one of the two original DNA strands.
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DNA Replication
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DNA Replication
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DNA Replication
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DNA Replication
• In Prokaryotes, remember that there is one circular DNA molecule.
-DNA Replication begins in one spot
- Two replication forks are formed and proceed in different directions until they meet.
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DNA Replication
• In Eukaryotic, DNA is very long and straight.
- Replication beings at many points, or origins, along the DNA
- at each origin, two replication forks move in opposite directions
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DNA Replication
• Any Error in DNA replication can lead to a mutation.