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Structure of DNA

  1. 1. Structure of DNA BIBHU PRASAD RATH LECT. IN BOTANY
  2. 2. What is DNA; Why it is there • DNA is a type of nucleic acids. • These are the structures those control the quality and quantity of protein synthesis in organisms. • These are present both inside and outside (mitochondria, chloroplast) the nucleus. • These biomolecules carry genes those are coded informations.
  3. 3. Components of DNA Polynucleotide chains of DNA carry nucleotides as monomers. Nucleotide comprises; a. Pentoses The ribose sugar in DNA is a 2 deoxyribose. b. Nitrogenous bases These are heterocyclic, planner, relatively water insoluble aromatic molecules. These are; pyrimidines and purines c. Phosphate [PO4 3-] It is an ion of phosphoric acid [H3PO4]. Phosphate groups are found in every nucleotide. As these carry negative charges frequently DNA is acidic. These can be attached to the 3̕̕ or 5̕ C of the pentose.
  4. 4. Pyrimidines; Single ring compounds, nitrogen in 1 and 3 positions of a benzene ring. In DNA two types of pyrimidines are found 1. Thymine ; 5-methyl, 2,4-dioxy pyrimidine 2. Cytosine ; 2-oxy, 4-amino pyrimidine Purines ; Double ring compounds, comprise a five membered imidazole ring and a pyrimidine ring joined at 4th and 5th position. These are of two types 1. Adenine ; 6-amino purine 2. Guanine ; 6-oxy, 2-amino purine
  5. 5. Watson-Crick model This model is based on the crystallographic studies • DNA is a helically twisted double stranded macromolecule. • The two strands are antiparallel to each other. • Spirally twisted DNA has two types of alternative grooves i.e. major and minor. • One turn of DNA has about 10 base pairs with a length of 34Å. Hence each base pair has a pitch of about 3.4 Å. • The sugars are joined together by phosphate groups that form phosphodiester bonds between the third and fifth carbon atoms of adjacent sugar rings. • Glycosidic Bond - In DNA, refers to the nitrogen-carbon linkage between the 9' nitrogen of purine bases or 1' nitrogen of pyrimidine bases and the 1' carbon of the sugar group. • The two strands are held together by weak hydrogen bonds. The two strands are complementary as the purines lie opposite to pyrimidines. Adenine forms two hydrogen bonds with thymine and cytosine forms three bonds with guanine. • Purine-pyrimidine pairing is about 20 Å thick.
  6. 6. ADENINE AND THYMINE BONDING
  7. 7. GUANINE AND CYTOSINE BONDING
  8. 8. Special characters of DNA • DNA is composed of polynucleotide chains • Two strands of the double helix are wound around each other in an anti-parallel orientation and have complementary sequences • Double helix is stabilized by base pairing and base stacking • Base can flip out from the double helix • DNA is usually right handed double helix • Double helix has major and minor grooves • Major groove is rich in chemical informations • Double helix exists in multiple conformations • DNA sometimes can form a left handed helix • DNA can denature and reassociate • Some DNA molecules are circular
  9. 9. Types of DNAs A - DNA B - DNA Z - DNA • COILING IS RIGHT HANDED • PITCH OF A HELIX TURN 28Å • MEAN BASE PAIR PER TURN 11 • RISE PER BASE PAIR 2.3Å • DIAMETER 26Å • RIGHT HANDED • 34Å • 10 • 3.4Å • 20Å • LEFT HANDED • 45Å • 12 • 3.8Å • 18Å
  10. 10. Factors affecting DNA stability This stability is important because it prevents the two DNA strands from breaking apart spontaneously and plays an important role in the way DNA is copied. a. Hydrogen bonding It contributes a little to the stability. b. Base stacking interaction Base pairs stack together through Van der Waal force of attraction. It contribute significantly to the overall stability. c. Ionic interaction Arrays of negative charges of the phosphate groups along the strands repel each other to keep the complementary backbones apart. Negative charges are on the exterior surface to minimize this repulsive effect. Divalent cations like Mg2+ bind to these anionic phosphates to shield them from one another.
  11. 11. MOLECULAR BIOLOGY BY JAMES D. WATSON STRUCTURE OF DNA

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