pneumoniae肺炎链球菌

1、Chapter 10The Structure, Replication and Repair of DNAKey Questions How did biologists discover what genes were made of and what they did? What is the structure of DNA? How does DNAs structure allow it to act as a template for its own replications? What is a mutation and why are mutations important?

2、Structure of DNA-OverviewEach nucleotide of DNA consists of: A sugar deoxyribose A phosphate A base there are 4 bases; 2 of the 4 bases are pyrimidines: cytosine (C) and thymine (T); The other 2 bases are purines: denine (A) and guanine (G)Structure of DNA-A Historic Story (1)Late 1800s scientists p

3、ostulated a biochemical basis Friedrich Miescher (1869): isolated DNA (called it nucleic acid) Researchers became convinced chromosomes carry genetic information1920s to 1940s expected the protein portion of chromosomes to be the genetic materialLate 1920s Frederick Griffith was working with Strepto

4、coccus pneumoniae肺炎鏈球菌肺炎鏈球菌 Strains that secrete capsules look smooth and can cause fatal infections in mice Strains that do not secrete capsules look rough and infections are not fatal in miceGriffiths experiments (next page) showed that: Genetic material from the heat-killed type S bacteria had be

5、en transferred to the living type R bacteria This trait gave them the capsule and was passed on to their offspringGriffith did not know the biochemical basis of his transforming principleRough strains (R) without capsule are not fatal No living bacteria found in bloodSmooth strains (S) with capsule

6、are fatal Capsule prevents immune system from killing bacteria Living bacteria found in bloodIf mice are injected with heat-killed type S, they surviveMixing live R with heat-killed S kills the mouse Blood contains living S bacteria TransformationGriffiths Bacterial TransformationsStructure of DNA-A

7、 Historic Story (2)Avery, MacLeod, and McCarty used purification methods to reveal that DNA is the genetic material 1940s interested in bacterial transformation Only purified DNA from type S could transform type R Purified DNA might still contain traces of contamination that may be the transforming

8、principle Added DNase, RNase and proteases RNase and protease had no effect With DNase no transformation DNA is the genetic material1952, Hershey and Chase studied T2 virus infecting Escherichia coli Bacteriophage or phage Phage coat made entirely of proteinDNA found inside capsidStructure of DNA-A

9、Historic Story (3)1952, Hershey and Chase studied T2 virus infecting Escherichia coli Bacteriophage or phage Phage coat made entirely of proteinBacteriophage A virus that infects bacteria Viruses are composed of protein and DNA (or RNA) Viruses are capable of forcing host cells to make more viruses

10、Viruses are not living organismsStructure of DNA-A Historic Story (3)Shearing force from a blender will separate the phage coat from the bacteria35S will label proteins only; 32P will label DNA onlyExperiment to find what is injected into bacteria- DNA or protein?DNA found inside capsid: Results sup

11、port DNA as the genetic materialHershey and Chase The Blender Guy/GalStructure of DNA-A Historic Story (3)Structure of DNA-A Historic Story (4)Chagaffs Rules:Found the proportions of the bases in many different speciesThe amount of A is equal to TThe amount of G is equal to CStructure of DNA-A Histo

12、ric Story (5)Solving DNA structure:1953, James Watson and Francis Crick, with Maurice Wilkins, proposed the structure of the DNA double helixWatson and Crick used Linus Paulings method of working out protein structures using simple ball-and-stick modelsRosalind Franklins X-ray diffraction results pr

13、ovided crucial information Erwin Chargoff analyzed base composition of DNA that also provided important informationRosalind Franklins ContributionCrystallographerAccurately measured the density of DNA, the number of water moleculesDiscovered that DNA had 2 slightly different structuresUsed Franklins

14、 data and work extensively, but did not cite itApplied Chargaffs RulesBuilt several models of DNAFound ball-and-stick model consistent with dataWatson and Crick awarded Nobel Prize in 1962Rosalind Franklin had died and the Nobel is not awarded posthumouslyStructure of DNA-A Historic Story (5)DNA Str

15、ucture LinkagesDNA is Double stranded Helical Sugar-phosphate backbone Bases on the inside Stabilized by hydrogen bonding Base pairs with specific pairing AT/GC or Chargoffs rule A pairs with T G pairs with C Keeps with consistent 10 base pairs per turn 2 DNA strands are complementary 5 GCGGATTT 3 3

16、 CGCCTAAA 5 2 strands are antiparallel One strand 5 to 3 Other stand 3 to 5Computer Generated Model of DNAFigure 10-9What Is a Gene?To Mendel, in 1865, it was just an abstraction Places on chromosomes Pure information Important to understand genes in chemical terms1908, Archbold Garrod proposed rela

17、tionship between genes and the production of enzymesStudied patients with metabolic defectsAlkaptonuria- patients body accumulates abnormal levels of homogentisic acid (alkapton)Hypothesized disease due to missing enzymeKnew it had a recessive pattern of inheritanceInborn error of metabolismWhat Cau

18、ses Alkaptonuria?Biochemical Importance of Genes Alkaptonuria black urine stains Garrod suspected that it might be inherited Caused by a recessive allele A crucial enzyme is missing; HA accumulates in the body and is excreted in the urine Suggested that genes might work by specifying enzymes1 Gene 1

19、 EnzymeIn the early 1940s, George Beadle and Edward Tatum rediscovered Garrods work, using Neurospora crassa (common bread mold), showed that 1 gene could specify 1 enzymeMinimum requirements for growth are carbon source (sugar), inorganic salts, and biotinMutant strains would be unable to grow unle

20、ss supplementedCompare to wild-type or normalA single mutation resulted in the requirement for a single type of vitaminStimulated research into other substances including arginine, an amino acidIsolated several mutants requiring arginine for growthExamined for ability to grow in the presence of prec

21、ursors3 groups based on requirementsBeadle and Tatum conclude that single gene controls the synthesis of a single enzyme One gene one enzyme hypothesis1 Gene 1 Enzyme1 Gene 1 PolypeptideSickle Cell AnemiaDifferences in gene has caused differences in the hemoglobin protein, not in the enzymeOne gene

22、one enzyme hypothesis has been modifiedEnzymes are only one category of cellular proteinsMore accurate to say one gene encodes a polypeptide Hemoglobin composed of 4 polypeptides required for functionOne gene one polypeptide theoryGenes influence phenotype by specifying polypeptidesDNA Replication3

23、different models for DNA replication proposed in late 1950s Semiconservative Conservative DispersiveNewly made strands are daughter strandsOriginal strands are parental strandsIn 1958, Matthew Meselson and Franklin Stahl devised experiment to differentiate among 3 proposed mechanismsNitrogen comes i

24、n a common light form (14N) and a rare heavy form (15N)Grew E.coli in medium with only 15NThen switched to medium with only 14NCollected sample after each generationOriginal parental strands would be 15N while newly made strands would be 14NResults consistent with semiconservative mechanismDNA Repli

25、cationSimultaneously Copying DNA StrandsDNA replication occurs in replication forksThese are Y-shaped regions of DNA where the 2 strands of the helix have come apartNucleotides add directly to the 3 end of an RNA primer; the other strand is produced in short fragments (Okazaki Fragments) that are jo

26、ined together by DNA ligaseIn the leading strand DNA primase makes one RNA primer DNA polymerase attaches nucleotides in a 5 to 3 direction as it slides forwardIn the lagging strand DNA synthesized 5 to 3 but in a direction away from the fork Okazaki fragments made as a short RNA primer made by DNA

27、primase at the 5 end and then DNA laid down by DNA polymeraseRNA primers will be removed by DNA polymerase and filled in with DNADNA ligase will join adjacent DNA fragmentsDNA PolymerizationDNA polymerase enzyme that strings together the nucleotides During replication 2 parental strands separate and

28、 serve as template strandsNew nucleotides must obey the AT/GC ruleEnd result 2 new double helices with same base sequence as originalDNA PolymerizationRNA PolymeraseAn RNA polymerase makes an RNA primer that provides a 3 end for DNA polymeraseThen DNA polymerase links together the nucleotides that assemble opposite the template into a new s