Presentation on theme: "DNA Structure and Function Chapter 13. Miescher Discovered DNA 1868 Johann Miescher investigated the chemical composition of the nucleus Isolated an organic."— Presentation transcript:
Miescher Discovered DNA 1868 Johann Miescher investigated the chemical composition of the nucleus Isolated an organic acid that was high in phosphorus He called it nuclein We call it DNA (deoxyribonucleic acid)
Mystery of the Hereditary Material Originally believed to be an unknown class of proteins Thinking was –Heritable traits are diverse –Molecules encoding traits must be diverse –Proteins are made of 20 amino acids and are structurally diverse
Structure of the Hereditary Material Experiments in the 1950s showed that DNA is the hereditary material Scientists raced to determine the structure of DNA 1953 - Watson and Crick proposed that DNA is a double helix Figure 13.2 Page 217
Griffith Discovers Transformation 1928 Attempting to develop a vaccine Isolated two strains of Streptococcus pneumoniae –Rough strain was harmless –Smooth strain was pathogenic
Griffith Discovers Transformation 1. Mice injected with live cells of harmless strain R. 2. Mice injected with live cells of killer strain S. 3. Mice injected with heat-killed S cells. 4. Mice injected with live R cells plus heat- killed S cells. Mice die. Live S cells in their blood. Mice live. No live R cells in their blood. Mice die. Live S cells in their blood. Mice live. No live S cells in their blood. Figure 13.3 Page 218
Transformation What happened in the fourth experiment? The harmless R cells had been transformed by material from the dead S cells Descendents of the transformed cells were also pathogenic
Oswald & Avery What is the transforming material? Cell extracts treated with protein- digesting enzymes could still transform bacteria Cell extracts treated with DNA-digesting enzymes lost their transforming ability Concluded that DNA, not protein, transforms bacteria
Bacteriophages Viruses that infect bacteria Consist of protein and DNA Inject their hereditary material into bacteria cytoplasm bacterial cell wall plasma membrane Figure 13.4b Page 219
Hershey & Chase’s Experiments Created labeled bacteriophages –Radioactive sulfur –Radioactive phosphorus Allowed labeled viruses to infect bacteria Asked: Where are the radioactive labels after infection?
virus particle labeled with 35 S virus particle labeled with 32 P bacterial cell (cutaway view) label outside cell label inside cell Hershey and Chase Results Figure 13.5 Page 219
Information Mon., 28 November –Chapter 14 and 16 highlights Wed., 30 November –Final exam review – BRING YOUR QUESTIONS! –Instructor evaluations Mon., 12 December, 2:15-4:15pm in C317 –Final Exam firstname.lastname@example.org Exam 3 will be returned at the end of this class.
Structure of Nucleotides in DNA Each nucleotide consists of –Deoxyribose (5-carbon sugar) –Phosphate group –A nitrogen-containing base Four bases –Adenine, Guanine, Thymine, Cytosine
Composition of DNA Chargaff showed: –Amount of A relative to G differs among species –Always: A=T and G=C
Rosalind Franklin’s Work Expert in X-ray crystallography Examined DNA fibers Concluded that DNA was some sort of helix
Watson-Crick Model of DNA 2 nucleotide strands –Run in opposite directions –Held together by H bonds between bases A binds with T and C with G Molecule is a double helix
DNA Structure Helps Explain How It Duplicates DNA is 2 nucleotide strands held together by H bonds H bonds between 2 strands are easily broken Each single strand then serves as template for new strand
DNA Replication new old Each parent strand remains intact Every DNA molecule is half “old” and half “new” Figure 13.9 Page 222
Base Pairing during Replication Each old strand serves as the template for complementary new strand Figure 13.10 Page 223
Enzymes in Replication Enzymes unwind the two strands DNA polymerase attaches complementary nucleotides DNA ligase fills in gaps Enzymes wind two strands together
A Closer Look at Strand Assembly Energy for strand assembly is provided by removal of two phosphate groups from free nucleotides newly forming DNA strand one parent DNA strand Figure 13.10 Page 223
Continuous and Discontinuous Assembly Strands can only be assembled in the 5’ to 3’ direction Figure 13.10 Page 223
DNA Repair Mistakes can occur during replication DNA polymerase can read correct sequence from complementary strand and, together with DNA ligase, can repair mistakes in incorrect strand
Information Mon., 28 November –Chapter 14 and 16 highlights Wed., 30 November –Final exam review – BRING YOUR QUESTIONS! –Instructor evaluation Mon., 12 December, 2:15-4:15pm in C317 –Final Exam email@example.com Exam 3 will be returned... Now!