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 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 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 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 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 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 Repeated the experiment using an enzymes that would break down DNA. Transformation did not occur. DNA was the transforming factor!
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 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 How the bacteriophage reprograms the bacterial cell to make viruses. Does the bacteriophage DNA, the protein, or both issue instructions to the bacteria?
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 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 Experiment Grew T2 with E. coli bacteria in a nutrient medium that contained radioactive sulfur ( 35 S) –The protein coat would incorporate the 35 S Grew T2 with E. coli bacteria in a nutrient medium that contained radioactive phosphorus ( 32 P) –The radioactive phosphorus would become part of the cell’s DNA
The Hershey-Chase Experiment 35 S-labeled & 32 P-labeled phages were used to infect two separate batches of E. coli bacteria
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 Nearly all the radioactivity in the bacteria was from phosphorus ( 32 P), 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.
Structure of DNA Double helix- two strands twisted around each other, like a winding staircase. Each strand is made of linked nucleotides.
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 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
How was the actual structure of DNA discovered?
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 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 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: 1.A purine on one strand of DNA is always paired with a pyrimidine on the opposite strand. 2.An adenine on one strand always pairs with a thymine on the opposite strand. 3.A guanine on one strand always pairs with a cytosine on the opposite strand.
Objectives 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 DNA Replication DNA Helicase Replication Fork DNA Polymerase
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 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 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 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 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 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.