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DNA Structure

  1. 1. Nucleic acid (DNA structure)
  2. 2. DNA •DNA forms the genetic material in most of the organisms. •Deoxyribonucleic acid
  3. 3. Nucleic acid  Friedrich Miescher in 1869 isolated what he called nuclein from the nuclei of pus cells  Nuclein was shown to have acidic properties, hence it became called nucleic acid  Two types of nucleic acid are found  Deoxyribonucleic acid (DNA)  Ribonucleic acid (RNA)
  4. 4. Nucleic acid structure  Nucleic acids are polynucleotides  Their building blocks are nucleotides
  5. 5. Nucleotide structure PHOSPATE SUGAR Ribose or Deoxyribose NUCLEOTIDE NITROGENOUS BASE PURINES PYRIMIDINES Adenine (A) Guanine(G) Cytocine (C) Thymine (T) Uracil (U)
  6. 6. Sugars (Pentose sugar)
  7. 7. Nitrogenous Bases
  8. 8. Nitrogenous Bases
  9. 9. Nucleotide O O=P-O O Phosphate Group N Nitrogenous base (A, G, C, or T) CH2 O C1 C4 C3 C2 5
  10. 10. •The bases are covalently attached to the 1’ position of a pentose sugar ring, to form a nucleoside Glycosidic bond R Ribose or 2’-deoxyribose Nucleosides =ribose/deoxyribose + bases 1
  11. 11. Adenosine, guanosine, cytidine, thymidine, uridine
  12. 12. Nucleotides = nucleoside + phosphate
  13. 13. BASES NUCLEOSIDES NUCLEOTIDES Adenine (A) Adenosine Adenosine 5’-triphosphate (ATP) Deoxyadenosine Deoxyadenosine 5’-triphosphate (dATP) Guanine (G) Guanosine Guanosine 5’-triphosphate (GTP) Deoxyguanosine Deoxy-guanosine 5’-triphosphate (dGTP) Cytosine (C) Cytidine Cytidine 5’-triphosphate (CTP) Deoxycytidine Deoxy-cytidine 5’-triphosphate (dCTP) Uracil (U) Uridine Uridine 5’-triphosphate (UTP) Thymine (T) Thymidine/ Deoxythymidie Thymidine/deoxythymidie 5’-triphosphate (dTTP)
  14. 14. 14 One Strand of nucleotide  One strand of DNA is a polymer of nucleotides.  One strand of DNA has many millions of nucleotides. nucleotide
  15. 15. Polymer of Nucleotide
  16. 16. The structure of a DNA chain can be concisely represented • The base chain is written in the 5’ →3’ direction
  17. 17. How DNA looks
  18. 18. James Watson Francis Crick Cambridge University Rosalind Franklin Maurice Wilkins King’s College
  19. 19. X-Ray Diffraction and the Structure of DNA Watson was shown this picture by Wilkins in early 1953. From the picture it was possible to calculate: 1) the distance between bases (3.4A°) 2) the length of the period (34A°) 3) the rise of the helix (36 degrees)
  20. 20. Chargaff’s Rule (Erwin Chargaff)  Molar ratio of purines is equal tomolar ratio of pyrimidines.
  21. 21. Base pairing A:T G:C 1 23 4 8 9 7 6 5 4 3 2 1
  22. 22. How Watson and Crick Solved the DNA Structure They already knew from Franklin and Wilkins’ work that DNA was in the form of a Double Helix They used Chargaff’s Rule to figure out how the 4 Bases match up in pairs
  23. 23. How Watson and Crick Solved the DNA Structure They discovered that: • The Phosphate Backbone was on the outside • Which protected the Bases on the inside • DNA acts as a Template or a Copying Mechanism for reproduction
  24. 24. Physical Structure (cont’d)  Chains are anti-parallel (i.e in opposite directions)  Diameter and periodicity are consistent  2.0 nm  10 bases/ turn  3.4 nm/ turn  Width consistent because of pyrimidine/purine pairing
  25. 25. 1. DNA contains two helical strands wound around a single axis. 2. Hydrophilic backbone containing deoxy ribose sugar and the phosphate group remain to the outside of the helix facing the aqueous phase. 3. The hydrohphobic purines and pyrimidines remain inside the double helix. 4. The purines of one strand pairs with the pyrimidines of the other strand by hydrogen bonding. There are 3 hydrogen bonds between guanine and cytosine and 2 hydrogen bonds between adenine and thymine. 5. The pairings of two strands create major groove and minor grooves on the surface of the DNA double helix. 6. The nucleotides in one strand are joined to each other by phosphor diester bond between 5’-phosphate of one nucleotide with the 3’- hydroxyl group of the next nucleotide. All the phosphodiester bonds are in the same orientation in a chain. 7. The two strands are antiparallel to each other i.e, the direction of 5’-3’ phosphodiester bond in the two strands are opposite. 8. Distance between two adjacent nucleotide pair in the double helix is 3.4 A°. 9. There are ten base pairs in each turn of the DNA double helix which accounts for 34 A° length.
  26. 26. Conformational variation in double-helical structure • B-DNA • A-DNA • Z-DNA
  27. 27. • B-form: the duplex structure proposed by Watson and Crick is referred as the B-form DNA. •It is the standard structure for DNA molecules. •A-form: at low humidity the DNA molecule will take the A-form: •The A-form helix is wider and shorter, with a shorter more compact helical structure, than the B-form helix. • Z-form: the Z-form DNA is adopted by short oligonucleotides. •It is a left-handed double helix in which backbone phosphates zigzag (Alternate purines and pyrimidines).
  28. 28. Thank you