Chapter 16 DNA is the Genetic Material 10 31 2012 Watson and Crick credited with structure of DNA People knew of DNA for a long time but it wasn t considered to be that complex so it couldn t hold genetic information so it would just help proteins out Protein was king and protein was going to be the molecule that was the genetic carrier All Cells Maintain a Genetic Program One cell becomes two Information directing this must be stored Information must be read out in a way that cells can understand Information must be transferred unchanged to the next generation and populations mother father child of Biology Central Dogma The information is DNA DNA RNA Protein Start off with DNA transcribed or put into format for Proteins Its transcribed into RNA then interpreted into a protein Nature of the Genetic Material Questions posed and answered in the next few lectures What is its structure How is it stored How is it converted into action How might it be changed over time Step 1 Identifying the Genetic Material Gregor Mendel Garden peas traits inherited independently behave as discrete units dominant or recessive Early 1900s Chromosome segregate at division Number halved during germ cell formation Expected if genes are on chromosomes T H Morgan Developed Drosophilia fruit flies as a model organism Proved experimentally that genes are on chromosomes Know that genes are on chromosomes Know that chromosomes are Protein and DNA Which is the gene Protein or DNA Frederick Griffith 1928 Streptococcus pneumonia Two strains of bacteria Smooth Colony S strain Smooth coat Rough Colony R strain He knew that immune system would kill R strain but S strain would cause pneumonia and kill you Griffith s transformation experiment S strain is encapsulated and virulent into mouse dead R strain is non encapsulated and non virulent into mouse live Heat killed S strain injected S strain into mouse live Heat killed virulent S strain R strain into mouse died Blood sample from dead mouse contains living S not heat killed S Why Genetic information could be moved from one organism to another genetic material is chemical and passed over bacteria through transformation Griffith is credited with idea of transformation Transformation Conversion of R cells into S cells called transformation Transformation change in genetic and physical composition as a result of assimilation of external DNA Something passed from nonliving cells to living cells Permanently changed the nonpathogenic cells into being pathogenic Heritable Initiated a 14 year search for the Transforming Principle Avery McCarty MacLeod 1944 Tried to understand Griffith s experiment Purified macromolecules from head killed S cell extracts Treated the extracts with various inactivating enzymes Proteas RNase DNase Mixed treated extracts with R cells Injected mice Proteas transformation RNase transformation DNase No transformation DNA is the transforming factor 1944 Avery determined the substance from the heat killed S cells had to be DNA and not a protein But Studies continued to experiment because they thought DNA wasn t complex enough and it had to be protein Hershey Chase Bacteria and Bacteriophages Phages contain only protein DNA 1952 Hershey Chase experiment uses phages to determine if genetic material is protein or DNA The Blender Experiment Used Bacteriophage Bacteria eaters Composed of DNA and protein only Make the protein radioactive and the DNA radioactive See where Protein floats away and is absorbed by the cell and the only thing the radioactive shows up that goes in is the DNA Radioactive label phage with bacteria agitate in blender and separate phage outside bacteria Centrifuge and measure radioactivity in pallet and supernatant DNA attached to S was all outside and DNA attached to P was all inside the cell This proved that DNA held genetic material DNA purines 2 rings and pyrimidines 1 ring Chargraff determined base content of DNA Chargraff s Rules 1 Amount of A T C G varies among species 2 In each species amount of A T and C G Each human chromosome contain an average of 140 million base pairs Many possible different sequences with base pairing Structure of DNA to determine the structure Once DNA was generally accepted as the genetic material the race was on Rosalind Franklin produced DNA pattern using X ray diffraction techniques showed two strands and a double helix Watson and Crick Won Nobel in 1962 1953 they determined the actual structure of DNA using Franklin s x ray Chargaff s rules Watson and Crick Base Pairing Purines make H bonds with Purines and Pyrimidines with Pyrimidines Double Helix sides of DNA is made up of sugars and phosphate ribose sugar because those are hydrophilic and then can stay in contact with the cell Every other one switches 5 to 3 Sugar attached to nitrogen base and phosphate attached on outside Watson and Crick Won the Race They won because More information than any other group Not afraid to ask smarter folks about stuff they didn t know Made some assumptions that ended up being correct Got lucky DNA Replication 10 31 2012 Duplicate DNA for cell reproduction and division Red blood cells have no nucleus white blood cells do Complementary base pairing is a fundamental principle Have to be antiparallel to match up and complement eachother Semi Conservative Replication Replication is when genetic material is copied passed on to the next generation during S phase of interphase in cell division Each parent strand is a template for a new complementary strand Region of replication of the DNA it starts to split new nucleotides are pairing with those of parental strands Semi Conservative Replication final product is one parental strand combined with one new strand conservative because still keeping half of the parent molecule DNA replication template strand and replicated strand 1 DNA is unwound by Helicase H bonds broken 2 New complementary base pairs are brought in by DNA Polymerase bing in matching complement of base pair and check to make sure its done proofreading make corrections as needed 3 Nucleotides join together H bonds reform Ligase sticking it together Pairing errors 1 in 100000 are usually corrected by DNA polymerase the proof reader Mutations occur after proof reading Usually only 1 in a billion base pairs are incorrectly paired DNA Replication Models CalTech Experiment Meselson and Stahl Semiconservative replication is not the only possible model Semiconservative Conservative Dispersive replication Must
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