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 ProgramOne 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)Central Dogma of BiologyThe information is DNADNA RNA ProteinStart off with DNA, transcribed or put into format for Proteins. Its transcribed into RNA then interpreted into a protein.Nature of the Genetic MaterialQuestions posed and answered in the next few lecturesWhat 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 MaterialGregor MendelGarden peas, traits inherited independently, behave as discrete units, dominant or recessiveEarly 1900sChromosome segregate at divisionNumber halved during germ cell formationExpected if genes are on chromosomesT.H. MorganDeveloped Drosophilia (fruit flies) as a model organismProved experimentally that genes are on chromosomesKnow that genes are on chromosomesKnow that chromosomes are Protein and DNAWhich is the gene? Protein or DNAFrederick Griffith: 1928Streptococcus pneumoniaTwo strains of bacteria:Smooth Colony (S strain): Smooth coatRough Colony (R strain)He knew that immune system would kill R strain but S strain would cause pneumonia and kill you.Griffith’s transformation experimentS strain is encapsulated and virulent into mouse deadR strain is non-encapsulated and non-virulent into mouse liveHeat killed S strain injected S strain into mouse liveHeat killed virulent S strain + R strain into mouse diedBlood sample from dead mouse contains living S not heat killed SWhy?! 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 transformationTransformationConversion of R cells into S cells called transformationTransformation: change in genetic and physical composition as a result of assimilation of external DNASomething passed from nonliving cells to living cellsPermanently changed the nonpathogenic cells into being pathogenicHeritableInitiated a 14 year search for the Transforming PrincipleAvery, McCarty & MacLeod: 1944Tried to understand Griffith’s experimentPurified macromolecules from head killed S cell extractsTreated the extracts with various inactivating enzymesProteasRNaseDNaseMixed treated extracts with R cellsInjected miceProteas transformationRNase transformationDNase No transformationDNA is the transforming factor1944- 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 BacteriophagesPhages contain only protein & DNA1952: Hershey & Chase experiment: uses phages to determine if genetic material is protein or DNAThe Blender ExperimentUsed BacteriophageBacteria eatersComposed of DNA and protein onlyMake the protein radioactive and the DNA radioactive. See where the radioactive shows up.Protein floats away and is absorbed by the cell and the only thing 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 materialDNA: purines (2 rings) and pyrimidines (1 ring)Chargraff- determined base content of DNAChargraff’s Rules:1. Amount of A,T,C,G varies among species2. In each species, amount of A=T and C=GEach human chromosome contain an average of 140 million base pairs. Many possible different sequences with base pairingStructure of DNAOnce DNA was generally accepted as the genetic material, the race was on to determine the structureRosalind Franklin produced DNA pattern using X-ray diffraction techniques showed two strands and a double helixWatson and Crick- Won Nobel in 19621953 they determined the actual structure of DNA using Franklin’s x ray & Chargaff’s rules.Watson and Crick Base PairingPurines make H bonds with Purines and Pyrimidines with PyrimidinesDouble 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 outsideWatson and Crick Won the RaceThey won because:More information than any other groupNot afraid to ask smarter folks about stuff they didn’t knowMade some assumptions that ended up being correctGot luckyDuplicate DNA for cell reproduction and division.Red blood cells have no nucleus, white blood cells do.Complementary base pairing is a fundamental principleHave to be antiparallel to match up and complement eachotherSemi Conservative ReplicationReplication 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 strandsSemi Conservative Replication- final product is one parental strand combined with one new strandconservative because still keeping half of the parent moleculeDNA replication- template strand and replicated strand1. DNA is unwound by Helicase- H bonds broken2. 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 needed3. 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-readingUsually only 1 in a billion base pairs are incorrectly pairedDNA Replication ModelsCalTech Experiment – Meselson and StahlSemiconservative replication is not the only possible
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