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Deoxyribonucleic Acid (DNA)
Deoxyribonucleic Acid (DNA)
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Structure of dna

  1. 1. Structure of DNA By: Meenakshi Gupta (M.Pharm. 2nd sem)
  2. 2. Nucleic acids • Discovered by: F. Meischer in nucleus of pus cell & called it ‘nuclein’. • Term nucleic acid was coined by ‘altman’. • Nucleic acids are polymers of nucleotides (=nitrogen bases+ pentose sugars+ phosphate). • On the basis of structure, N-Bases are broadly of two types: 1. Pyrimidines 2. Purines
  3. 3. 1. Pyrimidines: consist of one pyrimidine ring. Skeleton of ring composed of two nitrogen & four carbon atoms. Eg. Cytosine, thymine, uracil.
  4. 4. 2. Purines: consists of two rings i.e. one pyrimidine ring ( 2N+4C) and one imidazole ring (2N+3C) eg. Adenine, guanine
  5. 5. • Pentose sugars:-
  6. 6. • N-base forms bond with first carbon of pentose sugar to form a nucleoside. Nitrogen of third place (N3) forms bond with sugar in case of pyrimidines while in purine, nitrogen of ninth place (N9) forms bond with sugar. • Phosphates:
  7. 7. • Phosphate form ester bond (covalent bond) with fifth carbon of sugar to form a complete nucleotide. nucleoside nucleotide
  8. 8. Types of nucleosides & nucleotides1. Adenine + ribose = adenosine Adenosine + phosphate = adenylic acid 2. Adenine + deoxyribose = deoxy adenosine deoxy adenosine + P = deoxy adenylic acid 3. Guanine+ ribose = guanosine guanosine + phosphate = guanylic acid 4. Guanine + deoxyribose = deoxy guanosine deoxy guanosine + P = deoxy guanylic acid
  9. 9. 5. Cytosine + ribose = cytidine Cytidine + phosphate = cytidylic acid 6. Cytosine + deoxyribose = deoxy cytidine Deoxy cytidine + P = deoxy cytidylic acid 7. Uracil + ribose = uridine Uridine + phosphate = uridylic acid 8. Thymine + deoxyribose = deoxy thymidine deoxy thymidine + P = deoxy thymidylic acid
  10. 10. DNA • Discovered by- meischer • Term was given by- zacharis • In DNA pentose sugar is deoxyribose sugar and four types of nitrogen bases A,T,G,C. • Wilkins and franklin studied DNA molecule with the help of X-ray crystallography. • With the help of this study, watson and crick (1953) proposed a double helix model for DNA. For this model, watson, crick & wilkins were awarded by nobel prize in 1962.
  11. 11. • According to this model, DNA is composed of two polypeptide chains. • Both polynucleotide chains are complimentary and antiparallel to each other. • In both strand of DNA direction of phosphodiester bond is opposite ie. If the direction of phosphodiester bond in one strand is 3’-5’ then it is 5’-3’ in another strand. • Both the strands are held together by hydrogen bonds. These H-bonds are present between nitrogen bases of both strand.
  12. 12. • Adenine binds to thymine by two hydrogen bonds and cytosine binds to guanine by three hydrogen bonds.
  13. 13. Chargaff’s equivalency rule • In a double stranded DNA, amount of purine nucleotides is equals to amount of pyrimidine nucleotides. purine = pyrimidine [A] +[G] = [T] + [C] [A] +[G]/[T]+[C] = 1
  14. 14. Base ratio • A+T/ G+C= Constant for a given species. • In a DNA, if A+T> G+C= A-T Type DNA eg., eukaryotic DNA • In a DNA, if G+C > A+T= G-C type DNA eg., prokaryotic DNA • Melting point of DNA depends on G-C contents • Hence, M.P. of prokaryotic DNA> M.P. of eukaryotic DNA
  15. 15. Configuration of DN A molecule • Two strands of DNA are helically coiled like a revolving ladder. Backbone of this ladder (reiling) is composed of phosphates and sugars while steps (bars) are composed of pairs of nitrogen bases. • Distance between two successive steps is 3.4A. In one complete turn of DNA molecule there are such 10 steps . So the length of complete turn is 34A. This is called helix length.
  16. 16. • Diameter of DNA molecule ie. Distance between phosphates of two strands is 20A. • Distance between sugar of two strands is 11.1A. • Length of H-bonds between nitrogen bases is 2.8-3.0A. Angle between N-base & C1 carbon of pentose is 51⁰. • Molecular weight of DNA is 1000000 to 1000000000 dalton. • In nucleus of eukaryotes the DNA is associated with histone protein to form nucleoprotein. Histone forms an angle of 30⁰ with DNA.
  17. 17. • Bond between DNA and histone is salt linkage (mg2+).
  18. 18. Types of DNA On the basis of direction of twisting, there are two types of DNA. • Right handed DNA- Clockwise twisting eg. The DNA for which watson & crick proposed model was ‘B’ DNA. DNA Helix length No. of base pair Distance between two pairs diameter A 28A 11 pairs 2.56A 23A B 34A 10 pairs 3.4A 20A C 31A 9.33 pairs 3.32A 19A D 24.24A 8 pairs 3.03A 19A
  19. 19. • Left handed DNA Anticlockwise twisting eg. Z-DNA, discovered by Rich. Phosphate & sugar backbone is zig-zag, units of Z-DNA are dinucleotides (purine and pyrimidine in alternate order). Helix length – 45.6A Diameter – 18.4A No. of base pairs – 12 (6 dimers) Distance between 2 base- pairs – 3.75A
  20. 20. • Dehydration of DNA drives it into the A form, and this apparently protects DNA under conditions such as the extreme desiccation of bacteria. Protein binding can also strip solvent off of DNA and convert it to the A form, as revealed by the structure of a rod-shaped virus. • C-DNA also known as C form DNA. It is one of the many possible double helical structures of DNA. This form of DNA can be observed at some conditions such as relatively low humidity and the presence of certain ions, such as Li+ or Mg2+. • Z DNA is commonly believed to provide torsional strain relief (supercoiling) while DNA transcription occurs. The potential to form a Z- DNA structure also correlates with regions of active transcription.
  21. 21. Special points • DNA absorbs U.V. rays means 2600A wavelength. • Hyperchromicity: when a double stranded DNA molecule is denatured by heating then denatured DNA molecule absorbs more amount of light. • Hypochromicity: when denatured DNA molecule cool slowly then it becomes double stranded and it absorbs less amount of light
  22. 22. • If a normal DNA molecule is placed at high temperature (80⁰-90⁰C) then both the strands of DNA will separate from each other due to breaking of H- bonds. It is called denaturation. • When denatured DNA molecule is placed at normal temperature then both the strands of DNA molecule attaches & recoiled to each other. It is called renaturation. • DNA supercoiling refers to the over- or under-winding of a DNA strand, and is an expression of the strain on that strand. • Supercoiling occurs when the molecule relieves the helical stress by twisting around itself. Overtwisting leads to postive supercoiling, while undertwisting leads to negative supercoiling.
  23. 23. • Pallindromic DNA- wilson & thomas sequences of nucleotides same from both ends. • DNA molecule is dextrorotatory while RNA molecule is laevorotatory. • C-value: total amount of DNA in a haploid genome of organism.
  24. 24. Thank you for paying attention

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