2. Honors Biology 2006-2007 The History Of DNA

3. Honors Biology 2006-2007

4. Quick Review  Chromosomes are made up of DNA and Proteins.  Genes are a segment of the DNA molecule that carries the instructions for producing a specific traits.

5. Honors Biology 2006-2007 DNA or Proteins?  Prior to the 1950’s, scientists could not decide whether DNA or protein was the material responsible for heredity.  3 experiments would lead to the answer:  1. Frederick Griffith  2. Avery, McCarty & MacLeod  3. Hershey & Chase 1952)

6. Honors Biology 2006-2007 Griffith’s Experiment FFrederick Griffith was working to find a cure for Streptococcus pneumonia (bacteria) TTest Subjects: lab mice HHe used different strains (types) of bacterium in this experiment. 1928

7. Honors Biology 2006-2007 Strains of Bacteria Used  S Bacteria = Disease Causing (pathogenic)  Have protective outer coat that “hides” the bacteria from the body’s immune defenses.  R Bacteria = Do not cause disease (non- pathogenic)  Lacked the protective coat and thus the immune system can identify and destroy them.

8. Honors Biology 2006-2007 The Experimental Trials Transformation? something in heat-killed bacteria could still transmit disease-causing properties live pathogenic strain of bacteria live non-pathogenic strain of bacteria mice diemice live heat-killed pathogenic bacteria mix heat-killed pathogenic & non-pathogenic bacteria mice livemice die A.B. C. D.

9. Honors Biology 2006-2007 Griffith’s “Transforming Factor”  Why did the mice die that were given a mixture of heat-killed pathogenic bacteria and live non- pathogenic bacteria?  A substance had been passed from the dead bacteria to live bacteria = “Transforming Factor”

10. Honors Biology 2006-2007 Griffith’s Results  After examining blood samples, he found that somehow the uncoated bacteria had TRANSFORMED into bacteria with coats.  Transformation is a change in the genetic makeup caused when cells take up foreign genetic material. BUT, what was it? DNA or Protein??

11. Honors Biology 2006-2007 What Next?  Whatever the molecule, it had to have several properties in order to fit the bill:  It had to be duplicated whenever a cell divided, so it could be passed on unchanged.  It had to be in the form of an informational code  It had to be (mostly) stable and resistant to change

12. Honors Biology 2006-2007 Avery, McCarty & MacLeod Oswald Avery Maclyn McCarty Colin MacLeod 1944

13. Honors Biology 2006-2007 DNA causes Transformation  Avery, McCarty & MacLeod  purified both DNA & proteins from Streptococcus pneumonia bacteria  which will transform non-pathogenic bacteria?  Experiment 1:  injected protein into bacteria = no effect  injected DNA into bacteria = transformation!  Experiment 2:  added protein destroying enzymes to bacteria = transformation!  added DNA destroying enzymes to bacteria = no effect

14. Honors Biology 2006-2007 Hershey & Chase Alfred HersheyMartha Chase

15. Honors Biology 2006-2007 Confirmation of DNA  Hershey & Chase  “Blender” experiment using bacteriophages  viruses that infect bacteria  Half of the phages had their proteins tagged with radioactive sulfur ( 35 S)  The other half had their DNA tagged with radioactive phosphorus ( 32 P)  All phage were then allowed to infect bacteria! 1952 | 1969

16. Honors Biology 2006-2007

17. Protein coat labeled with 35 S DNA labeled with 32 P bacteriophages infect bacterial cells T2 bacteriophages are labeled with radioactive isotopes S vs. P bacterial cells are agitated to remove viral protein coats 35 S radioactivity found in the liquid 32 P radioactivity found in the bacterial cells Which radioactive marker is found inside the cell? Which molecule carries viral genetic info? Hershey & Chase

18. Honors Biology 2006-2007 Blender experiment Results  Radioactive phage & bacteria in blender  35 S phage  radioactive proteins were in liquid  therefore protein did NOT enter bacteria  32 P phage  radioactive DNA was in pellet  therefore DNA did enter bacteria Taaa-Daaa!

19. Honors Biology 2006-2007 Hershey & Chase’s Conclusion  Hershey & Chase Confirmed DNA is “transforming factor”  Proved that DNA AND NOT protein is the hereditary material in cells (i.e., genes are made up of DNA)

20. Honors Biology 2006-2007 Wilkin’s and Franklin’s Photographs  In 1952, Maurice Wilkins and Rosalind Franklin, developed high quality X-ray diffraction photographs of strands of DNA.  These photos were key pieces of data used to determine the helical structure of DNA molecules.

21. Honors Biology 2006-2007 Watson and Crick 1953 article in Nature

22. Honors Biology 2006-2007 The Structure of DNA

23. Honors Biology 2006-2007 Deoxyribonucleic Acid  DNA is the molecule of heredity.  DNA (Deoxyribonucleic Acid) is a double-stranded nucleic acid that determines an organism’s traits by controlling the production of proteins.  DNA holds the information for life!  Why do we study DNA??

24. Honors Biology 2006-2007 DNA’s Size  DNA is an extremely long molecule  DNA from one of your cells would measure 6 feet tall!  All of the DNA in your body could stretch to the sun and back 400 times!  5 million strands of DNA can fit through the head of one needle! The earth is 93 million miles from the sun!

25. Honors Biology 2006-2007 DNA’s Helical Structure  Watson & Crick discovered DNA is made up of 2 chains of nucleotides joined together by their nitrogen bases.  The 2 strands are twisted together, forming a Double-Helix.  Like a spiral staircase, twisted ladder, or zipper

26. Honors Biology 2006-2007 Subunits of DNA  When we group the deoxyribose sugar, phosphate group and nitrogen base together into a subunit it is called a Nucleotide.  One strand of DNA has many millions of nucleotides. nucleotide

27. Honors Biology 2006-2007 Nucleotides Continued Phosphate C C O O C C O -P O O O Nitrogenous base Deoxyribose C  Sugar = Deoxyribose.  The Phosphate Group = Phosphorus atom surrounded by 4 oxygen atoms.  The Nitrogen Bases are carbon ring structures that contain one or more atoms of nitrogen.

28. Honors Biology 2006-2007 What is DNA made of?  The backbone of the molecule is alternating phosphate groups and deoxyribose sugars.  The “rungs” of the DNA “ladder” are nitrogenous bases. phosphate deoxyribose bases

29. Honors Biology 2006-2007 Four nitrogenous bases  DNA has four different nitrogenous bases:  Adenine ( A)  Thymine ( T)  Cytosine ( C)  Guanine ( G)

30. Honors Biology 2006-2007 Two Classes of DNA Bases  Pyrimidines are single ring bases.  Thymine  Cytosine (Pyrimidins have a “y”) (Pyrimidins have a “y”)  Purines are double ring bases.  Adenine  Guanine C C C C N N O N C C C C N N N N N C

31. Honors Biology 2006-2007 Chargaff’s Rule  The amount of Adenine is always equal to the amount of Thymine.  Adenine and Thymine have a 1:1 ratio  The amount of Guanine is always equal to the amount of Cytosine.  Guanine and Cytosine have a 1:1 ratio

32. Honors Biology 2006-2007 Base pairing in DNA  Pairing  Adenine and Thymine always join together  Two Hydrogen Bonds  Cytosine and Guanine always join together  3 Hydrogen Bonds  Adenine-Thymine and Guanine- Cytosine are called Complimentary Base Pairs.

33. Honors Biology 2006-2007 How do the strands stick?  Hydrogen Bonds form between the nitrogenous bases.  Weak, but there are millions and millions of them in a single molecule of DNA.  Remember hydrogen bonds can form between hydrogen and any electronegative atom! hydrogen bonds

34. Honors Biology 2006-2007 Nucleotide Sequence  Nucleotide Sequence is extremely important.  Differences in organisms are due to the difference in sequence of the nucleotides along a strand of DNA.  Nucleotide sequence forms the unique genetic information of an organism.

35. Honors Biology 2006-2007 DNA Replication

36. Honors Biology 2006-2007 Copying DNA  Each cell in an organism has an exact copy of the DNA that was in the fertilized egg (Mitosis)  The DNA in the chromosome is copied in a process called DNA Replication.  If DNA wasn’t copied before cell division, new cells would only have half the DNA of their parents.

37. Honors Biology 2006-2007 DNA Replication  Base pairing allows each strand to serve as a pattern (template) for a new strand  Semi-Conservatice Model  2 molecules are formed.  Each has an original strand and one new strand.

38. Honors Biology 2006-2007 let ’ s meet the team … DNA Replication  Large team of enzymes coordinates replication: 1.DNA Helicase - unwinds the DNA so that it can be copied 2.DNA Polymerase(s) - Brings in the nucleotide building blocks to make the complimentary strand AND “proofreads” the finished strands. 3.DNA Ligase - Acts as “glue” cementing new pieces of DNA together to form a continuous strand.

39. Honors Biology 2006-2007 1st step: Unwinding single-stranded binding proteins  Unwind & unzip DNA  helicase enzyme  unwinds DNA helix  stabilized by single-stranded binding proteins  The areas where the double helix separate are called replication forks.

40. Honors Biology 2006-2007 Replication fork direction of replication

41. Honors Biology 2006-2007  DNA Polymerases bring in new nucleotides that match up to template strands Now, that ’ s a compliment! 2nd step - Adding Bases

42. Honors Biology 2006-2007 Results of DNA Replication  DNA Replication results in the formation of 2 DNA molecules, each identical to the original DNA molecule.  Each new molecule has one strand from the original DNA molecule and one new strand.  Semi-conservative!

43. Honors Biology 2006-2007 Checking for Errors  Sometimes errors occur when the wrong nucleotide is added to the new strand.  Luckily, DNA Polymerase has a “proofreading function.”  DNA Polymerase can backtrack and remove incorrect nucleotides.

44. Honors Biology 2006-2007 Fast & accurate!  It takes E. coli <1 hour to copy 5 million base pairs in its single chromosome  divide to form 2 identical daughter cells  Human cell copies its 6 billion bases & divide into daughter cells in only few hours  remarkably accurate  only ~1 error per 100 million bases  ~30 errors per cell cycle