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DNA: The Genetic Material. Chapter 9 Section 1. Who Was the First Person To Isolate DNA?. Friedrich Meischer 1870’s. Griffith’s Experiment. 1928 Fredrick Griffith Bacteriologist Trying to prepare a vaccine against pneumonia. Griffith’s Experiment.

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DNA: The Genetic Material

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dna the genetic material

DNA: The Genetic Material

Chapter 9 Section 1

griffith s experiment
Griffith’s Experiment
  • 1928
  • Fredrick Griffith
  • Bacteriologist
  • Trying to prepare a vaccine against pneumonia
griffith s experiment1
Griffith’s Experiment
  • Two types, or strains, of S. pneumoniae
  • First strain is enclosed in a capsule composed of polysaccharides.
    • Capsule protects the bacterium from the body’s defense system.
    • Forms smooth-edges (S) when grown in a petri dish
  • Helps make the microorganism virulent(able to cause disease).
griffith s experiment2
Griffith’s Experiment
  • Second strain lacks the polysaccharide capsule and does not cause disease.
    • Forms rough-edges (R) when grown in a petri dish
griffith s discovery
Griffith’s Discovery
  • The harmless R bacteria had changed and became virulent S bacteria.
  • Transformation is a change in genes caused when cells take up foreign material.
    • Genes: sections of DNA in a chromosome that code for traits
avery s experiment
Avery’s Experiment
  • 1944
  • Oswald Avery
    • Along with Colin MacLeod & Maclyn McCardy
  • Rockefeller Institute in New York
  • Repeated Griffith’s experiment to determine which molecule in heat-killed bacteria was most important for transformation.
avery s experiment1
Avery’s Experiment
  • Made an extract, or juice, from the heat-killed bacteria.
  • Treated the extract with enzymes that destroyed proteins, lipids, carbohydrates, and other molecules, including RNA.
  • Transformation still occurred
avery s discovery
Avery’s Discovery
  • Repeated the experiment using an enzymes that would break down DNA.
  • Transformation did not occur.
  • DNA was the transforming factor!
what scientists knew
What Scientists Knew
  • Avery’s experiment clearly indicated genetic material is composed of DNA
  • Many scientist remain skeptical
  • Proteins are important to many aspects of the cell structure & metabolism, so most suspected that proteins were the genetic material
  • Scientist knew very little about DNA
what scientists knew1
What Scientists Knew
  • Viruses are composed of DNA or RNA surrounded by a protective protein coat.
  • Bacteriophage (phage) is a virus that infects bacteria.
  • When phages infect bacterial cells, the pages are able to produce more viruses
    • Released when the bacterial cells rupture.
what scientists didn t know
What Scientists Didn’t Know
  • How the bacteriophage reprograms the bacterial cell to make viruses.
  • Does the bacteriophageDNA, the protein, or both issue instructions to the bacteria?
the hershey chase experiment
The Hershey-Chase Experiment
  • 1952
  • Alfred Hershey & Martha Chase
  • Scientists at Cold Spring Harbor Laboratory, in New York
  • Used the bacteriophage T2 to answer this question.
the hershey chase experiment1
The Hershey-Chase Experiment
  • Knew the only molecule in the phage that contains phosphorus is its DNA.
  • The only phage molecules that contain sulfur are the proteins in its coat.
the hershey chase experiment2
The Hershey-Chase Experiment
  • Grew T2 with E. coli bacteria in a nutrient medium that contained radioactive sulfur (35S)
    • The protein coat would incorporate the 35S
  • Grew T2 with E. coli bacteria in a nutrient medium that contained radioactive phosphorus (32P)
    • The radioactive phosphorus would become part of the cell’s DNA
the hershey chase experiment3
The Hershey-Chase Experiment
  • 35S-labeled & 32P-labeled phages were used to infect two separate batches of E. coli bacteria
the hershey chase experiment4
The Hershey-Chase Experiment
  • They waited a few minutes for the viruses to inject their genetic material
  • Next, they separated the viruses from the bacteria & tested the bacteria for radioactivity
hershey chase discovery
Hershey-Chase Discovery
  • Nearly all the radioactivity in the bacteria was from phosphorus (32P), the marker found in DNA.
  • Concluded that the DNA of viruses is injected into the bacterial cell, while most of the viral proteins remained outside.
  • Causes bacterial cells to produce more viral DNA and proteins.
  • DNA is the hereditary material.
the structure of dna

The Structure of DNA

Chapter 9 Section 2

structure of dna
Structure of DNA
  • Double helix- two strands twisted around each other, like a winding staircase.
  • Each strand is made of linked nucleotides.
  • 1920’s
  • The subunits that make up DNA.
  • 3 parts
    • Phosphate group
    • A 5-Carbon sugar molecule (deoxyribose)
    • Nitrogen-containing base
      • Any one of 4 different bases
purines pyrimidines
Purines & Pyrimidines
  • Purines are nitrogen bases made of 2 rings of carbon & nitrogen atoms
    • Adenine
    • Guanine
  • Pyrimidines are nitrogen bases made of a single ring of carbon & nitrogen atoms
    • Thymine
    • Cytosine
chargaff s observation
Chargaff’s Observation
  • 1947
  • Erwin Chargaff
  • The amount of adenine (A) always equaled the amount of thymine (T)
    • A = T
  • The amount of guanine (G) always equaled the amount of cytosine (C)
    • G = C
wilkins franklin s photographs
Wilkins & Franklin’s Photographs
  • 1952
  • Maurice Wilkins & Rosalind Franklin
  • King’s College in London
  • Developed high-quality X-ray diffraction photographs of strands of DNA
  • Suggested DNA molecule resembled a tightly coiled helix & was composed of 2 or 3 chains of nucleotides
james watson francis crick
James Watson & Francis Crick
  • 1953
  • Developed the first 3-D model of DNA
  • Had to take into account both Chargaff’s findings & Frankin and Wilkins’s X-ray diffraction data
base pairings

Watson & Crick determined:

  • A purine on one strand of DNA is always paired with a pyrimidine on the opposite strand.
  • An adenine on one strand always pairs with a thymine on the opposite strand.
  • A guanine on one strand always pairs with a cytosine on the opposite strand.
the replication of dna

The Replication of DNA

Chapter 9 Section 3

  • Summarize the process of DNA replication.
  • Describe how errors are corrected during DNA replication.
  • Compare the number of replication forks in prokaryotic and eukaryotic DNA.
key terms
Key Terms
  • DNA Replication
  • DNA Helicase
  • Replication Fork
  • DNA Polymerase
dna replication
DNA Replication
  • DNA replication is the process of making a copy of DNA.
  • Watson & Crick proposed that one DNA strand serves as a template, or pattern, on which the other strand is built.
dna replication1
DNA Replication
  • The double helix unwinds, caused by an enzyme (DNA helicase).
  • DNA helicases open the double helix by breaking the hydrogen bonds that link complementary base pairs.
  • Once separated additional proteins attach to the ends to keep them apart.
dna replication2
DNA Replication
  • At the replication fork, enzymes known as DNA polymerases move along each of the DNA strands
  • DNA polymerases add nucleotides to the exposed nitrogen bases, according to the base-pairing rules.
  • Two new double helixes are formed.
dna replication3
DNA Replication
  • Once DNA polymerase have begun adding nucleotides to a growing double helix, the process continues until all of the DNA has been copied & the polymerase is signaled to detach.
checking for errors
Checking for Errors
  • DNA polymerase has a “proofreading” role.
  • It can only add a new nucleotide if the previous nucleotide was correct.
  • If it is incorrect, the polymerase will go back and remove the incorrect nucleotide & replace it with the correct one.
  • Reduces errors in DNA replication to 1 error per 1 billion nucleotides!
rate of replication
Rate of Replication
  • The replication of a typical human chromosome with one pair of replication forks spreading from a single point, would take 33 days!
  • Each human chromosome is replicated in about 100 sections that are 100,000 nucleotides long, each section with its own starting point.
  • As a result, an entire human chromosome can be replicated in about 8 hours.