2. DNA Structure www.assignmentpoint.com

3. DNA STRUCTURE Each nucleotide is composed of (1) a Phosphate group (2) a five – carbon sugar (or Pentose), and (3) a cyclic nitrogen containing compound called a base. Nucleic acids first called “nuclein” because they were isolated from cell nuclei by F. Miescher in 1869 www.assignmentpoint.com

4. In DNA, the sugar is 2-deoxyribose (thus the name deoxyribonucleic acid) In RNA, the sugar is ribose (thus ribonucleic acid). www.assignmentpoint.com

5. There are four different bases commonly found in DNA: Adenine Guanine Thymine and Cytosine. RNA also contains adenine, guanine and cytosine, but has different base, uracil in the place of thymine. www.assignmentpoint.com

6. 6-aminopurine2-amino-6-oxypurine 4-amino-2- oxypyrimidine 2,4-oxypyrimidine2,4-oxy-5-pyrimidine Cytosine, thymine, and uracil are single-ring base called Pyrimidines. Adenine and Guanine are double ring base called Purines www.assignmentpoint.com

7. The Watson and Crick DNA Double helix The correct structure of DNA was first deduced by J.D. Watson and F.H.C.Crick in 1953. Their double helix model of DNA structure was based on two major kind of evidence. 1. Chargaff’s rule 2. X – ray diffraction patterns. www.assignmentpoint.com

8. Chargaff’s rule The composition of DNA from many different organisms was analyzed by E.Chargaff and his colleagues. It was observed that concentration of thymine was always equal to the concentration of adenine (A = T) And the concentration of cytosine was equal to the concentration of guanine (G = C). This strongly suggest that thymine and adenine as well as cytosine and guanine were present in DNA with fixed interrelationship. Also the total concentration of purines (A +G) always equal to the total concentration of pyrimidine (T +C). However, the (T+ A)/ (G+C) ratio was found to vary widely in DNAs of different species. www.assignmentpoint.com

10. X ray diffraction When X rays are focused through isolated macromolecules or crystals of purified molecules, the X ray are deflected by the atom of the molecules in specific patterns called diffraction patterns. It provides the information about the organization of the components of the molecules. Watson and Crick had X ray crystallographic data on DNA structure from the studies of Wilkins and Franklin and their coworkers. These data indicated that DNA was a highly ordered, multiple stranded structure with repeating sub structures spaced every 3.4 A o (1 Angstrom = 10 -10 m ) www.assignmentpoint.com

11. X-ray diffraction patterns of DNA – Rosalind Franklin and Maurice Wilkins The central cross shaped pattern as indicative of a helical structure. The heavy dark patterns (top and bottom) indicate that the bases are stacked perpendicular to the axis of the molecule. www.assignmentpoint.com

12. Double Helix Watson and Crick proposed that DNA exists as a double helix in which two polynucleotide chains are coiled above one another in a spiral. Each polynucleotide chain consists of a sequence of nucleotide linked together by Phosphodiester bonds. The two polynucleotide strands are held together in their helical configurations by hydrogen bonding. The base pairing is specific That is, adenine is always paired with thymine and guanine is always paired with cytosine Thus, all base-pairs consists of one purine and one pyrimidine. Once the sequence of bases in one strand of DNA double helix is known, it is possible to know the other strand sequence of base because of specific base pairing. www.assignmentpoint.com

14. In their most structural configuration, adenine and thymine form two hydrogen bonds, where as guanine and cytosine form three hydrogen bonds. The two strands of a DNA are complementary (not identical) to each other. It is this property, that makes DNA uniquely suited to store and transmitting the genetic information. The base-pairs in DNA are stacked 34A o apart with 10 base-pairs per turn (360 0 ) of the double helix The sugar – phosphate backbones of the two complementary strands are antiparallel, that is they have opposite chemical polority. www.assignmentpoint.com

15. As one move unidirectionally along a DNA double helix, the phosophodiester bonds in one bonds in one strand go from a 3’Carbon of one nucleotide to a 5’Carbon of the adjacent nucleotide. Where as those in complementary strand go from 5’Carbon to a 3’carbon. This opposite polarity of the complementary strands is very important in considering the mechanism of replication of DNA. The high degree of stability of DNA double helices results in part from the large number of hydrogen bonds between base pairs. www.assignmentpoint.com

16. Although each hydrogen bond by itself quite weak, since no. of hydrogen bonds are more, it can withstand. The planar sides of the base pair are relatively non polar and thus tend to be water insoluble (hydrophobic). The hydrophobic core stacked base-pairs contributes considerable stability to DNA molecules present in the aqueous protoplasms of living cells. www.assignmentpoint.com

17. Conformational Flexibility of DNA Molecule The vast majority of the DNA molecules present in the aqueous protoplasms of living cells almost certainly exists in the Watson – Crick double helix from just described. –This is the B form of DNA B form represent the 92% relative humidity. In fact, intracellular B-form DNA appears to have an average of 10.4 nucleotide-pairs per turn, rather than 10. www.assignmentpoint.com

18. In high concentration of salts or in a dehydrated state, (75% humidity) DNA exists in the A- form, which has 11 nucleotide-pairs per turn. Recently, certain DNA sequences have been shown to exist in a unique left handed, double helical form called Z-DNA. The helices of A and B form DNA are wound in a right handed manner. B-DNAA-DNAZ-DNA FormResiduesPitch Per Turn A 0 A 11 24.6 B 10 33.2 Z 12 45.6 www.assignmentpoint.com

20. Did you know? Each cell has about 2 m of DNA. The average human has 75 trillion cells. The average human has enough DNA to go from the earth to the sun more than 400 times. DNA has a diameter of only 0.000000002 m. The earth is 150 billion m or 93 million miles from the sun. www.assignmentpoint.com

21. Semiconservative Replication of DNA Living organism perpetuate their kind reproduction. This may simple fission as in bacteria or complex mode of reproduction as in higher plants or animals. In all cases, however reproduction entails the faithful transmission of genetic information of the progeny. Since the genetic information is stored in DNA, the replication of DNA is central to all biology www.assignmentpoint.com