1 DNA: The Genetic Material Chapter 14

2 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

3 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.

4 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.

5 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

6 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

7

8

9 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

10 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

11 Nucleotides

12 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 inter- twined double helix.  complementary base-pairing  purines pairing with pyrimidines  constant 2 nanometer diameter  antiparallel configuration

13 DNA Double Helix

14 Semi-Conservative Replication Each chain in the helix is a complimentary mirror image of the other. – double helix unzips and undergoes semi- conservative replication  each strand original duplex becomes one strand of another duplex  confirmed by Meselson-Stahl experiment

15

16 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

17 DNA Replication

18 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

19 DNA Synthesis

20 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.

21 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

22 DNA Replication Fork

23 Replisome Replisome is a macromolecular protein machine (replication organelle). – fast, accurate replication of DNA during cell division

24 Stages of Replication Initiation – always occurs at the same site Elongation – majority of replication spent in elongation Termination – exact details unclear

25 Eukaryotic DNA Replication Eukaryotes usually have multiple, large chromosomes. – multiple origins of replication

26 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.

27 One-Gene / One-Polypeptide

28 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

29