1. 1 History of DNA

2. 2 DNA Structure Rosalind Franklin took diffraction x-ray photographs of DNA crystals In the 1950’s, Watson & Crick built the first model of DNA using Franklin’s x-rays

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4. 4 Rosalind Franklin

5. 5 DNA Structure

6. 6 DNA Two strands coiled called a double helix Sides made of a pentose sugar Deoxyribose bonded to phosphate Center made of nitrogen bases bonded together by weak hydrogen bonds

7. 7 DNA Double Helix Nitrogenous Base (A,T,G or C) “Rungs of ladder” “Legs of ladder” Phosphate & Sugar Backbone

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9. 9 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

10. 10 DNA Nucleotide O O=P-O OPhosphate Group Group N Nitrogenous base (A, G, C, or T) (A, G, C, or T) CH2 O C1C1 C4C4 C3C3 C2C2 5 Sugar Sugar(deoxyribose)

11. 11 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

12. 12 Antiparallel Strands One strand of DNA goes from 5’ to 3’ (sugars) The other strand is opposite in direction going 3’ to 5’ (sugars)

13. 13 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

14. 14 Base-Pairings Purines only pair with Pyrimidines Three hydrogen bonds required to bond Guanine & Cytosine CG 3 H-bonds

15. 15 T A Two hydrogen bonds are required to bond Adenine & Thymine

16. 16 Chargaff’s Rule Adenine ThymineAdenine must pair with Thymine Guanine CytosineGuanine must pair with Cytosine The bases form weak hydrogen bonds G C TA

17. 17 Question: Adenine CytosineIf there is 30% Adenine, how much Cytosine is present?

18. 18 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

19. 19 DNA Replication

20. 20 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

21. 21 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.

22. 22 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’ 5’ 3’ 5’

23. 23 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 Eukaryotic chromosomes have MANY bubbles Prokaryotes (bacteria) have a single bubble Bubbles

24. 24 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

25. 25 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’ 3’ Direction of Replication

26. 26 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

27. 27 DNA Damage & Repair Chemicals & ultraviolet radiation damage the DNA in our body cells Cells must continuously repair DAMAGED DNA Excision repair occurs when any of over 50 repair enzymes remove damaged parts of DNA DNA polymerase and DNA ligase replace and bond the new nucleotides together

28. 28 Question: What would be the complementary DNA strand for the following DNA sequence? DNA 5’-CGTATG-3’

29. 29 Answer: DNA 5’-CGTATG-3’ DNA 3’-GCATAC-5’

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