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DNA Composition and Structure: Understanding the Genetic Material

Explore the discovery and composition of DNA, including the double helix structure, nucleotides, hydrogen bonding, and packaging within cells. Learn about DNA replication, transcription, and translation processes in genetic information flow.

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DNA Composition and Structure: Understanding the Genetic Material

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  1. DNA Structure/Composition Taryono Faculty of Agriculture Gadjah Mada University

  2. Flow of Genetic Information DNA RNA Protein RNA Transcription DNA Replication Protein Translation

  3. DNA • Discovery of the DNA double helix A. 1950’s B. Rosalind Franklin- X-ray photo of DNA. C. Watson and Crick- described the DNA molecule from Franklin’s X-ray.

  4. DNA is the Genetic Material • DNA encodes all the information in the cell • The composition of the DNA is the same in all cells within an organism • Variation among different cells is achieved by reading the DNA differently • DNA contains four bases that encode all the information to make a bacteria or a human • In some viruses the genetic material is RNA

  5. How is Information Encoded in DNA? • DNA Consists of four kinds of bases (A,C,G,T) joined to a sugar phosphate backbone • Bases carry the genetic information while the phosphate backbone is structural • Two complementary strands of bases (C-G) and (A-T)

  6. DNA is a Polymer of Deoxyribonucleotide Units DEOXYRIBONUCLEOTIDE DEOXYRIBONUCLEOTIDE

  7. Deoxyribonucleic Acid (DNA) Nucleotide: 1. Phosphate group 2. 5-carbon sugar 3. Nitrogenous base • ~2 nm wide

  8. Phosphate Group O O=P-O O 5 CH2 O N Nitrogenous base (A, G, C, or T) C1 C4 Sugar (deoxyribose) C3 C2 DNA Nucleotide

  9. “Rungs of ladder” Nitrogenous Base (A,T,G or C) “Legs of ladder” Phosphate & Sugar Backbone DNA Double Helix

  10. 5 O 3 3 O P P 5 5 C O G 1 3 2 4 4 2 1 3 5 O P P T A 3 5 O O 5 P P 3 DNA Double Helix

  11. A or G T or C Nitrogenous Bases • PURINES 1. Adenine (A) 2. Guanine (G) • PYRIMIDINES 3. Thymine (T) 4. Cytosine (C)

  12. 3 H-bonds G C BASE-PAIRINGS Base # of Purines Pyrimidines PairsH-Bonds Adenine (A)Thymine (T) A = T 2 Guanine (G)Cytosine (C) C G 3

  13. H-bonds G C A T BASE-PAIRINGS

  14. C T A G 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.

  15. Deoxy ribo nucleotide Ribose= Five Carbon Sugar Molecule 5´ 5´ 1´ 1´ 4´ 4´ 2´ 2´ 3´ 3´ Ribose (RNA) Deoxyribose (DNA) Backbone Sugar Molecules

  16. 5´ 1´ 3´ 2´ 5´ 1´ 3´ 2´ 5´ 1´ 3´ 2´ The DNA Backbone is a Deoxyribose Polymer Deoxyribose sugars are linked by Phosphodiester Bonds 5´-p 3´-OH 5´ 3´

  17. 5´ 3´ 3´ 5´ 5´ 5´ 1´ 3´ 2´ 5´ 1´ 3´ 2´ 5´ 1´ 3´ 2´ 3´

  18. Base 5´ 5´ 1´ 1´ 3´ 3´ 2´ 2´ Base 5´ 5´ 1´ 1´ 3´ 3´ 2´ 2´ Base 5´ 5´ 1´ 1´ 3´ 3´ 2´ 2´

  19. DeoxyRibonucleotide Deoxyadenosine 5´-triphosphate (dATP) Deoxyadenosine DeoxyRibonucleoside

  20. 9 9 Two Purines Adenine Guanine Two Pyrimidines 1 1 Thymine Cytosine DNA is Composed of Four Different Ribonucleotides

  21. T C T A 5´ 3´ G A 3´ 5´

  22. Base Pairing Follows: Chargaff’s Rule: • DNA has equal numbers of thiamines and adenines (A=T) and equal numbers of guanines and cytosines (G=C) Note that in each pair there is one purine and one pyrimidine A-T G-C

  23. Base Pairing Occurs Through Hydrogen Bonds G-C A-T

  24. G C = A T

  25. Advantages of Double-Stranded Nature of DNA • Forms a stable structure • Hydrophobic bases stack on top of one another away from solvent • Charged phosphate backbone is on the outside accessible to solvent • Each strand can serve as the template • For a new strand of DNA (replication) • For an RNA molecule (transcription)

  26. Double-stranded DNA Forms a Double Helix

  27. Native DNA Forms a B-DNA Helix • Two strands wind about each other in a right-handed manner • Diameter: ~20Å • Bases per turn: 10 (~34Å) • A major and a minor groove Major Minor 20Å

  28. DNA is Highly Packaged within the Cell • If the DNA from a single human cell were stretched out end to end, it would be ~2 meters in length DNA 2 M Cell Nucleus (magnified lots of times) 5 x 10-8 M

  29. Mechanisms of Packaging • Has to be Organized so that DNA can be Untangled for Replication and Transcription • Supercoiling • Wrapping around Proteins to Create Chromatin • Enzymes that Modulate the Packaging of DNA are called Topoisomerases

  30. Supercoiling • Most native DNA exists in a negatively supercoiled state • This means that it is slightly unwound and it is a bit easier to pull the two strands apart More supercoiling

  31. Topoisomerases Modulate Supercoiling • Topoisomerases act as Molecular Scissors • They can make a cut in the DNA and pass second strand through that break to untangle the DNA

  32. DNA is Coiled Around Histone Proteins • DNA is wrapped around abundant nuclear proteins called Histones • This forms a complex called a Nucleosome • Histones are H1, H2A, H2B, H3, H4

  33. DNA is Further Packaged

  34. Take Home Message • DNA is a double helix composed of a sugar-phosphate backbone and base pairs (a polymer of deoxyribonucleotides) • Composition/Structure • DNA is compacted to fit into the cell through: • Supercoiling • Extensive interactions with proteins • These factors all contribute to how the DNA is Read

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