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DNA Structure and Function Griffith • Griffith showed some heredity material could move into live harmless bacteria and make a lethal strain Griffith’s Experiment Mice injected with live cells of harmless strain R. Mice injected with live cells of killer strain S. Mice injected with heat-killed S cells. Mice injected with live R cells plus heat-killed 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. Heat killed S strain, but releases the killer genes that the R strain incorporated. Virus • • • • Basically only two parts DNA inside Protein Coat outside Carries genetic material – in which part? genetic material viral coat bacterial cell wall plasma membrane sheath base plate tail fiber cytoplasm Hershey-Chase • Experiment with viruses showed that the genetic information was in DNA, not protein. virus particle labeled with 35S virus particle labeled with 32P bacterial cell label outside cell Hershey and Chase showed DNA carries genetic information label inside cell The Hershey-Chase experiment: phages Fig. 9.5a Fig. 9.5bc Fig. 9.6a Fig. 9.6b Watson and Crick Rosalind Franklin’s X-ray Crystallography DNA • Deoxyribonucleic Acid = DNA • Made up of nucleotides • Nucleotides have three parts – Sugar – Phosphate group – Nitrogenous base • Sugar-phosphates make the DNA back bone that is covalently bonded phosphate group adenine (A) base with a double-ring structure guanine (G) base with a double-ring structure thymine (T) base with a single-ring structure cytosine (C) base with a single-ring structure sugar (ribose) Nitrogenous bases • • • • Four different nitrogenous bases Have one or two rings Form 2 or 3 hydrogen bonds Bases can only pair one way: – A-T – C-G • The sequence of nitrogenous bases carries the genetic information or or one base pair DNA Structure • Forms a double helix • Two complementary strands held together by hydrogen bonds Fig. 9.5a Fig. 9.5bc Meselson- Stahl • Heavier isotope falls to bottom of flask • Timed to capture each new generation of bacteria • Shows radiation diluted by half each generation, didn’t stay together. • Showed semi-conservative replication Fig. 9.6a Fig. 9.6b DNA replication • Semiconservative – one old and one new strand in each daughter molecule • Each original strand acts as a template to form a new complementary strand DNA Replication Three enzymes: • Helicase – unwinds DNA • DNA Polymerase adds new nucleotides off the template – Works in one direction only – One side makes separate fragments • Ligase seals up the fragments – Proofreads DNA, fixes mistakes Three Enzymes DNA Replication Helicase Unwinds helix Polymerase adds nucleotides Ligase Seals fragments continuous assembly on one strand newly forming DNA strand discontinuous assembly on other strand one parent DNA strand DNA Replication • Starts in several spots • Pretty rapid process. • Very accurate, few errors Chromosomes • DNA Replication forms the sister chromatids just before Mitosis or meiosis Fig. 9.10 Mutations • When cells are dividing, the DNA strands are apart. • A change in the DNA has no complementary strand to fix it. • These changes get incorporated into new strand • They are passed on in all the new divisions. • Dividing cells collect mutations, can become cancerous – Skin, lungs, liver Transcription DNA Translation protein RNA nucleus • DNA to RNA • Copies only select genes, not all at once • Each gene is on only one strand of DNA, not the complimentary strand cytoplasm • • • • RNA to Protein In cytoplasm Uses ribosome Can make multiple copies • Relatively short lived RNA • Always a single strand • Use Ribose as a sugar • Uses Uracil – and Adenine, Cytosine, Guanine • mRNA carries genetic info. From nucleus to cytoplasm • tRNA carries amino acids to ribosome, links the genetic code • rRNA makes up most of ribosome URACIL (U) base with a single-ring structure phosphate group sugar (ribose) DNA RNA protein Chromosome during transcription Transcription • At Initiation RNA polymerase binds start of gene and uncoils DNA. • At Elongation RNA polymerase moves along the gene briefly binding nucleotides to DNA (only about 10 nucleotides at a time), as the RNA nucleotides join together in a making a single complimentary strand • At Termination the mRNA moves out of nucleus, detaches and DNA recoils RNA polymerase DNA transcribed DNA winds up again newly forming RNA transcript DNA to be transcribed unwinds DNA template at the assembly site Fig. 9.11 growing RNA transcript 3’ 5’ 3’ 5’ direction of transcription 5’ 3’ m RNA modification • new pre-mRNA includes extra nucleotides called introns must be cut out. • The exons remain to go on to the cytoplasm carrying the information for the protein synthesis. Fig. 9.17 Translation • mRNA code directs sequence of amino acids in protein. • Uses ribosomes to assemble proteins • At Initiation a tRNA attaches to the mRNA and the ribosome subunits combine. – Start codon is AUG • At Elongation the ribosome moves down the mRNA assembling the amino acids – Only 6 nucleotides at at time – Each triplet codes for one amino acid • At Termination a stop codon causes the protein chain and the ribosome and mRNA to separate from each other. base sequence of gene region mRNA amino acids arginine glycine tyrosine tryptophan tyrosine Genetic Code uses triplets of Nucleotides to place amino acids in sequence Fig. 9.13 Fig. 9.14 Fig. 9.15 Fig. 9.16 Mutations • a Point Mutation is a single base pair nucleotide substitution – May cause a single amino acid change, or none • Insertions and Deletions (adding or removing nucleotides) reset the reading frame and change subsequent amino acids. – Missense makes a new amino acid chains – Nonsense adds stop codons and synthesis cuts off. Fig. 9.23 original base triplet in a DNA strand a base substitution within the triplet (red) During replication, proofreading enzymes make a substitution: possible outcomes: or original, unmutated sequence a gene mutation Mutations mRNA parental DNA arginine glycine tyrosine tryptophan asparagine amino acids altered mRNA arginine glycine leucine leucine glutamate DNA with base insertion altered aminoacid sequence Polyribosomes – make multiple copies of the protein at the same time on the same mRNA Fig. 9.18 Transcription mRNA rRNA protein subunits mRNA ribosomal transcripts subunits tRNA tRNA Translation amino acids, tRNAs, ribosomal subunits Protein From DNA to protein