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Ch 5 and16 A Close Look at the Hereditary Molecules • Protein sequence-->programmed by genes • Genes are made of DNA, a nucleic acid LE 5-25 Flow of genetic information DNA DNA Synthesis of mRNA in the nucleus mRNA RNA NUCLEUS CYTOPLASM Protein mRNA Movement of mRNA into cytoplasm via nuclear pore Ribosome Synthesis of protein Polypeptide Amino acids The Roles of Nucleic Acids • Two types: – Deoxyribonucleic acid (DNA) – Ribonucleic acid (RNA) • DNA provides directions for its own replication. • DNA directs synthesis of messenger RNA (mRNA) • mRNA controls protein synthesis. • Protein synthesis occurs on ribosomes. LE 5-26a 5 end Nucleic acid building block Nucleoside Nitrogenous base Phosphate group Nucleotide 3 end Polynucleotide, or nucleic acid Pentose sugar Nucleic Acid Structure Monomers nucleotide (3 parts) 1. nitrogenous base nucleoside 2. 5 C sugar 3. Phosphate Polymer polynucleotide or nucleic acid LE 5-26b Nitrogenous bases Pyrimidines Cytosine C Thymine (in DNA) Uracil (in RNA) U T Purines Adenine A Guanine G Pentose sugars Deoxyribose (in DNA) Nucleoside components Ribose (in RNA) Important Nucleic Acid Distinctions Two kinds of bases Pyrimidines-one ring (T,U,C) Purines- two rings (G,A) DNA the sugar = deoxyribose NO 2’ OH (hydroxyl) RNA the sugar= ribose YES 2’ OH Nucleotide Polymers • Nucleotides (nt) connect through phosphodiester bond 5’ Phosphate--> 3’OH • Creation of a sugar-phosphate backbone with bases as appendages. • Sequence of bases along DNA or mRNA polymer unique for each gene. LE 16-7 5 end Hydrogen bond 3 end 1 nm 3.4 nm 3 end 0.34 nm Key features of DNA structure 5 end Partial chemical structure Space-filling model Two DNA strands bind together through complementary base-pairing. Structure of DNA double helix: published in 1953 Francis Crick James Watson Watson JD, Crick FHC. 1953. Molecular structure of nucleic acids: a structure for deoxyribonucleic acids. Nature 171:738. LE 16-6 Partly based on Franklin’s x-ray diffraction data Rosalind Franklin Franklin’s X-ray diffraction photograph of DNA LE 16-8 Chargaff’s rules (1940s): Amount of A=T G=C LE 16-UN298 Watson & Crick: built model of DNA and tested possible combinations of bases Did model support Chargaff’s observations and Franklin’s x-ray diffraction data? Purine + purine: too wide Pyrimidine + pyrimidine: too narrow Purine + pyrimidine: width consistent with X-ray data LE 16-7 Antiparallel DNA strands 5 end Hydrogen bond 3 end 1 nm 3.4 nm 3 end 0.34 nm Key features of DNA structure 5 end Partial chemical structure Space-filling model Two DNA strands bind together through complementary base-pairing. The DNA Double Helix • Two polynucleotides (strands) base-paired together GC, AT (complementary base-pairing) • Double helix • Two sugar-phosphate backbones run in opposite 5´ to 3´ directions - antiparallel • One DNA molecule includes many genes Complementary base pairs A=T 2 H-bonds Sugar Adenine (A) Sugar Thymine (T) G=C 3 H-bonds Sugar Sugar Guanine (G) Cytosine (C) Behavior of DNA Draw a 10 base pair double-stranded DNA (dsDNA) that is rich in AT. Draw a 10 base pair double-stranded DNA (dsDNA) that is rich in GC. If these were placed in a tube of boiling water what would happen? DNA would become single stranded (ssDNA) (denatured or melted). Which DNA would denature first. Why? AT rich fragment less stable 2 H-bonds/bp versus 3 H-bonds/bp DNA Used as Evolutionary Ruler • Linear sequences of DNA in chromosomes – passed from parents to offspring • Two closely related species are more similar in DNA sequence than distantly related species • Similarity of DNA sequence – Determines evolutionary relatedness 1. Compare the human sequence to the frog and mouse. Which sequence is most similar to human? human 5’ GAACCTTCCAATTGATCT3’ 5’ GAACCAACCAATTAAACT3’ frog 5’ GAACCTTCGAATTGATCT3’ mouse 2. Write in the complementary strand for each. Earlier data suggested that DNA was hereditary material Model system: Drosophila melanogaster Investigator: Thomas Hunt Morgan (early 1900’s) Evidence: white eye phenotype associated with X-chromosome Model system: bacteria and viruses Investigators: Many Evidence: various Evidence That DNA Can Transform Bacteria Evidence for genetic role of DNA (Frederick Griffith,1928) Heat-killed pathogenic “S” Streptococcus pneumoniae + “R”non-pathogenic bacterial strain Some living bacteria became pathogenic Transformation of “R’ to ‘S”, How could one determine pathogenicity experimentally? LE 16-2 Living S cells (control) Living R cells (control) Heat-killed S cells (control) Mixture of heat-killed S cells and living R cells RESULTS Mouse dies Mouse healthy Mouse healthy Mouse dies Living S cells are found in blood sample What molecule was responsible for conferring a new phenotype into an organism? • Oswald Avery, Maclyn McCarty, and Colin MacLeod (1944) • Published results – Showed DNA from bacteria NOT protein--> caused transformation of “R” to “S” Independent confirmation • Alfred Hershey and Martha Chase (1952) – Used bacterial virus (bacteriophage) (T2) to ask whether DNA or protein was hereditary material LE 16-3 Phage head Tail Tail fiber Bacterial cell 100 nm DNA LE 16-4 Hershey & Chase labeling experiment Phage Radioactive protein Empty protein shell Radioactivity (phage protein) in liquid Bacterial cell Batch 1: Sulfur (35S) DNA Phage DNA Protein radiolabelled Centrifuge Pellet (bacterial cells and contents) Radioactive DNA Batch 2: Phosphorus (32P) DNA radiolabelled Centrifuge Pellet Radioactivity (phage DNA) in pellet Phage produced in and released from bacteria with radioactive DNA. Hershey & Chase results -Suggest that DNA, not protein, is transferred to bacteria by phage. -DNA programs the reproduction of more phage. Contains important genetic instructions. I’m a pretty cool molecule but I’ll still answer your questions.