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Structure of DNA

  1. 1. STRUCTURE OF DNA Part – 2 V.S.RAVI KIRAN
  2. 2. V.S.RAVIKIRAN, MSc., Department of Biochemistry, ASRAM Medical college, Eluru-534005.AP, India. vsravikiran2013@gmail.com
  3. 3. STRUCTURE OF DNA • Deoxyribonucleic acid (DNA) is composed of four deoxyribonucleotides, i.e. deoxyadenylate (A),  deoxyguany late (G), deoxycytidylate (C),  and thymidylate (T).
  4. 4. STRUCTURE OF DNA • These units are linked by 3′ to 5′ phosphodiester bonds to form a long polypeptide chain. • The nucleotide is formed by a combination of base + sugar + phosphoric acid. • The 3′-hydroxyl of one sugar is linked to the 5′-hydroxyl of another sugar through a phosphate group (Fig. 44.1).
  5. 5. STRUCTURE OF DNA • In this particular example, the thymidine is attached to cytidine and then cytidine to adenosine through phosphodiester linkages (Fig. 44.1). • Fig. 44.1: Polynucleotide
  6. 6. STRUCTURE OF DNA • In the DNA, the base sequence is of paramount importance. • The genetic information is encoded in the specific sequence of bases; if the base is altered, the information is also altered.
  7. 7. STRUCTURE OF DNA • The deoxyribose and phosphodiester linkages are the same in all the repeating nucleotides. • Therefore, the message will be conveyed, even if the base sequences alone are mentioned as shown: • 5′P--Thymine--Cytosine-Adenine-3′OH Or, 5′------T--C--A---3′ • This would convey all the salient features of the polynucleotide shown in Figure 44.1.
  8. 8. Polarity of DNA molecule • In the case of DNA, the base sequence is always written from the 5′ end to the 3′ end. • This is called the polarity of the DNA chain.
  9. 9. Watson-Crick Model of DNA Structure • The salient features of Watson-Crick model of DNA are given in Figures 44.2 and 44.3. Fig. 44.2: Watson-Crick model of double helical structure of DNA. Adjacent bases are separated by 0.34 nm. The diameter or width of the helix is 2 nanometers.
  10. 10. Watson-Crick Model of DNA Structure Fig. 44.3: Base pairing rule. Base pairing of A with T and G with C. Hydrogen bonds between bases
  11. 11. Watson-Crick Model of DNA Structure • Right Handed Double Helix • DNA consists of two polydeoxyribonucleotide chains twisted around one another in a right handed double helix. • The bases are located perpendicular to the helix axis, whereas the sugars are nearly at right angles to the axis.
  12. 12. Watson-Crick Model of DNA Structure • The Base Pairing Rule • Always the two strands are complementary to each other. So, the adenine of one strand will pair with thymine of the opposite strand, while guanine will pair with cytosine. • The base pairing (A with T; G with C) is called Chargaff’s rule, which states that the number of purines is equal to the number of pyrimidines.
  13. 13. Watson-Crick Model of DNA Structure • Hydrogen Bonding • The DNA strands are held together mainly by hydrogen bonds between the purine and pyrimidine bases. • There are two hydrogen bonds between A and T while there are three hydrogen bonds between C and G. • The GC bond is therefore stronger than the AT bond.
  14. 14. Watson-Crick Model of DNA Structure • The spatial alignment of the helix allows only purine to pyrimidine base pairing. • A-T and C-G base pairs have almost the same shape. • A mispairing will disturb the stable double helical structure.
  15. 15. Watson-Crick Model of DNA Structure • Antiparallel • The two strands in a DNA molecule run antiparallel, which means that one strand runs in the 5′ to 3′ direction, while the other is in the 3′ to 5′ direction. (Fig. 44.2). . (Fig. 44.2).
  16. 16. Watson-Crick Model of DNA Structure • Other Features • In the DNA, each strand acts as a template for the synthesis of the opposite strand during replication process. • The spiral has a pitch of 3.4 nanometers per turn. • Within a single turn, 10 base pairs are seen. • Thus, adjacent bases are separated by 0.34 nm. • The diameter or width of the helix is 1.9 to 2.0 nm.
  17. 17. Watson-Crick Model of DNA Structure • A major groove (1.2 nm) and a minor groove (0.6 nm) wind along the molecule, parallel to the phosphodiester backbone. • In these grooves, proteins interact with the exposed bases.
  18. 18. Watson-Crick Model of DNA Structure • The stacking of base pairs stabilize the double helix by the hydrophobic effect and conformation of the ring systems. • DNA is the storehouse of genetic information (Box 44.1).
  19. 19. Watson-Crick Model of DNA Structure
  20. 20. Denaturation of DNA Strands • Denaturation of DNA Strands • The double stranded DNA may be denatured and separated by heat. This is called as melting of DNA. • Tm or melting temperature is the temperature when half of the helical structure is denatured. • At lower temperature, the melted strands are re-associated; this is called annealing. • When DNA is denatured, unstacking of bases occur resulting in increased absorbance at 260 nm-hyperchromicity.
  21. 21. Higher Organization of DNA • In higher organisms, DNA is organized inside the nucleus. • Double stranded DNA is wound round histones to form nucleosomes (Fig. 44.4). Fig. 44.4: DNA wraps twice around histone octamer to form one nucleosome
  22. 22. Chromatin • Chromatin is a loose term employed for a long stretch of DNA in association with histones. • Chromatin is then further and further condensedto form chromosomes (Fig. 44.5). • Similarly, the DNA molecule is folded and compressed to 10,000 fold to generate chromosomes (Fig.44.5). Fig. 44.5: DNA condenses repeatedly to form chromosome
  23. 23. THANK YOU

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