1. 1 DNA Structure and Replication

2. 2 DNA Two strands coiled called a double helix Sides made of a pentose sugar Deoxyribose bonded to phosphate (PO 4 ) groups by phosphodiester bonds Center made of nitrogen bases bonded together by weak hydrogen bonds

3. 3 DNA Double Helix Nitrogenous Base (A,T,G or C) “Rungs of ladder” “Legs of ladder” Phosphate & Sugar Backbone

4. 4 Helix Most DNA has a right-hand twist with 10 base pairs in a complete turnMost DNA has a right-hand twist with 10 base pairs in a complete turn

5. 5 DNA Stands for Deoxyribonucleic acid nucleotidesMade up of subunits called nucleotides Nucleotide made of:Nucleotide made of: Phosphate group 1.Phosphate group 5-carbon sugar 2.5-carbon sugar Nitrogenous base 3.Nitrogenous base

6. 6 DNA P P P O O O 1 2 3 4 5 5 3 3 5 P P P O O O 1 2 3 4 5 5 3 5 3 G C TA

7. 7 Antiparallel Strands One strand of DNA goes from 5’ to 3’ (sugars) The other strand is opposite in direction going 3’ to 5’ (sugars)

8. 8 Nitrogenous Bases Double ring PURINESDouble ring PURINES Adenine (A) Guanine (G) Single ring PYRIMIDINESSingle ring PYRIMIDINES Thymine (T) Cytosine (C) T or C A or G

9. 9 Base-Pairings Purines only pair with Pyrimidines Three hydrogen bonds required to bond Guanine & Cytosine CG 3 H-bonds

10. 10 T A Two hydrogen bonds are required to bond Adenine & Thymine

11. Discovering the structure of DNA Rosalind Franklin’s DNA image “Chargoff’s rule” A = T & C = G 11

12. Rosalind Franklin (early 1950’s) used x-ray diffraction to get information about the structure of the DNA molecule 12 Photo shows DNA twisted around each other like coils of string  helical Angle of X suggests two strands in the structure

13. Chargaff’s Rules [G]=[C] –The percentages of G equal the percentages of C in DNA [A] = [T] –The percentages of A equal the percentages of T in DNA 13

14. 14 Question: Adenine CytosineIf there is 30% Adenine, how much Cytosine is present?

15. 15 Answer: CytosineThere would be 20% Cytosine Adenine (30%) = Thymine (30%)Adenine (30%) = Thymine (30%) Guanine (20%) = Cytosine (20%)Guanine (20%) = Cytosine (20%) Therefore, 60% A-T and 40% C-GTherefore, 60% A-T and 40% C-G

16. Discovering the structure of DNA Structure was discovered in 1953 by James Watson and Francis Crick 16

17. Functions of DNA DNA holds the genetic information in every cell DNA holds the instructions for making proteins The sequence of nitrogen bases in DNA determines the traits of an organism 17

18. 18 DNA Replication

19. 19 Replication Facts DNA has to be copied before a cell dividesDNA has to be copied before a cell divides DNA is copied during the S or synthesis phase of interphaseDNA is copied during the S or synthesis phase of interphase New cells will need identical DNA strandsNew cells will need identical DNA strands

20. 20 Synthesis Phase (S phase) S phase during interphase of the cell cycle Nucleus of eukaryotes Mitosis -prophase -metaphase -anaphase -telophase G1G1 G2G2 S phase interphase DNA replication takes place in the S phase.

21. 21 DNA Replication Begins at Origins of ReplicationBegins at Origins of Replication Two strands open forming Replication Forks (Y-shaped region)Two strands open forming Replication Forks (Y-shaped region) New strands grow at the forksNew strands grow at the forks ReplicationFork Parental DNA Molecule 3’3’ 5’5’ 3’3’ 5’5’

22. 22 DNA Replication As the 2 DNA strands open at the origin, Replication Bubbles formAs the 2 DNA strands open at the origin, Replication Bubbles form Prokaryotes (bacteria) have a single bubble Eukaryotic chromosomes have MANY bubbles Bubbles

23. 23 DNA Replication Enzyme Helicase unwinds and separates the 2 DNA strands by breaking the weak hydrogen bondsEnzyme Helicase unwinds and separates the 2 DNA strands by breaking the weak hydrogen bonds Single-Strand Binding ProteinsSingle-Strand Binding Proteins attach and keep the 2 DNA strands separated and untwisted

24. 24 DNA Replication Before RNA primersBefore new DNA strands can form, there must be RNA primers present to start the addition of new nucleotides PrimasePrimase is the enzyme that synthesizes the RNA Primer DNA polymerase can then add the new nucleotides

25. 25

26. 26 DNA Replication DNA polymerase can only add nucleotides to the 3’ end of the DNADNA polymerase can only add nucleotides to the 3’ end of the DNA This causes the NEW strand to be built in a 5’ to 3’ directionThis causes the NEW strand to be built in a 5’ to 3’ direction RNAPrimer DNA Polymerase Nucleotide 5’5’ 5’5’ 3’3’

27. 27 Synthesis of the New DNA Strands The Leading Strand single strandThe Leading Strand is synthesized as a single strand from the point of origin toward the opening replication fork RNAPrimer DNA Polymerase Nucleotides 3’3’5’5’ 5’5’

28. 28 Synthesis of the New DNA Strands The Lagging Strand is discontinuouslyThe Lagging Strand is synthesized discontinuously against overall direction of replication This strand is made in MANY short segments It is replicated from the replication fork toward the origin RNA Primer Leading Strand DNA Polymerase 5’5’ 5’5’ 3’3’ 3’3’ Lagging Strand 5’5’ 5’5’ 3’3’ 3’3’

29. 29 Lagging Strand Segments Okazaki Fragments - lagging strandOkazaki Fragments - series of short segments on the lagging strand Must be joined together by an enzymeMust be joined together by an enzyme Lagging Strand RNAPrimerDNAPolymerase 3’3’ 3’3’ 5’5’ 5’5’ Okazaki Fragment

30. 30 Joining of Okazaki Fragments The enzyme Ligase joins the Okazaki fragments together to make one strandThe enzyme Ligase joins the Okazaki fragments together to make one strand Lagging Strand Okazaki Fragment 2 DNA ligase DNA ligase Okazaki Fragment 1 5’5’ 5’5’ 3’3’ 3’3’

31. 31 Replication of Strands Replication Fork Point of Origin

32. 32 Proofreading New DNA DNA polymerase initially makes about 1 in 10,000 base pairing errorsDNA polymerase initially makes about 1 in 10,000 base pairing errors Enzymes proofread and correct these mistakesEnzymes proofread and correct these mistakes The new error rate for DNA that has been proofread is 1 in 1 billion base pairing errorsThe new error rate for DNA that has been proofread is 1 in 1 billion base pairing errors

33. 33 Semiconservative Model of Replication Idea presented by Watson & CrickIdea presented by Watson & Crick TheThe two strands of the parental molecule separate, and each acts as a template for a new complementary strand New DNA consists of 1 PARENTAL (original) and 1 NEW strand of DNA Parental DNA DNA Template New DNA

34. The Results of DNA Replication DNA replication produces two DNA molecules that are identical to the original DNA molecule Each new DNA molecule has one “old” strand and one “new” strand 34

35. Functions of DNA Polymerase Adds nucleotides to the new strands of DNA according to base-pairing rules Proof reads the new strands of DNA and corrects any errors 35

36. What is a Complementary Strand of DNA? A DNA strand that has the base sequence that matches up exactly with the original strand of DNA according to base- pairing rules 36

37. 37 Question: What would be the complementary DNA strand for the following DNA sequence? DNA 5’-CGTATG-3’

38. 38 Answer: DNA 5’-CGTATG-3’ DNA 3’-GCATAC-5’

39. Importance of DNA Replication Helps cells prepare for cell division Ensures that every cell in your body has a complete, accurate copy of your DNA 39

40. DNA replication 40

41. DNA replication 41

42. DNA replication 42

43. DNA replication 43

44. DNA replication Original DNA strands 44

45. DNA replication Newly assembled DNA strands 45

46. DNA replication Semi-conservative replication 46