Chapter 16 Chromatin made up of DNA and Protein Ideas about genetic material circa 1940 s The case for proteins o All the different variations The case against nucleic acids o Only 5 nucleic acids o Structure is carbon rings with nitrogen bases o Purines and Pyrimidine o Deoxyribose sugar attached to DNA o Ribose sugar attached to RNA o Phosphate is also attached Building the Case for Nucleic Acids 1928 Griffith recognized a principle Process Proposes Heritable substance transforms cells Transformation change the genotype and phenotype due to assimiliation of external DNA Pathogenic strain o Living S smooth Strain o Kills the mouse Non Pathogenic Strain o Living R rough cells o Mouse healthy Heat killed S cells control o Mouse healthy Mixture of heat killed S cells and living R cells o Mouse dies o Living S cells are in the mouse 1944 Avery McCarty MacLeod experiments o Only DNA transforms cells o Living R S cell protein Mouse healthy o Living R S cell RNA Mouse Healthy o Living R S cell DNA Mouse DIES Only DNA transforms bacteria 1950 Erwin Chargraff o adenines thymines o guanines cytosines Building the case for Nucleic Acids Hershey and Chase 1952 figure 16 4 o Viral DNA can program cells Wilkins and Rosaline Franklin X ray crystallography o X ray diffraction pass X rays through aligned DNA fibers produces shadows of structure Watson and Crick 1953 o Radically different structure 2 helical chains made of phosphate sugars bases located on the inside o Novel features paried bases purine pyramidine H bonds hold together 2 between A T 3 between G C o Other important points Ribose sugar would not fit Structure suggests replication mechanism o Components Phosphate Sugar Deoxyribose Nitrogenous bases ATGC o Directionality 5 w phosphate 3 w OH on sugar Antiparallel Orientation Replication of DNA Semi conservative model of replications proposed by Watson and Crick 1954 Evidence provided by Meselson and Stahl 1958 Experimental Setup Fig 16 11 Used heavy and light isotopes of nitrogen 15N and 14N to label strands of DNA in separate rounds of replication DNA Structure Anti Parallel Helix sequence of bases Covalent bonds H bonds When does DNA replication occur S phase of Cell Cycle DNA Replication Initiation Helicase unwind and separate DNA Single Strand binding protein stabilize unwound DNA Primase synthesizes RNA primer from parental DNA Synthesizing the new strand DNA polymerase III o Requires template primer o ONLY synthesizes from 5 to 3 o Incorporates one nucleotide at time mathes parent nucleotide forms covalent bond with 3 end of previous sugar Two methods of elongation 1 Continuous synthesis on LEADING strand a DNA Pol III Elongates 2 Synthesize series of fragments on LAGGING Strand Chromosomes get shorter after every replication Telomeres allow limited cell divisions w o eroding genes