DNA: The Genetic Material

1. DNA: The Genetic Material Chapter 9 Section 1

2. Who Was the First Person To Isolate DNA? • Friedrich Meischer • 1870’s

3. Griffith’s Experiment • 1928 • Fredrick Griffith • Bacteriologist • Trying to prepare a vaccine against pneumonia

4. 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).

5. Griffith’s Experiment • Second strain lacks the polysaccharide capsule and does not cause disease. • Forms rough-edges (R) when grown in a petri dish

6. Griffith’s Experiment

7. 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

8. 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.

9. 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

10. Avery’s Discovery • Repeated the experiment using an enzymes that would break down DNA. • Transformation did not occur. • DNA was the transforming factor!

11. 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

12. 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.

13. 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?

14. 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.

15. 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.

16. 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

17. The Hershey-Chase Experiment • 35S-labeled & 32P-labeled phages were used to infect two separate batches of E. coli bacteria

18. 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

19. 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.

20. The Structure of DNA Chapter 9 Section 2

21. Structure of DNA • Double helix- two strands twisted around each other, like a winding staircase. • Each strand is made of linked nucleotides.

22. 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

23. 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

24. Nitrogen Bases

25. How was the actual structure of DNA discovered?

26. 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

27. 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

28. 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

29. 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.

30. Complementary Base Pairs

31. What is the complementary base pair? TCGAACT AGCTTGA

32. The Replication of DNA Chapter 9 Section 3

33. 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.

34. Key Terms • DNA Replication • DNA Helicase • Replication Fork • DNA Polymerase

35. 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.

36. 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.

37. Replication Forks

38. 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.

39. 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.

40. 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!

41. 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.

42. Replication Forks