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DNA structure
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DNA structure

  1. 1. 1 Presented By Dr. T,Ugandhar Asst.Prof of Botany GDCMAHABUBABAD
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  4. 4. Timeline 1800’s F Miescher - nucleic acids 1928 F. Griffith - Transforming principle 1952 Avery, McCleod & McCarty- Transforming principle is DNA 1944 Hershey-Chase ‘blender’ experiment 1949 Erwin Chargaff – base ratios
  5. 5. DNA (Deoxyribonucleic Acid)
  6. 6. A HISTORY OF DNA • Discovery of the DNA double helix A. Frederick Griffith – Discovers that a factor in diseased bacteria can transform harmless bacteria into deadly bacteria (1928) B. Rosalind Franklin - X-ray photo of DNA. (1952) C. Watson and Crick - described the DNA molecule from Franklin’s X-ray. (1953) SEE p. 292-293
  7. 7. X-ray diffraction patterns produced by DNA fibers – Rosalind Franklin and Maurice Wilkins
  8. 8. The Watson-Crick Model: DNA is a double helix • 1951 – James Watson learns about x-ray diffraction pattern projected by DNA • Knowledge of the chemical structure of nucleotides (deoxyribose sugar, phosphate, and nitrogenous base) • Erwin Chargaff’s experiments demonstrate that ratio of A and T are 1:1, and G and C are 1:1 • 1953 – James Watson and Francis Crick propose their double helix model of DNA structure
  9. 9. Watson & Crick proposed… •DNA had specific pairing between the nitrogen bases: ADENINE – THYMINE CYTOSINE - GUANINE •DNA was made of 2 long stands of nucleotides arranged in a specific way called the “Complementary Rule”
  10. 10. Deoxyribonucleic Acid (DNA): • Deoxyribonucleic acid, also abbreviated as DNA, is the principal informational macromolecule of the cell, which stores, translates and transfers the genetic information. • In the prokaryotes, the DNA is found mostly in the nuclear zone. • In eukaryotes it is found in the nucleus, mitochondria and chloroplast. • The present understanding of the storage and utilization of the cell’s genetic information is based upon the discovery of the structure of DNA by Watson and Crick in 1953. 10
  11. 11. Structure of DNA: DNA is made of two helical chains coiled around the same axis, to form a right-handed double helix. The two chains in the helix are anti-parallel to each other, i.e., the 5′-end of one polynucleotide chain and the 3′-end of the other polynucleotide chain is on the same side and close together. The distance between each turn is 3.6 nm (formerly 3.4 nm). There are 10.5 nucleotides per turn (formerly 10 nucleotides). 11
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  14. 14. Chargaff’s Rule • Adenine must pair with Thymine • Guanine must pair with Cytosine • Their amounts in a given DNA molecule will be about the same. G CT A
  15. 15.  The spatial relationship between the two strands creates major and minor grooves between the two strands. In these grooves some proteins interact.  The hydrophilic backbones of alternating deoxyribose and negatively charged phosphate groups are on the outside of the double helix.  The hydrophobic pyrimidine and purine bases are inside the double helix, which stabilizes the double helix of the DNA.  The double helix is also stabilized by inter-chain hydrogen bond formed between a purine and pyrimidine base.  A particular purine base, pairs by hydrogen bonds, only with a particular pyrimidine base, i.e., Adenine (A) pairs with Thymine (T) and Guanine (G) pairs with Cytosine (C) only. 15
  16. 16. 16  Two hydrogen bonds pairs Adenine and Thymine (A = T), whereas three hydrogen bonds pairs Guanine and Cytosine (G ≡ C).  The base pairs A = T and G ≡ C are known as complementary base pairs.  Due to the presence of complementary base pairing, the two chains of the DNA double helix are complementary to each other.  Hence the number of A’ bases are equal to the number of T’ bases (or ‘G’ is equal to ‘C) in a given double stranded DNA.  One of the strands in the double helix is known as sense strand, i.e., which codes for RNA/proteins and the other strand is known as antisense strand.
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  18. 18. Genetic material of cells… • GENES – units of genetic material that CODES FOR A SPECIFIC TRAIT • Called NUCLEIC ACIDS • DNA is made up of repeating molecules called NUCLEOTIDES
  19. 19. DNA Nucleotide O O=P-O O Phosphate Group N Nitrogenous base (A, G, C, or T CH2 O C1 C4 C3 C2 5 Sugar (deoxyribose)
  20. 20. DNA Double Helix Nitrogenous Base (A,T,G or C) “Rungs of ladder” “Legs of ladder” Phosphate & Sugar Backbone
  21. 21. DNA Double Helix P P P O O O 1 2 3 4 5 5 3 3 5 P P P O O O 1 2 3 4 5 5 3 5 3 G C T A
  22. 22. Nitrogenous Bases • PURINES 1. Adenine (A) 2. Guanine (G) • PYRIMIDINES 3. Thymine (T) 4. Cytosine (C) T or C A or G
  23. 23. BASE-PAIRINGS CG H-bonds T A
  24. 24. Genetic Diversity… • Different arrangements of NUCLEOTIDES in a nucleic acid (DNA) provides the key to DIVERSITY among living organisms.
  25. 25. The Code of Life… • The “code” of the chromosome is the SPECIFIC ORDER that bases occur. A T C G T A T G C G G…
  26. 26. DNA is wrapped tightly around histones and coiled tightly to form chromosomes See p. 297
  27. 27. DNA Replication • DNA must be copied • The DNA molecule produces 2 IDENTICAL new complementary strands following the rules of base pairing: A-T, G-C •Each strand of the original DNA serves as a template for the new strand See p. 298
  28. 28. DNA Replication • Semiconservative Model: 1. Watson and Crick showed: the two strands of the parental molecule separate, and each functions as a template for synthesis of a new complementary strand. . Parental DNA DNA Template New DNA
  29. 29. Essential features of B-DNA • Right twisting • Double stranded helix • Anti-parallel • Bases on the inside (Perpendicular to axis) • Uniform diameter (~20A) • Major and minor groove • Complementary base pairing
  30. 30. • Structurally, purines (A and G pair best with pyrimidines (T and C) • Thus, A pairs with T and G pairs with C, also explaining Chargaff’s ratios
  31. 31. Maybe because RNA but not DNA is prone to base- catalysed hydrolysis Why DNA evolved as the genetic material but not RNA?
  32. 32. B-DNA Biologically dominant Right-handed double helix planes of base pairs are nearly perpendicular to the helix axis. helix axis passes through the base pairs and hence B-DNA has no internal spaces B-DNA has a wide and deep major groove and a narrow and deep minor groove
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  35. 35. DNA conformations B-DNA: – right-handed double helix with a wide and narrow groove. A-DNA – major groove is very deep and the minor groove is quite shallow Z-DNA – consists of dinucleotides, each with different conformations 4 stranded DNA – Telomeric DNA
  36. 36. DNA conformations both form right-handed double helices B-DNA helix has a larger pitch and hence a smaller width than that of A In B-DNA, the helix axis passes through the base pairs and hence B-DNA has no internal spaces, whereas that of A-DNA has a 6 Angstrom diameter hole along its helical axis. The planes of the base pairs in B- DNA are nearly perpendicular to the helix axis, whereas in A-DNA, they are inclined from this. Therefore, B-DNA has a wide and deep major groove and a narrow and deep minor groove, whereas A- A DNA B DNA
  37. 37. DNA conformations B-DNA forms a right-handed double helix in which the repeating unit is a nucleotide, whereas Z-DNA forms a left- handed double helix in which the repeating unit is a dinucleotide. The Z-DNA helix has a larger pitch and is therefore narrower than that of B- DNA. B-DNA has a wide and deep major groove and a narrow and deep minor groove, whereas Z-DNA has a narrow and deep minor B DNAZ DNA
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  44. 44. linear human chromosomes Double stranded DNA Genetic material may be DNA Single stranded DNA circula r linear circula r Prokaryotes Mitochondria Chloroplasts Some viruses (pox viruses) Parvovirus adeno-associated viruses
  45. 45. reoviruses Double stranded RNA Genetic material may be RNA Single stranded RNA Retroviruses like HIV
  46. 46. RNA / DNA hybrids e.g. during retroviral replication
  47. 47. AMAZING DNA FACTS… • DNA from a single human cell extends in a single thread for almost 2 meters long!!! • It contains information equal to some 600,000 printed pages of 500 words each!!! (a library of about 1,000 books)
  48. 48. Functions of DNA: • The base sequence of the DNA constitutes the informational signal called the genetic material. This nucleotide base sequence enables the DNA to function, store, express and transfer the genetic information. • Hence it programs and controls all the activities of an organism directly or indirectly throughout its life cycle. • (a) DNA stores the complete genetic information required to specify (form) the structure of all the proteins and RNA’s of each organism. 48
  49. 49.  DNA is the source of information for the synthesis of all cellular body proteins. Some of the proteins are structural proteins and some are enzymes. These enzymes arrange micro-molecules to form macromolecules. It determines the activities of an organism throughout its life cycle, i.e., the period of gestation, birth, maturity, senescence and death. It determines the individuality and identity of a given organism. It duplicates (replicates to form two daughter DNA) itself and transfers one of the copy to the daughter cell during cell division, thus maintaining the genetic material from generation to generation. 49
  50. 50. THANK YOU