CHAPTER 14:  DNA THE GENETIC MATERIAL

I. History of DNA (deoxyribonucleic acid)

            A. Needed to fit criteria:

                        1. store information to control development and metabolism

                        2. can be replicated during cell division and from one generation to the

                            next

                        3. mutations: rare changes that provide genetic variability

            B. Griffith (1931): pneumonia bacterium (S and R) and mice, did not know what

                the substance was that transformed the R to live S strain

            C. Avery: discovered that DNA is the transforming substance that Griffith worked

                 with

            D. Bacteriophages: viruses that attack bacteria

            E. Hershey and Chase (1952): labeled viral DNA with radioactive ³²P, it went

                inside the bacteria, not the radioactive ³⁵S for the protein coat, proved DNA is

                the genetic material

II. Structure of DNA

            A. 4 Nucleotides

                        1. purines: adenine (A), guanine (G)

                        2. pyrimidines: thymine (T), cytosine (C)

            B. Chargaff's rules

                        1. The amount of A,T,G, and C in DNA varies from species to species

                        2. In each species, the amount of A=T and G=C

            C. Rosalind Franklin: used X-ray diffraction, showed DNA is a helix

            D. Watson and Crick model (1950's)

                        1. double helix

                                    a. sugar-phosphate backbones outside

                                    b. nucleic acid base pairs on the inside

                        2. complementary base pairing: A=T, C=G

III. DNA replication

            A. semiconservative replication: 2 old strands make 2 new DNA molecules, each

                 have 1 old and 1 new molecule

                        1. unwinding: DNA "unzipped" (H bonds broken) by helicase enzyme

                        2. complementary base pairing: new nucleotides pair up with exposed

                            bases by DNA polymerase

                        3. joining: complementary nucleotides link together forming 2 new DNA

                            molecules, each have 1 old and 1 new strand

            B. Meselson and Stahl: (1958) centrifuged DNA in heavy nitrogen (¹⁵N), then put

                 in light nitrogen (¹⁴N), new strands had half heavy and half light strands

            C. Replication order: DNA strands run in opposite directions (5' and 3'), grows

                 from 5' end

            D. Replication errors: proofreading enzyme, some mistakes still occur

            E. In Prokaryotes:

                        1. circular loop of DNA, attached to cell membrane

                        2. replicates, separates in binary fission

            F. In Eukaryotes

                        1. replication bubbles spread in 2 directions along DNA strand until done

                        2. precedes cell division, during interphase

            G. Jumping genes or transposons, can move, provide variability

                        1. can cause localized mutations

                        2. can carry a copy of host genes when they jump, causing translocations,

                            deletions, and inversions

                        3. can leave copies before jumping, causing duplication

                        4. can cause resistance to antibiotics in bacteria