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DNA: Structure and Function The DNA Revolution • 1940s-1960s • Griffith & Avery—DNA transformed pneumococcus bacteria. • Encouraged the study of prokaryotic chromosomes. • Hershey and Chase—Bacteriophage experiment. DNA from viruses is injected to host bacteria cells. Griffith and Avery Hershey & Chase Hershey & Chase Hershey & Chase The Search for the Structure of the DNA Molecule • 1951—Rosalind Franklin—X-ray crystallography • Chargaff—Chargaff’s rules. Ratio of nitrogenous bases in DNA. Complimentary bases. • Watson & Crick--1953 Franklin Chargaff Watson & Crick Model of DNA • • • • • Nucleotides Sugar—Deoxyribose Phosphate Group Nitrogenous Bases Adenine & Thymine are complementary. • Cytosine and Guanine are complimentary. • Purines—Adenine & Guanine • Pyrimidines— Cytosine & Thymine • 5’ End—Phosphate side • 3’ End—Other side Watson & Crick Model • DNA is composed of 2 chains of nucleotides that form a double helix shape. • The two strands are antiparallel. • The backbone of the DNA molecule is composed of alternating phosphate groups and sugars. • The complimentary nitrogenous bases form hydrogen bonds between the strands. • A is complimentary to T and G is complimentary to C. DNA Model DNA • • • • • Functions 1. Storage of genetic information 2. Self-duplication & inheritance. 3. Expression of the genetic message. DNA’s major function is to code for proteins. • Information is encoded in the order of the nitrogenous bases. Chromosomes • • • • • Prokaryotic Circular Very small 1 chromosome per cell Some enzymes and proteins are associated with the DNA. • Not housed in a nucleus. • • • • • • • • Eukaryotic Linear Fairly long Several chromosomes per cell. Histone proteins--”spools”. Same in all eukaryotes Housed in a nucleus. Nucleosome—2 loops of DNA wrapped around 8 histone proteins. Unity theme. Central Dogma of Genetics • • • • DNA mRNA Proteins Traits • • • • Replication Unzip Template Floating nucleotides • Template • Sense strand • Semiconservative Replication • DNA Polymerase • Proofreading functions • 5’ to 3’ only • Replication fork • Discontinuous assembly • Okazaki fragments RNA • • • • 3 types—mRNA, tRNA, rRNA Ribose Uracil replaces Thymine Single stranded helix Transcription • • • • • • • RNA polymerase Messenger RNA mRNA Unzipping of DNA helix. Sense strand or template strand Codons Genetic code Translation • • • • • • • • Ribosomes rRNA Large and small subunits Codons Initiator or start codon Methionine (AUG) Stop codons tRNA • • • • • Initiation Chain Elongation Peptide bonds Chain termination Polysome