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Dna structure slide share


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This power point presentation explains double helical structure of DNA as proposed by Watson and Crick (1953).Attempts have also been made to high light the valuable contributions made by Rosalind Franklin and Wilkins. Brief details of different types of DNA have also been included.

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Dna structure slide share

  1. 1. STRUCTURE OF NUCLEIC ACIDS :DNA (DE-OXY RIBO NUCLEICACID )ByDr. Ichha PurakUniversity ProfessorDepartment of BotanyRanchi Women’s College,Ranchi
  2. 2. Discovery of DNAFriedrick Miescher (1869) , a Swiss chemist first identifiednuclein from nuclei of human white blood cells which waslater on renamed as Nucleic acid . It was rich in phosphorushaving no sulphur so was different than protein.Russian Biochemist Phobebus Levene (1910) is credited todiscover Phosphate-Sugar-Base as three components ofNucleotide . He also mentioned DNA as polynucleotide (Anyone of four adenine,Cytocine,Thymine /Guanine ).He also stated difference between Ribose and DeoxyriboseSugar
  3. 3. HISTORY OF DNAHISTORY OF DNAWilliam Astbury (1938) detected a periodicity of 3.4angstromsRosalind Franklin (1952) performed - X- ray diffractionanalysis of DNA crystalFranklin and Wilkins (1950-1953) confirmed 3.4 periodicityand noted uniform diameter of 20A○(2nm)The images of DNA taken by Franklin gave clue to Watson andCrick about the width of double helix and spacing of N-basesWatson and Crick (1953 ) proposed the DNA double helicalmodel based on Franklin’s X-ray crystallography Analysisand other evidences
  4. 4. 1962: Nobel Prize in Physiologyand MedicineJames D.WatsonFrancis H.CrickMaurice H. F.Wilkins Rosalind Franklin
  5. 5. X-ray Crystallography of DNA by Franklin and Wilkins(1950-52) showing helical symmetryFranklin’s X ray diffractionPhotograph of DNA
  6. 6. •DNA is one of the two nucleic acids ( RNA and DNA )•DNA is long, unbranched and spirally coiled inEukaryotes and circular in prokaryotes as well asmitochondria and plastids• In Prokayotes DNA is present only in Nucleoid and isMonocistronic, but in Eukaryotes DNA is present inNucleus, Mitochondria and Plastids and isPolycistronic.•DNA is genetic material ,carries heredity charactersover generations through DNA Replication ( DNA DNA )and Transcription ( DNA RNA ) followed byTranslation
  7. 7. •Nucleic Acids are fibre like molecules ( Length ismany times the breadth )•Nucleic Acids are polymers of Nucleotides.•DNA is a macromolecule of high Molecular Weight•Nucleotides consists of sugar, Phosphoric Acid andbases ( Purine /Pyrimidine)•
  9. 9. J D Watson and Francis Crick (1953 ) haveproposed a double helical structure (Model) of DNA ,for which they have received the Nobel Prize of1962.According to this Model some facts about DNA are asfollows : DNA has a double helical structure made bytwisting around of 2 very long thin polymeric strandsaround a common (imaginary) axis.
  10. 10. The original DNA model by Watson and Crick.Photo: Cold Spring Harbor LaboratoryArchivesImaginary axis is shownby a line passingthrough the centrelongitudinallyDNA has two sugar phosphate back bones with pairedbases in between
  11. 11. DNA DoubleHelix
  12. 12.  Two strands are antiparallel in their back bones,complementary in sequence of bases and arejoined by Hydrogen (H) bonds
  13. 13. OO=P-OOPhosphatePhosphateGroupGroupNNitrogenous baseNitrogenous base(A, G, C, or T)(A, G, C, or T)CH2OC1C4C3C25SugarSugar(deoxyribose)(deoxyribose) Each strand is polymer of deoxy ribonucleotideshaving phosphate ,sugar and N-base123456
  14. 14. Each Deoxyribonucleotide is made up of aDeoxyribose sugar, a phosphate, a NitrogenousBase, which may be either a Purine orPyrimidineDeoxy ribose sugar has formula C5 H10 O4 andis derived from Ribose sugar C5H10O5 by loss ofone oxygen atom at C-2 positionPhosphate (PO4) is derived from Phosphoricacid (H3PO4)
  15. 15. Figure : Nucleotides have three components.A nucleotide consists of a phosphate group, a pentose sugar (ribose or deoxyribose), and a nitrogen-containing base, all linked together by covalent bonds. The nitrogenous bases have two differentchemical forms: purines have two fused rings, and the smaller pyrimidines have a single ring.© 2008 by Sinauer Associates, Inc. All rights reserved. Used with permission.
  16. 16. 17Deoxyribose – C5 H10 O4Ribose- C5H10O5Adenine-6-amino PurineGuanine- 2-amino 6 oxy PurineCytocine- 6- amino pyrimidineThymine- 5- methyl 6 oxy pyrimidineUracil- 6-oxy pyrimidine
  17. 17. Nitrogenous Bases are either Pyrimidine -6 memberedring (Basic Skeleton 4C-2N ) or Purine- two rings (BasicSkeleton 5C-4N ) .Cytosine ( C ) and ( T ) are pyrimidines of DNA andAdenine ( A ) and Guanine (G ) are the Purines of DNAIn a deoxyribonucleotide phosphate is attached at C-3or C-5 of deoxy ribose sugar and base is attached toC-1 of sugarA) Sugar+ Base forms Nucleoside ( S-B )B) Sugar + Base + Phosphate forms Nucleotide ( P-S-B )
  18. 18. Sugar and Phosphate of thenucleotides are linked togetherforming exterior of frame work ofthe strands.
  19. 19. PO4 are linkedbetween two sugarmolecules at C3 &C5 position byformation ofPhosphodiesterbonds. PO4 areexposed to exterioras they are ( - ) velycharged and arehydrophilic.
  20. 20. In DNA deoxyribonucleotides are heldtogether byphosphodiesterbonds.
  21. 21. What chemical forces hold the twoDNA strands together? Two strands are connected to each other by meansof base pairing which comprise the steps of theladder. Base pairing takes place between one purineand one pyrimidine by H bonds. Although H bondsare weak but many H bonds give stability to thedouble helical structure.
  22. 22. The two strands of DNA are oriented in oppositedirections. One strand runs in 3’-5’ direction and theother in 5’-3’ direction. This antiparallel orientationalso supports the double helical nature of DNAmolecule.There lies close similarity of measurements of AT andGC pairs, distance of A-T is about 1.11 nm and that ofG-C is 1.08nm. The angle between C-1 of deoxyribosesugar and N of base is about 51°.
  23. 23. Base Pairing• Hydrogen bondsIndividually weakelectrostatic bondsbut collectivelybecome strong andprovide stability todouble helix
  24. 24. Hydrogen bonds between basesAlso important thatthe purine-pyrimidine basepairs are of similarsize.
  25. 25. DNA strands also held together by base stacking:Van der Waals interactions with neighboring basepairsThe evidence for this opinion is based on interferenceby urea and foramide ,are unable to separate doublestrand themselves but require heat in additionDouble stranded helix structure is also promoted byhaving phosphates on outside ,interact with water andK+and Mg+ +ions
  26. 26. 30
  27. 27.  Bases are projected inwards and lie perpendicularto the long sugar & phosphate chains. Bases are attached to C -1 of sugar and forattachment N at 3 position of pyrimidine and N at 9position of Purine is used. During base pairing Adenine always pairs withThymine by 2 H bonds and Guanine pairs withCytosine b y 3 H bonds.A=T G Ξ C
  28. 28.  Diameter of double helical structure is constantbecause of strict pyrimidine (small ) and Purine (large )base pairing. Diameter of B- DNA is 20 A ° (2 nm)For one complete turn 34 A° space is required and10 bp (base pairs ) are present in each complete turn.Each base pair lies at an angle of 36 ° from next basepairTwo strands of DNA run in antiparallel direction. Onein 3’ →5’ direction and other in 5’ →3’ direction. Thedirection depends on attachment of first PO4 group toC3 or C5 of deoxyribose sugar.
  29. 29. Two strands are coiled around a common axis in sucha way ( like a rope ) that deep (major ) and shallow(Minor) grooves are resulted. Proteins can bind withDNA at these locations. Major grooves are 22A° andMinor grooves are 12 A°
  30. 30. 34 Å3.4 Å20 ÅMinorGrooveMajorGrooveGCCGATTACGGCATTATAATGCCGGCStrands areantiparallel
  31. 31.  Two strands are coiled around each other in sucha way that they can not be separated unless endsare permitted to revolve ( just like a rope ) Suchcoiling is known as Plectonic coiling. Due to strict base pairing rules , two strands arecomplementary to each other in sequence ofbases. This feature helps in determining hereditythrough DNA Replication and Transcription.• DNA code consists of only 4 symbols A T G C
  32. 32. Erwin Chargaff (1949-1953) Digested manyDNAs and subjected products tochromatographic separation RESULTS The sum of Purines is equal to sum of PyrimidinesA = T, C = G , A + G = C + T (purine = pyrimidine)but A+T ≠ G +C ( Not equal ) Base ratio A + T/ G+C varies from one species toother and is not always equal to one but is constantfor a species.
  33. 33. Chargaff’s RuleChargaff’s Rule AdenineAdenine must pair with ThymineThymine GuanineGuanine must pair with CytosineCytosineThe amount of Adenine must be equal to amountof Thymine and amount of Guanine must be equal toCytosine in a given DNA moleculeG CT A
  34. 34. Figure : Chargaffs rule.In DNA, the total abundance of purines is equal to the totalabundance of pyrimidines.© 2008 by Sinauer Associates, Inc. All rights reserved. Used with permission
  35. 35.  DNA with high percentage of G ≡ C pairing( Mitochondrial DNA ) have more density than thosewith high A = T pairing. (Nuclear DNA )Upon heating upto 80-90 º C or more 2 strands ofDNA uncoil and separate( DNA Denaturation ). On cooling the strands comecloser and are held together ( DNA Renaturation orAnnealing ) The two separated strands can act astemplate for synthesis of new complementarystrand.This property is used in PCR ( PolymeraseChain Reaction) which is in vitro process for DNAamplification
  36. 36.  DNA is generally double stranded, but is singlestranded exceptionally in some Viruses viz.- 174 & S-13ȹ Both strands of DNA are right handed spiralsexcept Z DNA ( Left handed spiral )Alternative Forms of DNADNA can exist in several conformational isomersB form is the “normal” conformationA form is found in high salt concZ form Left-handed helix and 12 bp/turn (Z for zigzag)
  37. 37. DNAs are of various types A B C D E & Z , of whichB DNA is the most common form . The differenttypes of DNA differ in number of base pairs per turn,pitch, angle, diameter of Helix and handed nessetc.TYPE HANDEDNESSBASEPAIRS/TURNPITCH DIAMETERAngleBetween2 bpA RIGHT 11 28º 23Aº 32.7ºB RIGHT 10 34º 20Aº 36ºZ LEFT 12 69º 18Aº 30ºC RIGHT 9.33 31º 19Aº 38.6ºTYPES OF DNA
  38. 38. A11B10Z12Residues/TurnClock -wiseTurn Anti Clock-wise Turn
  39. 39. Figure : DNA can assume several different secondary structures.These structures depend on the base sequence of the DNA and theconditions under which it is placed.Used with permission. © 2005 by W. H. Freeman and Company. All rights reserved
  40. 40. Right vs. Left Handed HelicesB Z
  41. 41. DFFERENCES BETWEEN DNA AND RNAS N DNA RNA1 DNA is polymer ofDeoxyribonucleotidesRNA is polymer ofRibonucleotides2 DNA has Deoxyribose sugar RNA has Ribose sugar3 Deoxyribose Sugar has OH- onlyat C3 & C5Ribose sugar (C5H10O5) has OH-at C2,C3 &C54 DNA is Generally DoubleStrandedException In Bacteriophage Ф-174 and S-13RNA is generally single strandedException tRNA & rRNA at someplaces are double stranded5 DNA has Pyrimidines Cytosine andThymineRNA has Pyrimidines Cytosine andUracil6 DNA is Genetic Material RNA generally Non Genetic but insome Viruses it is Genetic Material
  42. 42. 7 DNA is stable in alkaline condition RNA is not stable8 DNA is Synthesized by Replicationusing both strands of DNA astemplates during InterphaseRNA is synthesized by transcriptionusing one strand of DNA asTemplate9 A- T and C-G are the Base Pairs In helical regions A-U and C-G arethe Base Pairs10 The helix geometry of DNA is of B-FormProtects itself against enzymesThe helix geometry of RNA is of A-FormIs destroyed by enzymes andproduced again11 DNA performs long term storage andtransfer of Genetic InformationRNA performs the function of amessenger between DNA and theproteins.
  43. 43. RIBOSE1OHOCH2HHOHHOHHOH2345OHOCH2HHOHHOHHH123452-DEOXY-RIBOSEONH NOHCH3THYMINEONHNOHURACILRNA DNAMolecular Differences between Ribonucleic Acid (RNA)& 2-deoxy-ribonucleic acid (DNA).Ribose replaces deoxyribose; uracil replaces thymineRNA
  44. 44. Nucleosides and Nucleotides
  45. 45. SUMMARY OF THE PRESENTATIONDNA STRUCTURE DNA ( De oxy Ribo Nucleic Acid ) is one of the twoNucleic Acid The Repeating Units of DNA are De-oxy Ribonucleotides Each Deoxy Ribonucleotide has 3 components DeoxyRibose Sugar, Phosphate and Any one N Base ( A/G/T/C) DNA is double helical in Nature , two strands are coiledaround a common imaginary axisDouble helical structure is like a ladder having stepsmade by base pairs which are projected inwards
  46. 46. Successive De oxy ribonucleotides are linked witheach other by Phospho diester bond which is formedby linkage of phosphate with two OH groups of twosugar residuesAmount of Purines is always equal to amount ofPyrimidines(Charguff’s Rule)Adenine always pairs with Thymine by two Hydrogenbonds and Guanine always pairs with Cytosine by threeHydrogen bonds
  47. 47. Two strands run in opposite directions. Two strands are complementary to each other insequence of bases because of strict base pairing rules ,A=T, G Ξ C. This feature helps in maintaining thediameter of double helix constant ( Pyrimidine→PurinePairing) and also making DNA as Genetic Materialthrough Replication and Transcription.DNA with high percentage of G ≡ C pairing ( Mitochondrial DNA )have more density than those with high A = T pairing. (Nuclear DNA )DNA with high percentage of G ≡ C pairing ( Mitochondrial DNA )have more density than those with high A = T pairing. (Nuclear DNA )DNA may have millions of Nucleotides