Molecular biology and genetics final reviewDNA structure—lectures 1 and 2Avery—work proves that DNA is the genetic material•Provided proof that DNA mediates transformation•Fractionated extracts from heat killed IIIS cells and tested for transformation activity by mixing with IIR cells•Could remove proteins, lipids, polysaccharides and ribonucleic acid without affecting transformation activity•Destruction of DNA with DNase destroyed the transforming activity•Prepared DNA from dead IIIS cells•Treated the DNA with different enzymes•Mixed treated DNA to cultures of IIR cells•Therefore DNA was the “transforming Principle”Hershey-Chase experiment—work shows that DNA is the genetic material of phage T2Bacteriophage T21.50% protein and 50% DNA2.Phage tail binds E. coli3.Progeny viruses produced in the infected cell4.Bacterium lyses to release progeny virusKnew from other work that•Phages attach to bacteria by their tails•Osmotic shock ruptures the phage producing an empty-headed phage ‘ghost’ that is non infectious•Agitation of phage and bacteria in a waring blender prevents infection•It is the phage protein or phage DNA thatis responsible for the generation of progeny phage particles“These facts show that the bulk of the sulfur remains at the cell surface during infection, and takes no part in the multiplication of intracellular phage. The bulk of the phage DNA, on the other hand, enters the cell soon after adsorption of phage to bacteria”Therefore genetic material in phage T2 is DNA notproteinDNA structural propertiesDNA is composed of nucleotides—nitrogenous base + sugar + phosphatePyrimidines are single ringed nitrogenous bases: Uracil, cytosine and ThyminePurines are double ringed nitrogenous bases: Adenine and GuanineNucleotides are joined together to form a DNA polymerJoined by phosphodiester bondsLinks the C-3 of one sugar to C-5 of another sugarThe molecule has a 5’ to 3’ polarity5’ phosphate—3’ hydroxylSame bond is found in RNANumber of guanine equals the number of cytosine (GC)
Number of adenine equals the number of thymine (AT)DNA has a sugar-phosphate backboneThe bases are joined by hydrogen bonds (2 H-bonds between T and A) (3 H-bonds between C and G)DNA has a hydrophilic exterior (likes water)Base pairs are hydrophobic (do not like water)Strands are complementary—each strand contains the information to produce the other strandDNA PackingSupercoilingSupercoiled DNA is more compactRequires cutting, rotation and rejoiningCollapse into coilsProkaryotic DNA is supercoiled and folded onto an RNA scaffoldEukaryotic DNA is wrapped around nucleosomesNucleosomes are composed of 4 different histones•H2a•H2b•H3•H4Each are present as dimersHistones•High content of basic amino acids•Highly conserved amino acid sequence in all eukaryotes•Replaced by protamines in sperm•
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- Fall '13