2Who Was the First Person To Isolate DNA? Friedrich Meischer1870’s
3Griffith’s Experiment 1928Fredrick GriffithBacteriologistTrying to prepare a vaccine against pneumonia
4Griffith’s Experiment Two types, or strains, of S. pneumoniaeFirst 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 dishHelps make the microorganism virulent (able to cause disease).
5Griffith’s Experiment Second strain lacks the polysaccharide capsule and does not cause disease.Forms rough-edges (R) when grown in a petri dish
7Griffith’s DiscoveryThe 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
8Avery’s Experiment 1944 Oswald Avery Rockefeller Institute in New York Along with Colin MacLeod & Maclyn McCardyRockefeller Institute in New YorkRepeated Griffith’s experiment to determine which molecule in heat-killed bacteria was most important for transformation.
9Avery’s ExperimentMade 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
10Avery’s Discovery DNA was the transforming factor! Repeated the experiment using an enzymes that would break down DNA.Transformation did not occur.DNA was the transforming factor!
11What Scientists KnewAvery’s experiment clearly indicated genetic material is composed of DNAMany scientist remain skepticalProteins are important to many aspects of the cell structure & metabolism, so most suspected that proteins were the genetic materialScientist knew very little about DNA
12What Scientists KnewViruses 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 virusesReleased when the bacterial cells rupture.
13What Scientists Didn’t Know How the bacteriophage reprograms the bacterial cell to make viruses.Does the bacteriophage DNA, the protein, or both issue instructions to the bacteria?
14The Hershey-Chase Experiment 1952Alfred Hershey & Martha ChaseScientists at Cold Spring Harbor Laboratory, in New YorkUsed the bacteriophage T2 to answer this question.
15The 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.
16The Hershey-Chase Experiment Grew T2 with E. coli bacteria in a nutrient medium that contained radioactive sulfur (35S)The protein coat would incorporate the 35SGrew 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
17The Hershey-Chase Experiment 35S-labeled & 32P-labeled phages were used to infect two separate batches of E. coli bacteria
18The Hershey-Chase Experiment They waited a few minutes for the viruses to inject their genetic materialNext, they separated the viruses from the bacteria & tested the bacteria for radioactivity
19Hershey-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.
26Chargaff’s Observation 1947Erwin ChargaffThe amount of adenine (A) always equaled the amount of thymine (T)A = TThe amount of guanine (G) always equaled the amount of cytosine (C)G = C
27Wilkins & Franklin’s Photographs 1952Maurice Wilkins & Rosalind FranklinKing’s College in LondonDeveloped high-quality X-ray diffraction photographs of strands of DNASuggested DNA molecule resembled a tightly coiled helix & was composed of 2 or 3 chains of nucleotides
28James Watson & Francis Crick 1953Developed the first 3-D model of DNAHad to take into account both Chargaff’s findings & Frankin and Wilkins’s X-ray diffraction data
29Base-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.
35DNA ReplicationDNA 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.
36DNA ReplicationThe 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.
38DNA ReplicationAt the replication fork, enzymes known as DNA polymerases move along each of the DNA strandsDNA polymerases add nucleotides to the exposed nitrogen bases, according to the base-pairing rules.Two new double helixes are formed.
39DNA ReplicationOnce 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.
40Checking 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!
41Rate of ReplicationThe 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.