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Structure of DNA Watson and Crick used two sets of observations to infer the structure of DNA X-ray crystallography: DNA is a helix Subunit analysis Subunits of nucleic acids (DNA) (RNA) Polymerization of DNA subunits by condensation T Deoxyribose The double helix “Complementary” base pairs stick together through H-bonds Note: antiparallel The double helix The helical twist pulls the hydrophobic bases close together DNA chains can be very long ! E. coli chromosome: 4.6x106 base pairs: 4.6x 106 x .34 nm = 1.5x106 nm = 1.5 mm ! Human DNA: 6x10-12 g/cell x 1/660 mol bp/g x 6.023x1023 bp/mol bp x 0.34x10-9 m/bp = 1.9 m ! Bacterial, viral DNA “chromosomes” are circles ! DNA in human chromosomes (and DNA of all eukaryotes) are linear strands ! “Z-DNA”, left-handed double helix, is also possible Melting and re-association of DNA ! H-bonds are weak bonds, broken by high temperature ! DNA “melts”--strands separate--at 85-95oC ! When cool, the strands can re-associate, if they have complementary base sequences ! Use this technique to recognize specific sequences ! make radioactive DNA of particular sequence ! fix unknowns to membrane ! add radioactive “probe” ! slowly cool to allow re-association ! wash off unassociated DNA ! measure radioactivity Question: Is the gene for Starlink Bt toxin present in my taco shells? Sample 1 Sample 2 Sample 3 32P: radioactive phosphorus In eukaryotes, DNA chains coil to become shorter and thicker Telomere s Centromere Replication of chromosomes ! Eukaryotes: chromosomes are linear (two to several hundred per cell; humans = 46) ! Distinguished visually by length, position of “centromere” ! Replication during S phase gives two pieces (“chromatids”), connected by centromere ! Progeny and parent DNA have identical base sequences G1 G2 Why do parent and progeny DNAs have identical base sequences? •Parent DNA serves as a template for the synthesis of progeny DNA DNA synthesis: the molecular mechanism •Separate the template strands •Add subunits: deoxyribonucleotide triphosphates •Connect (form new sugar-phosphate bonds) Complications: ! Splitting H-bonds ! Priming (DNA synthesis needs a primer) ! Anti-parallel templates Complications: ! Splitting H-bonds: unwinding enzyme, DNA helicase, binding proteins ! Priming (DNA synthesis needs a primer): RNA "primase" makes RNA; DNA added ! Antiparallel templates: Okazaki fragments of new DNA on one strand (for a short time) Summary ! The information in DNA is in the base sequence. ! The complementary strands of DNA represent two copies of all the information in the cell. ! Through hybridization, one strand can be used to identify a complementary strand. ! Duplication of DNA information is needed before a cell reproduces. ! DNA polymerase (together with other proteins) uses the two existing DNA strands as templates to synthesize two new strands.