1. DNA History and Structure

2. Why is it important that we know about the structure (shape) of DNA?
Allows us to understand how DNA copies itself for cell division (mitosis and meiosis)
Understand how we grow and heal
Understand how we reproduce
Helps diagnose diseases
Helps genetically engineer more productive and healthy food

3. DNA Deoxyribonucleic Acid
DNA is the molecule that contains our genetic information
originally proteins were thought to possibly be responsible for genes

4. Frederick Griffith
Frederick Griffith (1941) was the first to propose that cells could change their genotypes by taking in foreign material

6. Transformation
a change in genotype caused when cells take up foreign genetic material

7. Oswald Avery
Oswald Avery (1944) interpreted Griffith’s experiment after Griffith’s death
tried to destroy transformation using enzymes
transformation was not stopped by protein destroying enzymes, but was stopped by DNA-destroying enzymes
proved DNA was the molecule of heredity

8. Polymer DNA is a polymer molecule made of many repeating parts
made of monomers called nucleotides

9. Nucleotide a Nucleotide is made up of: a 5-carbon sugar (deoxyribose)
a phosphate group
one of the four nitrogen-containing bases

10. DNA Structure

11. DNA Structure

12. Name the 4 Nitrogen Containing Bases
the four bases are (A) __________, (T) __________, (C) __________, and (G) __________

13. Name the 4 Nitrogen Containing Bases
the four bases are (A) adenine, (T) thymine, (C) cytosine, and (G) guanine

15. Erwin Chargaff
Erwin Chargaff (1949) noted that different organisms had different amounts of the four bases

16. Chargaff’s Rules
however, Chargaff notes some consistencies in all organisms
A = T and C = G
A + G = T + C

17. Purines nitrogen bases made of two rings of carbon and nitrogen
adenine and guanine

18. Pyrimidines nitrogen bases made of one ring of carbon and nitrogen
thymine and cytosine

20. Franklin and Wilkins
Rosalind Franklin and Maurice Wilkins (1952) used X-ray diffraction to study the structure of DNA
bounced X-rays off a molecule which are then scattered on a film

21. DNA Structure they studied the patterns on the film
like shining a light on an object and studying its shadow
determined that DNA was made of some type of helix

22. DNA Structure they studied the patterns on the film
like shining a light on an object and studying its shadow
determined that DNA was made of some type of helix

23. Helix
a structure that spirals around a central axis

24. What childhood toy is shaped like a helix?
A slinky!

25. Watson and Crick
James Watson and Francis Crick (1953) used Franklin and Wilkins’ information to determine DNA was a double stranded, right-handed helix

26. Right-Handed Helix
a helix whose upward movement curves in the direction of the fingers of a right hand when the right thumb is pointing upward
as the helix curves counterclockwise, the strands move upward

27. Watson-Crick Model
DNA molecule is double stranded molecule called a _____ _____ (twisted ladder)

28. Watson-Crick Model
DNA molecule is double stranded molecule called a double helix (twisted ladder)

29. Nitrogen containing bases
(A,T,G,C)
phosphate
One nucleotide
Sugar (deoxyribose)

30. Minor Groove
Major Groove

31. Watson-Crick Model
the sides (backbone) of the molecule are made of alternating groups of _____ (____________) and ________

32. Watson-Crick Model
the sides (backbone) of the molecule are made of alternating groups of sugar (deoxyribose) and phosphate
sugars are larger than phosphates

33. DNA Structure

34. Watson-Crick Model
the rungs (steps) of the molecule are made of ___________ _________ _____ of which there are four kinds

35. Watson-Crick Model
the rungs (steps) of the molecule are made of nitrogen containing bases of which there are four kinds
A, C, T, & G

36. Watson-Crick Model
the bases are attached to the _____ of the backbone

37. Watson-Crick Model
the bases are attached to the sugar of the backbone

38. Watson-Crick Model 2-nanometer diameter overall
One nanometer = 1 billionth (10-9) of a meter
0.34-nanometer distance between each pair of bases
3.4-nanometer length of each full twist of the double helix
In all respects shown here, the Watson–Crick model for DNA structure is consistent with the known biochemical and x-ray diffraction data.
The pattern of base pairing (A only with T, and G only with C) is consistent with the known composition of DNA (A = T, and G = C).

39. Watson-Crick Model
DNA close up

40. Watson-Crick Model
sugars and phosphates are connected to each other by covalent bonds

41. Watson-Crick Model
the N containing bases are connected to the sugar by a covalent bond

42. Watson-Crick Model
DNA close up

43. Covalent Bonds
bonds connecting atoms that share electrons

44. Draw arrows pointing to the covalent bonds.

45. How are the bases connected to each other?
the N bases are linked to each other by hydrogen bonds

46. Watson-Crick Model
DNA close up

47. Hydrogen bonds vs. Covalent bonds in water

48. Hydrogen Bonds
weak chemical attractions between small positive and negative charges
much weaker than covalent bonds
this weakness is shown in diagrams as dotted lines instead of solid lines

49. Draw an arrow pointing to the hydrogen bonds.

50. H bonding between bases and Complementary Base Pairing
Nitrogen-containing bases pair up based on their ability to form hydrogen bonds
A always pairs with T
2 hydrogen bonds occur between adenine and thymine
C always pairs with G
3 hydrogen bonds occur between cytosine and guanine
this pairing pattern explains Chargaff’s Rules

51. Antiparallel Strands Anti – opposite Parallel – never touch
the 2 strands of a DNA double helix never cross each other and run in opposite directions

52. How is the antiparallel strand property of DNA show in this picture?

53. Anti
The letters and shapes are upside down on the strand on the right.
Parallel
The strands never cross.

54. Summary Identify the function (job) of DNA.
Contains our genetic information
Instructions for how our bodies are made and how they work
Also how those instructions are passed on to the next generation
Identify the basic structure (shape) of DNA.
Right-handed double helix