1 DNA and Replication

2 History of DNA

3 Early scientists thought protein was the cell’s hereditary material because it was more complex than DNA Proteins were composed of 20 different amino acids in long polypeptide chains

4 History of DNA Chromosomes are made of both DNA and protein Experiments on bacteriophage viruses by Hershey & Chase proved that DNA was the cell’s genetic material Radioactive 32 P was injected into bacteria!

5 Discovery of DNA Structure Erwin Chargraff showed the amounts of the four bases on DNA ( A,T,C,G) In a body or somatic cell: A = 30.3% T = 30.3% G = 19.5% C = 19.9%

6 Chargaff’s Rule Adenine Thymine A - TAdenine must pair with Thymine A - T Guanine Cytosine G - CGuanine must pair with Cytosine G - C The bases form weak hydrogen bonds G C TA

7 Rosalind Franklin Rosalind Franklin took diffraction x- ray photographs of DNA crystals

8 Watson and Crick In the 1950’s, Watson & Crick built the first model of DNA using Franklin’s x-rays

9 DNA Structure

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

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

12 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

13 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)

14 DNA P P P O O O P P P O O O G C TA

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

16 DNA Replication

17 Replication Facts DNA has to be copied before a cell dividesDNA has to be copied before a cell divides DNA is copied during interphase of mitosisDNA is copied during interphase of mitosis New cells will need identical DNA strandsNew cells will need identical DNA strands

Three Phases of Replication Initiation: Unzipping DNA Elongation: Making new DNA strand Termination: Ends at telomeres (specific repeated bases) Zipping DNA back together.

19 DNA Replication Begins at Origins of ReplicationBegins at Origins of Replication Two strands open forming Replication Forks (Y-shaped region) The “unzipping of the DNA strand is carried out by an enzyme DNA helicase.Two strands open forming Replication Forks (Y-shaped region) The “unzipping of the DNA strand is carried out by an enzyme DNA helicase. New strands grow at the forksNew strands grow at the forks ReplicationFork Parental DNA Molecule 3’ 5’ 3’ 5’

20 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

21 DNA Replication Before RNA primersBefore new DNA strands can form, there must be RNA primers present to start the addition of new nucleotides DNA polymerase can then add the new nucleotides The joining of RNA primers and DNA polymerase creates an Okazaki fragment. Only occurs on lagging strand

Leading and Lagging Strands Leading strand: starts at the 3’ end of the parent DNA forming the complementary strand of 5’ to 3’ Lagging strand: is discontinuous (patchwork for DNA); requires many RNA primers for Okazaki fragments

Okazaki Fragment 23

24 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

Finishing the New DNA Strands DNA Ligase “zips up” the lagging DNA strand –This is done by joining the old and new strands of DNA with hydrogen bonds. 25

26 Remember the Strands are Antiparallel P P P O O O P P P O O O G C TA

27 Replication of Strands Replication Fork Point of Origin

28 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

29 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

30 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

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

32 Answer: DNA 5’-GCGTATG-3’ DNA 3’-CGCATAC-5’

33 DNA by the Numbers Each cell has about 2 m of DNA. The average human has 75 trillion cells. The average human has enough DNA to go from the earth to the sun more than 400 times. DNA has a diameter of only m. The earth is 150 billion m or 93 million miles from the sun.