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DNA: The Genetic Material Chapter 14 1 Outline • • • • • • Genetic Material Experiments Chemical Nature of Nucleic Acids Three-Dimensional Structure of DNA – Watson and Crick Replication – Semi Conservative – Replication Process Eukaryotic DNA Replication One-Gene/One-Polypeptide Hypothesis 2 Hammerling Experiment • Hammerling Experiment – Cells of green alga (Acetabularia) were cut into pieces and observed to see which were able to express hereditary information. Discovered hereditary information is stored in the cell’s nucleus. 3 Transplantation Experiments • Briggs and King (1952), and Steward (1958) conducted several experiments that conclusively determined each nucleus in a eukaryotic cell contains a full set of genetic instructions. 4 Transplantation Experiments • Several experiments were required to conclusively determine which substance made up genes. – Griffith experiment documented movement of genes from one organism to another (transformation) movement of material can alter the genetic makeup of the recipient cell 5 Avery and Hershey-Chase Experiments • • Avery experiment – removed almost all protein from bacteria, and found no reduction in transforming activity Hershey-Chase – labeled DNA and protein with radioactive isotope tracer determined hereditary information was DNA, not protein 6 7 8 Chemical Nature of Nucleic Acids • DNA made up of nucleic acids – Each nucleotide is composed of a five carbon sugar, a phosphate group, and an organic base. nucleotides distinguished by the bases reaction between phosphate group of one nucleotide and hydroxyl group of another is dehydration synthesis phosphodiester bond 9 Chemical Nature of Nucleic Acids • • Purines - large bases – adenine and guanine Pyrimidines - small bases – cytosine and thymine Chargaff’s rule A = T and G = C 10 Nucleotides 11 Three-Dimensional Structure of DNA • X-ray diffraction suggested DNA had helical shape with a 2 nanometer diameter. – Watson and Crick deduced DNA is an intertwined double helix. complementary base-pairing purines pairing with pyrimidines constant 2 nanometer diameter antiparallel configuration 12 DNA Double Helix 13 Semi-Conservative Replication • Each chain in the helix is a complimentary mirror image of the other. – double helix unzips and undergoes semiconservative replication each strand original duplex becomes one strand of another duplex confirmed by Meselson-Stahl experiment 14 15 Replication Process • Replication of DNA begins at one or more sites (replication origin). – DNA polymerase III and other enzymes add nucleotides to the growing complementary DNA strands. require a primer can only synthesize in one direction endonucleases exonucleases 16 DNA Replication 17 Replication Process • • DNA polymerase cannot link the first nucleotides in a newly synthesized strand. – RNA polymerase (primase) constructs an RNA primer. DNA polymerase adds nucleotides to 3’ end. – Leading strand replicates toward replication fork. – Lagging strand elongates from replication fork. Okazaki fragments 18 DNA Synthesis 19 Replication Process • • DNA ligase attaches fragment to lagging strand. – Because synthesis of the leading strand is continuous and the lagging strand is discontinuous, the overall replication of DNA is referred to as semi-discontinuous. DNA gyrase removes torsional strain introduced by opening double helix. 20 Replication Process • • • • • Opening DNA double helix – initiating replication – unwinding duplex – stabilizing single strands – relieving torque Building a primer Assembling complementary strands Removing the primer Joining Okazaki fragments 21 DNA Replication Fork 22 Replisome • Replisome is a macromolecular protein machine (replication organelle). – fast, accurate replication of DNA during cell division 23 Stages of Replication • • • Initiation – always occurs at the same site Elongation – majority of replication spent in elongation Termination – exact details unclear 24 Eukaryotic DNA Replication • Eukaryotes usually have multiple, large chromosomes. – multiple origins of replication 25 One-Gene/One-Polypeptide Hypothesis • Genes produce their effects by specifying the structure of enzymes. – Each gene encodes the structure of one enzyme (Beadle and Tatum). Many enzymes contain multiple polypeptide subunits, each encoded by a separate gene. 26 One-Gene / One-Polypeptide 27 Summary • • • • • • Genetic Material Experiments Chemical Nature of Nucleic Acids Three-Dimensional Structure of DNA – Watson and Crick Replication – Semi Conservative – Replication Process Eukaryotic DNA Replication One-Gene/One-Polypeptide Hypothesis 28 29