DNA What is the major compon onent nt of all cells? s? PROTEIN - PDF document

2/18/2013 DNA What is the major compon onent nt of all cells? s? PROTEIN INS Why would protein synthesis be important? What substance directs protein synthesis? • cellular structures • enzymes DNA • cell membrane structures • organelles • direct all other cellular activities DNA DISCOVERY OF THE GENETIC CODE molecule responsible for all cell activities and contains the genetic code 1928: Frederick Griffith Genetic tic Code (studied how bacteria cause pneumonia) method cells use to store the program that is passed from one generation to another 1

2/18/2013 Griffith Experiment 1 Griffith Experiment 2 1. Grew 2 strains of bacteria on 1. Injected mice with heat plates killed virulent strain - smooth colonies- caused 2. Injected mice with disease (viru rule lent) non -virulent strain + heat killed virulent strain - rough edge colonies-did not cause disease (aviru rule lent) Results: - heat killed: lived 2. Injected into mice - mixed strains: mice developed pneumonia Results: - smooth colonies: died Conclusion: - rought colonies: lived heat killed virulent strain passed disease causing Conclusion: abilities to non virulent strain bacteria didn’t produce a toxin to kill mice 1944: Avery (et al) After Experiment 1. Repeated Griffith’s experiment with same results. Cultured bacteria from dead mice and they grew virulent strain. - result: transformation occurred 2. Did a second experiment using enzymes that would destroy RNA. Griffith hypothesized that a factor was - result: transformation occurred transferred from heat killed cells 3. Did third experiment using enzymes that would destroy DNA. to live cells . - result: no transformation TRANS NSFOR FORMA MATIO ION CONCLUSION DNA was transforming factor 1952: Hershey / Chase How Bacteriophages Work - studied how viruses (bacteriophage) affect bacteria. 1. bacteriophage attaches to surface of bacteria and Bacterio riophage ge injects DNA Virus composed of DNA core and protein coat 2. bacteria makes phage DNA 3. bacterial cell bursts 4. sends out new phages to infect more bacteria animation 2

2/18/2013 Hershey Chase Experiment DISCOV OVERY RY OF STRUCTU TURE OF DNA 1. They labeled virus protein coat with radioactive sulfur 2. They labeled virus DNA with radioactive phosphorous Result lt observed that bacteria had phosphorous *** virus injected bacterial cells with its phosphorous labeled DNA*** Conclu clusion DNA carried genetic code since bacteria made new DNA. Early 1950’s: Rosalind Franklin (English) 1950’s Watson (American) & Crick (English) x ray eviden dence: **double helix model** X pattern showed that fibers of DNA twisted and won Nobel prize in 1962 molecules are spaced at regular intevals on length fiber. Maurice Wilkins: x ray diffraction, worked with Franklin DNA Same time period: - double strand of nucleotides - may have 1000’s of nucleotides in 1 strand Chargaff (American biochemist) (very long molecule) - bases join up in specific (complementary) pairs: Chargaff’s Rule: • complementary pairs (base pairing rules) 1 purin ine bonds with 1 pyrimid idin ine on one rung of the ladder connected by a weak H bond C - G A – T Order r of nucle leotid ides not import rtant, proper r comple lementary ry bases must be paire red. 3

2/18/2013 Nucleotide Structure STRUCT CTURE URE OF DNA Composed of: Purines Pyrimidines A. Phosphate B. Deoxyribose sugar (5 C) C. 4 Nitrogenous bases - Purines Adenine A Guanine G - Pyrimidines Thymine T Cytosine C Sugar Base D bases attached to sugar Phosphate E. bases attached to each other by weak H bond REPLIC ICATION ION OF DNA DNA REPLICATION Process of duplication of DNA - Before cell can divide a new copy of DNA must be made for the new cell - Semicon onser ervative ive replication ion: each strand acts as a template (pattern) for new strand to be made End Result lt: one old strand, one new daughter strand Models of DNA Replication Steps of Replication 1. Enzyme DNA helica icase attaches to DNA molecule and unwinds 2 strands at various points on the strand (breaks H bonds so strand unwinds) - replica licatio ion forks ks: two areas on either end of the DNA where double helix separates - forms replica icatio ion bubble le: “bubble” under electron microscope 4

2/18/2013 2. Enzyme DNA polymera rase moves DNA Directionality along each of DNA strand and adds complementary bases of nucleotides floating freely in nucleus A. DNA polymerase begins synthesis at RNA primer segment - enzyme RNA Prima imase lays down this section on DNA strand - RNA primer segment signals beginning of replication - directionality: DNA polymerase reads the template in the 3’ to 5’ direction But if there exist no DNA polymerases capable of polymerizing DNA in the Daughter DNA strand 3' to 5' direction, how could this be? (since it is complementary) must be synthesized in the 5’ to 3’ direction Strands are antiparallel. Discontinuous synthesis - synthesis only occurs when a large amount of single strand D. DNA ligase stitches DNA is present together Okazaki fragments into a - daughter DNA is then synthesized single, unfragmented in 5’ to 3’ direction daughter molecule - leading and lagging strands: E. enzyme chops off - lead ading strand – continuously RNA primer and synthesized DNA strand replaces it with DNA - lagging strand - delayed, fragmented, daughter DNA - Okazaki aki fragments- discontinuous fragmented DNA segments 5

2/18/2013 End Replication Problem - On one end, RNA primer cannot be replaced with DNA because it is a 5’ 3. DNA polymerase catalyzes (DNA polymerase can only read from 3’ to 5’) formation of H bonds between nucleotides of template and newly arriving nucleotides - Causes daughter DNA’s to be shorter with each which will form daughter DNA replication (cell division) 4. Once all DNA is copied, 3’__________________________________ 5’ daughter DNA detaches 5’ -------------------------------------------------- 3’ animation 5’__________________________________ 3 3’ ------------------------------------------------- 5’ Speed of Replication Solution to End Replication Problem • Multiple replication forks- replication occurs simultaneously on many telomere res: regions of repeated non coding sequences at end of points of the DNA molecule chromosomes (protective sacrificial ends) - become shorter with repeated cell divisions • Would take 16 days to replicate 1 strand from one end to the other on - once telomeres are gone, coding sections of chrom. a fruit fly DNA without multiple forks are lost and cell does not have enough DNA to function • Actually takes ~ 3 minutes / 6000 sites replicate at one time • Human chromosome replicated in about 8 hours with multiple ***telomere theory of aging*** replication forks working together - telomera rase: special enzyme that contains an RNA template molecule so that telomeres can be added back on to DNA (rebuilds telomeres) ** found in: Cancer cells - immortal in culture Stem cells ** not found in most differentiated cells Accuracy and Repair Importance of DNA • Cell has proofreading functions 1. Controls formation of all substances in the cell by the genetic code • DNA polymerase can remove damaged nucleotides and replace with new ones for accurate replication 2. Directs the synthesis of specific strands of m RNA to make proteins • RNA does not have this ability- reason RNA viruses mutate so much RNA (Ribonucleic acid) • DNA damaged by heat, radiation, chemicals and other factors Another nucleic acid takes orders from DNA Used in protein synthesis 6