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Transcript
Miescher Discovered DNA • 1868 DNA Structure and Function Chapter 13 • 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 Griffith Discovers Transformation • 1928 • Attempting to develop a vaccine Figure 13.2 Page 217 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 heatkilled S cells. 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. Mice die. Live S cells in their blood. • Isolated two strains of Streptococcus pneumoniae – Rough strain was harmless – Smooth strain was pathogenic Figure 13.3 Page 218 1 Transformation Oswald & Avery • 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 Hershey & Chase’s Experiments Bacteriophages • Viruses that infect bacteria • Consist of protein and DNA • Inject their hereditary material into bacteria • What is the transforming material? • Cell extracts treated with proteindigesting enzymes could still transform bacteria • Cell extracts treated with DNA-digesting enzymes lost their transforming ability • Concluded that DNA, not protein, transforms bacteria • Created labeled bacteriophages bacterial cell wall plasma membrane – Radioactive sulfur – Radioactive phosphorus • Allowed labeled viruses to infect bacteria cytoplasm • Asked: Where are the radioactive labels after infection? Figure 13.4b Page 219 virus particle labeled with 35S Hershey and Chase Results virus particle labeled with 32P Structure of Nucleotides in DNA • Each nucleotide consists of bacterial cell (cutaway view) – Deoxyribose (5-carbon sugar) – Phosphate group – A nitrogen-containing base label outside cell label inside cell • Four bases – Adenine, Guanine, Thymine, Cytosine Figure 13.5 Page 219 2 Nucleotide Bases Composition of DNA ADENINE (A) phosphate group GUANINE (G) – Amount of adenine relative to guanine differs among species deoxyribose THYMINE (T) • Chargaff showed: CYTOSINE (C) – Amount of adenine always equals amount of thymine and amount of guanine always equals amount of cytosine A=T and G=C Figure 13.6 Page 220 Rosalind Franklin’s Work • Was an expert in X-ray crystallography • Used this technique to examine DNA fibers Watson-Crick Model • DNA consists of two nucleotide strands • Strands run in opposite directions • Strands are held together by hydrogen bonds between bases • Concluded that DNA was some sort of helix Watson-Crick Model • A binds with T and C with G • Molecule is a double helix DNA Structure Helps Explain How It Duplicates • DNA is two nucleotide strands held together by hydrogen bonds • Hydrogen bonds between two strands are easily broken • Each single strand then serves as template for new strand Figure 13.7 Page 221 3 Base Pairing during Replication DNA Replication • Each parent strand remains Each old strand serves as the template for complementary new strand intact • Every DNA molecule is half new old old new “old” and half “new” Figure 13.10 Page 223 Figure 13.9 Page 222 A Closer Look at Strand Assembly Enzymes in Replication • Enzymes unwind the two strands Energy for strand assembly is provided by removal of two phosphate groups from free nucleotides • DNA polymerase attaches complementary nucleotides • DNA ligase fills in gaps • Enzymes wind two strands together Continuous and Discontinuous Assembly newly forming DNA strand one parent DNA strand Figure 13.10 Page 223 DNA Repair • Mistakes can occur during replication Strands can only be assembled in the 5’ to 3’ direction • DNA polymerase can read correct sequence from complementary strand and, together with DNA ligase, can repair mistakes in incorrect strand Figure 13.10 Page 223 4 Cloning Dolly: Cloned from an Adult Cell • Making a genetically identical copy of an individual • Showed that differentiated cells could be used to create clones • Researchers have been creating clones for decades • Sheep udder cell was combined with enucleated egg cell • These clones were created by embryo splitting • Dolly is genetically identical to the sheep that donated the udder cell More Clones • • • • • Mice Cows Pigs Goats Guar (endangered species) 5