1 DNA Structure copyright cmassengale

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 copyright cmassengale

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

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 Left twisted DNA is called Z-DNA or southpaw DNALeft twisted DNA is called Z-DNA or southpaw DNA Hot spots occur where right and left twisted DNA meet producing mutationsHot spots occur where right and left twisted DNA meet producing mutations copyright cmassengale

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 copyright cmassengale

DNA Structure Song To the tune of “Row your boat” We love DNA Made of nucleotides Sugar, phosphate, and a base Bonded down the sides copyright cmassengale6

7 DNA Nucleotide 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) O copyright cmassengale

8 Pentose Sugar Carbons are numbered clockwise 1’ to 5’ CH2 O C1C1 C4C4 C3C3 C2C2 5 Sugar Sugar(deoxyribose) copyright cmassengale

9 DNA P P P O O O P P P O O O G C TA

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

11 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 copyright cmassengale

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

13 T A Two hydrogen bonds are required to bond Adenine & Thymine copyright cmassengale

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

Rosalind Franklin (early 1950’s) used x-ray diffraction to get information about the structure of the DNA molecule 15 Photo shows DNA twisted around each other like coils of string  helical Angle of X suggests two strands in the structure

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 copyright cmassengale16

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

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 copyright cmassengale

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

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 copyright cmassengale20

21 DNA Replication copyright cmassengale

22 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 copyright cmassengale

23 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. copyright cmassengale

24 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’ copyright cmassengale

25 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 copyright cmassengale

26 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 copyright cmassengale

27 DNA Replication Enzyme Topoisomerase relieve stressDNA moleculeEnzyme Topoisomerase attaches to the 2 forks of the bubble to relieve stress on the DNA molecule as it separates Enzyme DNA Enzyme copyright cmassengale

28 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 copyright cmassengale

29copyright cmassengale

30 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’

31 Remember HOW the Carbons Are Numbered! 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) copyright cmassengale

32 Remember the Strands are Antiparallel P P P O O O P P P O O O G C TA copyright cmassengale

33 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’ copyright cmassengale

34 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’ copyright cmassengale

35 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 copyright cmassengale

36 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’ copyright cmassengale

37 Replication of Strands Replication Fork Point of Origin copyright cmassengale

38 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 copyright cmassengale

39 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 copyright cmassengale

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

41 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 copyright cmassengale

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

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

44 Question: What would be the complementary DNA strand for the following DNA sequence? DNA 5’-CGTATG-3’ copyright cmassengale

45 Answer: DNA 5’-CGTATG-3’ DNA 3’-GCATAC-5’ copyright cmassengale

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

DNA replication

Original DNA strands

DNA replication Newly assembled DNA strands

DNA replication Semi-conservative replication

54copyright cmassengale