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Chemistry of DNA
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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.

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 NUCLEIC ACID ) By Dr. Ichha Purak University Professor Department of Botany Ranchi Women’s College,Ranchi http://www.dripurak.com/
  2. 2. Discovery of DNA Friedrick Miescher (1869) , a Swiss chemist first identified nuclein from nuclei of human white blood cells which was later on renamed as Nucleic acid . It was rich in phosphorus having no sulphur so was different than protein. Russian Biochemist Phobebus Levene (1910) is credited to discover Phosphate-Sugar-Base as three components of Nucleotide . He also mentioned DNA as polynucleotide (Any one of four adenine,Cytocine,Thymine /Guanine ). He also stated difference between Ribose and Deoxyribose Sugar
  3. 3. HISTORY OF DNAHISTORY OF DNA William Astbury (1938) detected a periodicity of 3.4 angstroms Rosalind Franklin (1952) performed - X- ray diffraction analysis of DNA crystal Franklin and Wilkins (1950-1953) confirmed 3.4 periodicity and noted uniform diameter of 20A○ (2nm) The images of DNA taken by Franklin gave clue to Watson and Crick about the width of double helix and spacing of N-bases Watson and Crick (1953 ) proposed the DNA double helical model based on Franklin’s X-ray crystallography Analysis and other evidences
  4. 4. 1962: Nobel Prize in Physiology and Medicine James D. Watson Francis H. Crick Maurice H. F. Wilkins Rosalind Franklin
  5. 5. X-ray Crystallography of DNA by Franklin and Wilkins (1950-52) showing helical symmetry Franklin’s X ray diffraction Photograph of DNA
  6. 6. •DNA is one of the two nucleic acids ( RNA and DNA ) •DNA is long, unbranched and spirally coiled in Eukaryotes and circular in prokaryotes as well as mitochondria and plastids • In Prokayotes DNA is present only in Nucleoid and is Monocistronic, but in Eukaryotes DNA is present in Nucleus, Mitochondria and Plastids and is Polycistronic. •DNA is genetic material ,carries heredity characters over generations through DNA Replication ( DNA  DNA )and Transcription ( DNA RNA ) followed by Translation
  7. 7. •Nucleic Acids are fibre like molecules ( Length is many times the breadth ) •Nucleic Acids are polymers of Nucleotides. •DNA is a macromolecule of high Molecular Weight •Nucleotides consists of sugar, Phosphoric Acid and bases ( Purine /Pyrimidine) •
  8. 8. POLYNUCLEOTIDE
  9. 9. J D Watson and Francis Crick (1953 ) have proposed a double helical structure (Model) of DNA , for which they have received the Nobel Prize of 1962. According to this Model some facts about DNA are as follows :  DNA has a double helical structure made by twisting around of 2 very long thin polymeric strands around a common (imaginary) axis.
  10. 10. The original DNA model by Watson and Crick. Photo: Cold Spring Harbor Laboratory Archives Imaginary axis is shown by a line passing through the centre longitudinally DNA has two sugar phosphate back bones with paired bases in between
  11. 11. DNA Double Helix
  12. 12.  Two strands are antiparallel in their back bones, complementary in sequence of bases and are joined by Hydrogen (H) bonds
  13. 13. O O=P-O O PhosphatePhosphate GroupGroup N Nitrogenous baseNitrogenous base (A, G, C, or T)(A, G, C, or T) CH2 O C1 C4 C3 C2 5 SugarSugar (deoxyribose)(deoxyribose)  Each strand is polymer of deoxy ribonucleotides having phosphate ,sugar and N-base 1 2 3 4 5 6
  14. 14. Each Deoxyribonucleotide is made up of a Deoxyribose sugar, a phosphate, a Nitrogenous Base, which may be either a Purine or Pyrimidine Deoxy ribose sugar has formula C5 H10 O4 and is derived from Ribose sugar C5H10O5 by loss of one oxygen atom at C-2 position Phosphate (PO4) is derived from Phosphoric acid (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 different chemical 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. 17 Deoxyribose – C5 H10 O4 Ribose- C5H10O5 Adenine-6-amino Purine Guanine- 2-amino 6 oxy Purine Cytocine- 6- amino pyrimidine Thymine- 5- methyl 6 oxy pyrimidine Uracil- 6-oxy pyrimidine
  17. 17. Nitrogenous Bases are either Pyrimidine -6 membered ring (Basic Skeleton 4C-2N ) or Purine- two rings (Basic Skeleton 5C-4N ) . Cytosine ( C ) and ( T ) are pyrimidines of DNA and Adenine ( A ) and Guanine (G ) are the Purines of DNA In a deoxyribonucleotide phosphate is attached at C-3 or C-5 of deoxy ribose sugar and base is attached to C-1 of sugar A) Sugar+ Base forms Nucleoside ( S-B ) B) Sugar + Base + Phosphate forms Nucleotide ( P-S-B )
  18. 18. Sugar and Phosphate of the nucleotides are linked together forming exterior of frame work of the strands.
  19. 19. PO4 are linked between two sugar molecules at C3 & C5 position by formation of Phosphodiester bonds. PO4 are exposed to exterior as they are ( - ) vely charged and are hydrophilic.
  20. 20. In DNA deoxyribo nucleotides are held together by phosphodiester bonds.
  21. 21. What chemical forces hold the two DNA strands together?  Two strands are connected to each other by means of base pairing which comprise the steps of the ladder. Base pairing takes place between one purine and one pyrimidine by H bonds. Although H bonds are weak but many H bonds give stability to the double helical structure.
  22. 22. The two strands of DNA are oriented in opposite directions. One strand runs in 3’-5’ direction and the other in 5’-3’ direction. This antiparallel orientation also supports the double helical nature of DNA molecule. There lies close similarity of measurements of AT and GC pairs, distance of A-T is about 1.11 nm and that of G-C is 1.08nm. The angle between C-1 of deoxyribose sugar and N of base is about 51°.
  23. 23. Base Pairing • Hydrogen bonds Individually weak electrostatic bonds but collectively become strong and provide stability to double helix
  24. 24. Hydrogen bonds between bases Also important that the purine- pyrimidine base pairs are of similar size.
  25. 25. DNA strands also held together by base stacking: Van der Waals interactions with neighboring base pairs The evidence for this opinion is based on interference by urea and foramide ,are unable to separate double strand themselves but require heat in addition Double stranded helix structure is also promoted by having phosphates on outside ,interact with water and K+ and Mg+ + ions
  26. 26. 30
  27. 27.  Bases are projected inwards and lie perpendicular to the long sugar & phosphate chains.  Bases are attached to C -1 of sugar and for attachment N at 3 position of pyrimidine and N at 9 position of Purine is used.  During base pairing Adenine always pairs with Thymine by 2 H bonds and Guanine pairs with Cytosine b y 3 H bonds. A=T G Ξ C
  28. 28.  Diameter of double helical structure is constant because 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 and 10 bp (base pairs ) are present in each complete turn. Each base pair lies at an angle of 36 ° from next base pair Two strands of DNA run in antiparallel direction. One in 3’ →5’ direction and other in 5’ →3’ direction. The direction depends on attachment of first PO4 group to C3 or C5 of deoxyribose sugar.
  29. 29. Two strands are coiled around a common axis in such a way ( like a rope ) that deep (major ) and shallow (Minor) grooves are resulted. Proteins can bind with DNA at these locations. Major grooves are 22A° and Minor grooves are 12 A°
  30. 30. 34 Å 3.4 Å 20 Å Minor Groove Major Groove GC CG AT TA CG GC AT TA TA AT GC CG GC Strands are antiparallel
  31. 31.  Two strands are coiled around each other in such a way that they can not be separated unless ends are permitted to revolve ( just like a rope ) Such coiling is known as Plectonic coiling.  Due to strict base pairing rules , two strands are complementary to each other in sequence of bases. This feature helps in determining heredity through DNA Replication and Transcription. • DNA code consists of only 4 symbols A T G C
  32. 32. Erwin Chargaff (1949-1953) Digested many DNAs and subjected products to chromatographic separation RESULTS  The sum of Purines is equal to sum of Pyrimidines A = 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 to other and is not always equal to one but is constant for a species.
  33. 33. Chargaff’s RuleChargaff’s Rule  AdenineAdenine must pair with ThymineThymine  GuanineGuanine must pair with CytosineCytosine The amount of Adenine must be equal to amount of Thymine and amount of Guanine must be equal to Cytosine in a given DNA molecule G CT A
  34. 34. Figure : Chargaff's rule. In DNA, the total abundance of purines is equal to the total abundance 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 those with high A = T pairing. (Nuclear DNA ) Upon heating upto 80-90 º C or more 2 strands of DNA uncoil and separate ( DNA Denaturation ). On cooling the strands come closer and are held together ( DNA Renaturation or Annealing ) The two separated strands can act as template for synthesis of new complementary strand.This property is used in PCR ( Polymerase Chain Reaction) which is in vitro process for DNA amplification
  36. 36.  DNA is generally double stranded, but is single stranded exceptionally in some Viruses viz. - 174 & S-13ȹ  Both strands of DNA are right handed spirals except Z DNA ( Left handed spiral ) Alternative Forms of DNA DNA can exist in several conformational isomers B form is the “normal” conformation A form is found in high salt conc Z 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 which B DNA is the most common form . The different types of DNA differ in number of base pairs per turn, pitch, angle, diameter of Helix and handed ness etc. TYPE HANDED NESS BASE PAIRS /TURN PITCH DIAMETE R Angle Between 2 bp A RIGHT 11 28º 23Aº 32.7º B RIGHT 10 34º 20Aº 36º Z LEFT 12 69º 18Aº 30º C RIGHT 9.33 31º 19Aº 38.6º TYPES OF DNA
  38. 38. A 11 B 10 Z 12Residues/Turn Clock -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 the conditions under which it is placed. Used with permission. © 2005 by W. H. Freeman and Company. All rights reserved
  40. 40. Right vs. Left Handed Helices B Z
  41. 41. DFFERENCES BETWEEN DNA AND RNA S N DNA RNA 1 DNA is polymer of Deoxyribonucleotides RNA is polymer of Ribonucleotides 2 DNA has Deoxyribose sugar RNA has Ribose sugar 3 Deoxyribose Sugar has OH- only at C3 & C5 Ribose sugar (C5H10O5) has OH- at C2,C3 &C5 4 DNA is Generally Double Stranded Exception In Bacteriophage Ф- 174 and S-13 RNA is generally single stranded Exception tRNA & rRNA at some places are double stranded 5 DNA has Pyrimidines Cytosine and Thymine RNA has Pyrimidines Cytosine and Uracil 6 DNA is Genetic Material RNA generally Non Genetic but in some Viruses it is Genetic Material
  42. 42. 7 DNA is stable in alkaline condition RNA is not stable 8 DNA is Synthesized by Replication using both strands of DNA as templates during Interphase RNA is synthesized by transcription using one strand of DNA as Template 9 A- T and C-G are the Base Pairs In helical regions A-U and C-G are the Base Pairs 10 The helix geometry of DNA is of B- Form Protects itself against enzymes The helix geometry of RNA is of A- Form Is destroyed by enzymes and produced again 11 DNA performs long term storage and transfer of Genetic Information RNA performs the function of a messenger between DNA and the proteins.
  43. 43. RIBOSE 1 OHOCH2 H H OH H OH H OH 23 4 5 OHOCH2 H H OH H OH H H 1 23 4 5 2-DEOXY-RIBOSE O N H N O H CH3 THYMINE O N HN O H URACIL RNA DNA Molecular Differences between Ribonucleic Acid (RNA) & 2-deoxy-ribonucleic acid (DNA). Ribose replaces deoxyribose; uracil replaces thymine RNA
  44. 44. Nucleosides and Nucleotides
  45. 45. SUMMARY OF THE PRESENTATION DNA STRUCTURE  DNA ( De oxy Ribo Nucleic Acid ) is one of the two Nucleic Acid  The Repeating Units of DNA are De-oxy Ribonucleotides  Each Deoxy Ribonucleotide has 3 components Deoxy Ribose Sugar, Phosphate and Any one N Base ( A/G/T/C)  DNA is double helical in Nature , two strands are coiled around a common imaginary axis Double helical structure is like a ladder having steps made by base pairs which are projected inwards
  46. 46. Successive De oxy ribonucleotides are linked with each other by Phospho diester bond which is formed by linkage of phosphate with two OH groups of two sugar residues Amount of Purines is always equal to amount of Pyrimidines(Charguff’s Rule) Adenine always pairs with Thymine by two Hydrogen bonds and Guanine always pairs with Cytosine by three Hydrogen bonds
  47. 47. Two strands run in opposite directions.  Two strands are complementary to each other in sequence of bases because of strict base pairing rules , A=T, G Ξ C. This feature helps in maintaining the diameter of double helix constant ( Pyrimidine→Purine Pairing) and also making DNA as Genetic Material through 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
  48. 48. PART OF DNA DOUBLE THANK YOU

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