![Franklin specialized in X-ray crystallography – a technique in which x-ray
beams are shot through a crystal to project physical interference onto a screen.
Maurice Wilkins, the deputy head of the laboratory, was particularly jealous and without
permission, he took Franklin’s Photo 51, and showed them to two of his science colleagues –
Francis Crick and James Watson of Cambridge University.
In the epilogue Watson writes; "Since my initial impressions about [Franklin],
both scientific and personal (as recorded in the early pages of this book) were
often wrong I want to say something here about her achievements." He goes on
to describe her superb work, and, despite this, the enormous barriers she
faced as a woman in the field of science. He also acknowledged that it took
years to overcome their bickering before he could appreciate Franklin's
generosity and integrity.](https://image.slidesharecdn.com/class456-structureofdna-240825113045-3db1c484/85/Structure-of-DNA-it-s-general-characteristics-functions-and-topology-8-320.jpg)
Structure of DNA , it's general characteristics, functions and topology
- 2. DNA Organization and Molecular structure Molecular Biology of the Gene - James D Watson
- 3. DNA was first identified in the late 1860s by Swiss chemist Friedrich Miescher. As his pet cell for investigation, Miescher chose white blood cells, because they occurred as individual cells and were easy to obtain. Discarded bandages from a nearby surgical clinic provided pus from which intact white blood cells could be harvested. When Miescher exposed the cells to various salt solutions, they burst and a slimy porridge emerged that refused further analysis. As white blood cells were known to contain large nuclei, Miescher devised a method to further purify the cells and eventually obtain their nuclei. When he then subjected the nuclei to the same alkaline solutions as before, he noticed a precipitate that differed from previously characterised proteinaceous compounds in several ways. The precipitate was insoluble in the solutions proteins could normally be dissolved in; in contrast to proteins, the substance was rich in phosphorus; and, most notably, it was resistant to protein-digesting enzymes. Miescher concluded that he had discovered a new class of cellular substance, different from proteins, which he named, given its provenance, ‘nuclein’. https://www.cell.com/fulltext/S0960-9822(04)00543-3 Miescher died in 1895, aged only 51. Forty-nine years were to pass before it would be realised that the slimy substance isolated from pus was what carried genetic information. The story begins in 1844, one hundred years before Avery and his colleagues realised the significance of DNA as the genetic material (see accompanying article), with the birth in Basel of Friedrich Miescher, who went on to discover DNA at the age of 25.
- 4. The organic chemist Alexander Todd had determined that the backbone of the DNA molecule contained repeating phosphate and deoxyribose sugar groups Scottish biochemist whose research on the structure and synthesis of nucleotides, nucleosides, and nucleotide coenzymes gained him the Nobel Prize for Chemistry.
- 5. Phoebus Levene - A physician turned chemist Levene is credited with many firsts. For instance, he was the first to discover the order of the three major components of a single nucleotide (phosphate-sugar-base); the first to discover the carbohydrate component of RNA (ribose); the first to discover the carbohydrate component of DNA (deoxyribose); and the first to correctly identify the way RNA and DNA molecules are put together. Levene proposed that nucleic acids were composed of a series of nucleotides, and that each nucleotide was in turn composed of just one of four nitrogen-containing bases, a sugar molecule, and a phosphate group.
- 6. Erwin Chargaff After developing a new paper chromatography method for separating and identifying small amounts of organic material, Chargaff reached two major conclusions (Chargaff, 1950). • First, he noted that the nucleotide composition of DNA varies among species. In other words, the same nucleotides do not repeat in the same order, as proposed by Levene. • Second, Chargaff concluded that almost all DNA--no matter what organism or tissue type it comes from--maintains certain properties, even as its composition varies. • In particular, the amount of adenine (A) is usually similar to the amount of thymine (T), and the amount of guanine (G) usually approximates the amount of cytosine (C). • In other words, the total amount of purines (A + G) and the total amount of pyrimidines (C + T) are usually nearly equal. (This second major conclusion is now known as "Chargaff's rule.") https://www.nature.com/scitable/topicpage/discovery-of-dna-structure-and-function-watson-397/ During analysis of the DNA of an organism having 5386 nucleotides find out A=29%, G=17%, C=32%, T=17%. What is the nature of DNA molecule? A.It is single stranded DNA B.It is doubled stranded linear DNA C.Both A and B D.It is doubled stranded circular DNA
- 7. Putting the Evidence Together: Watson and Crick Propose the Double Helix Chargaff's realization that A = T and C = G, combined with some crucially important X-ray crystallography work by English researchers Rosalind Franklin and Maurice Wilkins, contributed to Watson and Crick's derivation of the three-dimensional, double-helical model for the structure of DNA. James Watson, Francis Crick, Maurice Wilkins, e Rosalind Franklin Photo 51 taken by Rosalind Franklin and RG Gosling Watson, Crick and Maurice Wilkins were awarded the 1962 Nobel Prize in Physiology or Medicine "for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material".
- 8. Franklin specialized in X-ray crystallography – a technique in which x-ray beams are shot through a crystal to project physical interference onto a screen. Maurice Wilkins, the deputy head of the laboratory, was particularly jealous and without permission, he took Franklin’s Photo 51, and showed them to two of his science colleagues – Francis Crick and James Watson of Cambridge University. In the epilogue Watson writes; "Since my initial impressions about [Franklin], both scientific and personal (as recorded in the early pages of this book) were often wrong I want to say something here about her achievements." He goes on to describe her superb work, and, despite this, the enormous barriers she faced as a woman in the field of science. He also acknowledged that it took years to overcome their bickering before he could appreciate Franklin's generosity and integrity.
- 10. Watson and Crick posed in the early 1990s with a model like the one they made to first describe the structure of DNA. Courtesy Susan Lauter, CSHL.
- 11. Helical Structure of DNA Space-Filling model Schematic model Has two polynucleotide chain Antiparallel Backbone- Alternate sugar and phosphate Bases project inward 1 helical turn = 34 A° or 3.4 nm 10.5 bases pairs per helical turn. Major groove and Minor groove Bonds: Glycosidic bond – between sugar and bases. Phosphodiester bond-Joins two nucleotides Hydrogen bond – Between two bases 1 Angstroms = 0.1 nm
- 12. Formation of nucleotide by removal of water Nucleotide Tri Phosphate ∆G for the hydrolysis of one mole of ATP into ADP and Pi is 7.3 kcal/mole ( 30.5 − − kJ/mol) ∆G for the hydrolysis of one mole of ATP in a living cell = 14 kcal/mol (−57 kJ/mol). Free energy released during this process is lost as heat Energy coupling – Eg., Sodium-Potassium Pumps
- 15. Purines & Pyrimidines Nitrogen in Purine and pyrimidine rings are amino form, rarely imino form Oxygen in Guanine and Thymine is Keto form rarely enol configuration TAUTOMERIC STATE OF BASES Double ringed structure N9-glycosidic bond Single ringed structure N1-glycosidic bond
- 16. Hydrogen Bonding between bases G-C Base pair: (3 hydrogen bonds) 1. Exocyclic NH2 at C2 of Guanine + carbonyl group at C2 of Cytosine 2. Hydrogen bond between N1 of Guanine + N3 of Cytosine 3. Carbonyl group at C6 of Guanine + exocyclic NH2 at C4 of Cytosine A-T Base pair (2 hydrogen bonds) 1. Exocyclic amino group at C6 on Adenine + carbonyl at C4 in Thymine 2. N1 of Adenine + N3 of Thymine
- 17. Stability of DNA Stability of DNA Hydrogen bonding: Gives thermodynamic stability (Increased entropy in organic molecule is stabilized by hydrogen bonding between bases) Base stacking: Electron cloud interaction between bases increases stability Van der Walls attraction (distance- dependent interaction) Specificity of bases Hydrophobic nature of bases Backbone angle is 30°
- 19. Methylation of bases Removal of damaged bases Homologous recombination DNA repair Base Flipping Protrusion of base from double helix
- 20. DNA is usually a right- Handed Helix (Handedness rule from Physics) Each of the polynucleotide chains in the double helix is right-handed Each base pair is twisted from the previous one by about 36 ° Complete helical periodicity 10 bp and one complete helix is 360 °
- 21. Double helix has Major and Minor groves The angle at which the two sugars protrude from the base pairs (the angle between glycosidic bonds: Narrow angle – 120 ° (Minor groove) Wide angle - 240 ° (Major groove)
- 22. Minor groove Minor groove Minor groove Minor groove Major groove Major groove Major groove Major groove A – hydrogen bond acceptors D – hydrogen bond donors H – Non polar hydrogen M - Methyl group Chemical groups exposed in the major and minor grooves from the edges of the base pair Minor Groove: AHA – A:T AHA (R) – T:A ADA – G:C ADA(R) – C:G Major groove: ADAM – A:T AADH – G:C MADA – T:A HDAA – C:G
- 23. a b Propeller twist Two flat bases counter rotate relative to each other along the long axis of the base pair. Two members of each base pair do not lie in the same plane. DNA is not perfectly regular
- 24. Left handed helix: (Z – form) Purine – syn Pyrimidine - anti Right handed helix: (B - form) Anti conformation Syn: Purine: N3 is above the sugar Anti Purine: H8 is above the sugar