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UCSD BILD 1 - Lecture

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LE 5-25NUCLEUSDNACYTOPLASMmRNAmRNARibosomeAminoacids Synthesis ofmRNA in the nucleus Movement ofmRNA into cytoplasmvia nuclear pore Synthesis of proteinPolypeptideCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsOverview: Life’s Operating Instructions• In 1953, James Watson and Francis Crickintroduced an elegant double-helical model forthe structure of deoxyribonucleic acid, or DNA• DNA, the substance of inheritance, is the mostcelebrated molecule of our time• Hereditary information is encoded in DNA andreproduced in all cells of the body• This DNA program directs the development ofbiochemical, anatomical, physiological, and (tosome extent) behavioral traitsCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsConcept 16.1: DNA is the genetic material• Early in the 20th century, the identification ofthe molecules of inheritance loomed as a majorchallenge to biologistsCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsThe Search for the Genetic Material: ScientificInquiry• When Morgan’s group showed that genes arelocated on chromosomes, the two componentsof chromosomes—DNA and protein—becamecandidates for the genetic material• The key factor in determining the geneticmaterial was choosing appropriateexperimental organisms• The role of DNA in heredity was firstdiscovered by studying bacteria and theviruses that infect themCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsEvidence That DNA Can Transform Bacteria• The discovery of the genetic role of DNA began withresearch by Frederick Griffith in 1928• Griffith worked with two strains of a bacterium, apathogenic “S” strain and a harmless “R” strain• When he mixed heat-killed remains of the pathogenicstrain with living cells of the harmless strain, someliving cells became pathogenic• He called this phenomenon transformation, nowdefined as a change in genotype and phenotype dueto assimilation of foreign DNALE 16-2Living S cells(control)Living R cells(control)Heat-killedS cells (control)Mixture of heat-killedS cells and livingR cellsMouse diesLiving S cellsare found in blood sampleMouse healthyMouse healthyMouse diesRESULTSCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• In 1944, Oswald Avery, Maclyn McCarty, andColin MacLeod announced that thetransforming substance was DNA• Their conclusion was based on experimentalevidence that only DNA worked in transformingharmless bacteria into pathogenic bacteria• Many biologists remained skeptical, mainlybecause little was known about DNACopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsEvidence That Viral DNA Can Program Cells• More evidence for DNA as the genetic materialcame from studies of a virus that infectsbacteria• Such viruses, called bacteriophages (orphages), are widely used in molecular geneticsresearchLE 16-3BacterialcellPhageheadTailTail fiberDNA100 nmCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• In 1952, Alfred Hershey and Martha Chaseperformed experiments showing that DNA isthe genetic material of a phage known as T2• To determine the source of genetic material inthe phage, they designed an experimentshowing that only one of the two componentsof T2 (DNA or protein) enters an E. coli cellduring infection• They concluded that the injected DNA of thephage provides the genetic informationLE 16-4Bacterial cellPhageDNARadioactiveproteinEmptyprotein shellPhageDNARadioactivity(phage protein)in liquidBatch 1:Sulfur (35S)RadioactiveDNACentrifugePellet (bacterialcells and contents)PelletRadioactivity(phage DNA)in pelletCentrifugeBatch 2:Phosphorus (32P)Copyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsAdditional Evidence That DNA Is the GeneticMaterial• In 1947, Erwin Chargaff reported that DNAcomposition varies from one species to thenext• This evidence of diversity made DNA a morecredible candidate for the genetic material• By the 1950s, it was already known that DNAis a polymer of nucleotides, each consisting ofa nitrogenous base, a sugar, and a phosphategroupLE 16-5Sugar–phosphatebackbone5′ endNitrogenousbasesThymine (T)Adenine (A)Cytosine (C)DNA nucleotidePhosphate3′ endGuanine (G)Sugar (deoxyribose)Copyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsBuilding a Structural Model of DNA: Scientific Inquiry• After most biologists became convinced thatDNA was the genetic material, the challengewas to determine how its structure accounts forits role• Maurice Wilkins and Rosalind Franklin wereusing a technique called X-ray crystallographyto study molecular structure• Franklin produced a picture of the DNAmolecule using this techniqueLE 16-6Franklin’s X-ray diffractionphotograph of DNARosalind FranklinLE 5-24aPhotographic filmDiffracted X-raysX-raysourceX-raybeamX-raydiffraction patternCrystalLE 5-24bNucleic acid3D computer modelX-ray diffraction patternProteinCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• Cytosine makes up 38% of the nucleotides in asample of DNA from an organism. What percentof the nucleotides in this sample will be thymine?– A. 12– B. 24– C. 31– D. 38– E. It cannot be determined from the information provided.Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• Franklin’s X-ray crystallographic images ofDNA enabled Watson to deduce that DNA washelical• The X-ray images also enabled Watson todeduce the width of the helix and the spacingof the nitrogenous bases• The width suggested that the DNA moleculewas made up of two strands, forming a doublehelixCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• Watson and Crick built models of a double helix toconform to the X-rays and chemistry of DNA• Franklin had concluded that there were twoantiparallel sugar-phosphate backbones, with thenitrogenous bases paired in the molecule’s interior• At first, Watson and Crick thought the bases pairedlike with like (A with A, and so on), but such pairingsdid not result in a uniform width• Instead, pairing a purine with a pyrimidine resulted in auniform width consistent with the X-rayLE 16-UN298Purine + purine: too widePyrimidine + pyrimidine: too narrowPurine + pyrimidine: widthconsistent with X-ray dataCopyright © 2005


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