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Transcript
Ch. 16 The Molecular Basis of Life Nov 15 5:19 PM 1 Essential Question: On a molecular basis, how does information get passed on from one cell to another? Nov 15 5:20 PM 2 DNA Nov 15 5:22 PM 3 History behind discovery of DNA Morgan early 1900's genes located on chromosomes chromosomes made of DNA and Protein DNA or proteins could be genetic Nov 15 5:23 PM 4 Griffith (1928) studied Streptococcus pneumoniae virulent/nonvirulent strain conclusion: living bacteria had been transformed into pathogenic bacteria by an unknown, heritable substance Nov 15 5:33 PM 5 up to 1940's believed to be proteins specific in function essential little known about nucleic acids Nov 15 5:29 PM 6 Avery, McCarty and MacLeod 1944 purified molecules of heat killed pathogenic bacteria tried to transform live nonpathogenic bacteria with each type only DNA worked scientists still skeptical because believed in proteins Nov 15 5:33 PM 7 Hershey/Chase 1952 used bacteriophages (viruses that infect bacteria), T2 = phage that infects Escherichia coli knew T2 contained DNA and protein knew viruses infected bacteria to make more viruses experiment was to see if it was the protein or the DNA that could reprogram a host cell to make viruses Nov 15 5:38 PM 8 Nov 15 8:30 PM 9 Dec 310:31 AM 10 Hershey/Chase proved it is DNA that is the genetic material, not proteins for viruses Erwin Chargaff 1947 knew that DNA was polymer of nucleotides made of sugar (deoxyribose), a nitrogenous base and a phosphate group DNA differs from one organism to another saw ratios of A, C, G, A, T C = G and A= T (Chargaff's rules) Nov 15 8:31 PM 11 Also known: DNA doubled before mitosis equally distributed in daughter cells each species: diploid set of chromosomes has double amount of DNA as haploid set Once DNA was believed to be genetic material the race was on to find the structure of it Nov 15 9:08 PM 12 1950's knew arrangement of covalent bonds in nucleotide Nov 15 9:23 PM 13 Players in the race to find the structure: Linus Pauling of Cal Tech Nov 15 9:24 PM 14 Maurice Wilkins and Rosalind Franklin at King's College in London worked on xray crystallography Nov 15 9:27 PM 15 James Watson and Francis Crick at Cambridge http://nobelprize.org/nobel_prizes/medicine/laureates/1962/watsonbio.html http://nobelprize.org/nobel_prizes/medicine/laureates/1962/crickbio.html used model building techniques Nov 15 9:27 PM 16 Possible pairings of purines and pyrimidines Dec 47:04 AM 17 base pairing in DNA Dec 47:05 AM 18 DNA found to be double helix structure nitrogen bases on the inside alternating sugar and phosphates on the outside adenine and guanine are purines (double ring structure) cytosine and thymine are pyrimidines (single ring structures) hydrogen bonds hold the strands together 3 bonds between G and C 2 bonds between A and T explained Chargaff's rules Nov 15 9:28 PM 19 Dec 47:03 AM 20 April 1953 Watson and Crick published in Nature their structure of DNA Nobel Peace Prize was shared by Watson, Crick, and Wilkins Nov 15 9:36 PM 21 How does the structure of DNA affect its function? "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material" (Watson and Crick in Nature) Nov 15 9:39 PM 22 How does DNA replicate? Watson and Crick's basic idea for replication Nov 15 9:42 PM 23 Three models for DNA replication Nov 15 9:44 PM 24 Meselson and Stahl 1950's tested the 3 hypotheses Nov 15 9:46 PM 25 Meselson and Stahl proved that DNA replication is semiconservative work on studying DNA replication is done in bacteria because they have fewer base pairs and they reproduce quickly (every 20 minutes for E. coli) Nov 15 9:50 PM 26 Procedure for replication 1. begins at origins of replication helicase recognizes certain DNA sequences and attach to those sites has many sites to speed up replication Nov 15 9:54 PM 27 2. helicase causes DNA strands to untwist and separate to form a replication bubble 3. When strands untwist become tighter farther down helix. Topoisomerase relieves some of the strain. Dec 47:08 AM 28 4. singlestrand binding protein binds to the unpaired strands and keeps them stable until new strands can be synthesized 5. DNA polymerase adds new nucleotides by adding a nucleoside triphosphate. When added it loses two phosphate groups (pyrophosphate) which then gets hydrolyzed to two phosphate molecules DNA polymerase adds the nucleotides to the 3' end of the strand (sugar end) only, never to 5' end Nov 15 10:02 PM 29 Some of the proteins involved in the initiation of DNA replication Feb 84:28 PM 30 Dec 47:10 AM 31 6. Initial nucleotide chain added is called a primer (can be DNA or RNA) = short piece (510 nucleotides long) of RNA with a 3' end 7. primase joins RNA nucleotides together one at a time making a primer complementary to the DNA template 8. DNA polymerase III adds a DNA nucleotide to the 3' end of the RNA primer and then adds more DNA nucleotides to the growing strand only one primer is needed for DNA to make strand Nov 15 11:04 PM 32 5' and 3' ends Dec 47:11 AM 33 this happens with the leading strand = the strand of the DNA gets read easily in the 3' to 5' direction 9. For the 5' to 3' strand (lagging strand) DNA polymerase works away from replicating fork gets synthesized in short fragments (called Okazaki fragments 100200 nucleotides long in eukaryotes) same scenario as before with primer, primase, DNA polymerase III, but each Okazaki fragment must be primed separately Nov 15 11:14 PM 34 Synthesis of the leading strand during DNA replication Feb 84:33 PM 35 Priming Dec 47:11 AM 36 10. For Okazaki fragments, DNA polymerase I replaces the RNA nucleotides of the primers with DNA versions add to 3' end of adjacent Okazaki fragment 11. DNA ligase joins the sugarphosphate backbones of the two Okazaki fragments together. Nov 15 11:23 PM 37 Replication Nov 15 11:28 PM 38 Summary of Bacterial DNA replication Feb 84:35 PM 39 Dec 47:16 AM 40 can have errors in the replication of DNA 1 in 100,000 basepairs DNA polymerase proofreads the nucleotide with the template if an error is found, DNA polymerase removes the nucleotide and then continues synthesis if error is missed can get mismatch repair enzymes fix the incorrectly paired nucleotides cell continually monitors and repairs genetic material over 130 DNA repair enzymes in humans found so far Nov 15 11:30 PM 41 in nucleotide excision repair: Nuclease = enzyme that cuts out the damaged DNA segment DNA polymerase and ligase are responsible for filling the empty spot back in. ex. for skin cells ultraviolet light can cause thymine dimers = two adjacent thymines that covalently link together and cause buckling of DNA interferes with replication if not repaired can cause skin cancer xeroderma pigmentosum hypersensitivity to light get skin cancer very easily Nov 15 11:44 PM 42 Nucleotide excision What are the functions of nuclease, DNA polymerase and DNA ligase here? Dec 47:17 AM 43 Replicating process problem at end of replication, have no way of completing 3' end of lagging strand because an RNA primer is there therefore after replication, the strands would get shorter and shorter only problem in eukaryotes, Why? Nov 15 11:51 PM 44 Shortening of the ends of linear DNA molecules Nov 15 11:56 PM 45 Eukaryotes have telomeres do not contain genes contain multiple repetitions of a short nucleotide chain TTAGGG number of repetitions can be 100 to 1,000 telomeres postpone the shortening of DNA and the loss of genes on the ends of the DNA molecules in normal cells also prevent cell death shortening of teleomeres may have something to do with aging of certain tissues Nov 15 11:58 PM 46 For germ cells (give rise to gametes) telomerase catalyzes the lengthening of the telomeres in germ cells restores original length uses a short RNA molecule that acts as a template for new telomere synthesis have found telomerase in cancer cells that are not germ cells if we can find a way to stop the telomerase in tumor cells, could possibly target chemotherapy towards this Nov 16 12:04 AM 47 Telomeres on mouse Chromosomes Dec 47:19 AM 48 So what is a chromosome? Prokaryote circular strand of DNA with small amount of protein Eukaryote linear strand of DNA with large amount of protein (histones) Feb 183:18 PM 49 nucleoid region = dense region of DNA in bacteria, not membrane bound in eukaryote = chromatin = strand of a complex of DNA and proteins Feb 183:30 PM 50 heterochromatin = clumped chromatin in interphase Euchromatin = "true chromatin", less compact, more dispersed • genes can be expressed from this structure histones help change chromatin organization help chromosomes condense Feb 183:33 PM 51 So...... On a molecular basis, how does information get passed on from one cell to another? Feb 183:43 PM 52