Organism

1. Organism Organs Tissues Cells (DNA) DNA is the molecular repository of genetic information. The ability to store and transmit genetic information from one generation to the next is a fundamental condition for life.

2. What is the genetic material? • Chromosomes are made of protein and DNA. So, one or other or combination of both should be the genetic material. • Fundamental property required of the genetic material is that it must be able to exist in an infinite variety of forms: • Each cell contains a large number of different genes • Each gene controls different heritable trait • Each gene should have different structure • These speculations led biologists conclude that protein (not DNA)was the genetic material. DNA was believed to be small invariant molecule, whereas proteins were known to be macromolecules made of long polymers of amino acids.

3. Genetic Material • By late 1930sDNA was a long polymer and, like protein, could exist in an almost infinite number of variable forms • Identification of the chemical nature of the transforming principle: • Transforming principle can change the bacterium Streptococcuspneumoniae, from one form to another (avirulent to virulent form). • Frederick Griffith 1928 bacterial transformation Cell membrane Cell wall Slime layer or capsule S = Smooth; with slime layer or capsule; virulent R = Rough; without capsule; avirulent

4. Griffith’s Experiment (bacterial transformation) IS IIR IS Heat killed IS + living IIR mouse mouse Δ 600C for 3h death alive mouse mouse alive death - Virulent S bacteria can be obtained from the mouse postmortem - Some property of the dead IS bacteria can transform the live IIR into IS - The transforming principle is the genetic material

5. The transforming principle is DNA Oswald Avery, Colin Macleod and Maclyn McCarty 1944 Filterate from heat-killed S cells Ribonuclease (degrade RNA) Deoxyribonuclease (degrade DNA) Trypsin, chemotrypsin (degrade protein) SIII enzyme (degrade polysaccharide) transformation transformation transformation No transformation

6. Components of DNA Nucleotides (the monomer in DNA) - the sugar component (pentose; 2’-deoxyribose) - the nitrogenous base (double-ring purines A & G; single-ring pyrimidines T & C) - the phosphoric acid component (three phosphate groups can be attached) HOCH2 OH 5’ O H H 4’ 1’ H 5 carbons in the pentose sugar C1’ base is attached C2’ ribose or deoxyribose C3’ point of attachment for more nucleotides C4’ completes ring via oxygen which bridges to the C1 C5’ hangs away from the ring; point of attachment for its –PO4 H 3’ 2’ HO H

7. Structural elements of the nucleotides

8. Polynucleotides: • Nucleotides are joined together by phosphodiester bonds • Polynucleotides have distinct ends (5’-P terminus and 3’-OH terminus) • Polynucleotides can be any length and have any sequence

9. The double helix J. Watson and F. Crick (Nobel Prize in 1962) 1953 discovered double helix using model building based on all the available information. E. Chargaff 1945-50demonstrated that the base ratios in DNA are constant. Result: A=T and G=C; total purines (A+G) is equal to total pyrimidines (T+C); but A+T ≠ G+C; GC content varies from species to species

10. Structure of DNA • DNA is double-stranded • Sugar-phosphate backbone is on the outside, and the bases are stacked on the inside of the helix • Bases of the two strands interact by hydrogen bonding (A-T and G-C) • Ten base pairs per turn of the helix; pitch of the helix is 34A0 i.e. spacing between adjacent base pairs is 3.4A0 ; the helix is 20A0 in diameter. • The two strands are antiparallel • The double helix has two different grooves; major and minor • The double helix is right-handed DNA movie

11. Circumfrance of Earth = 4 X 104 Km Sun – Earth distance = 1.5 X 108 Km Total length of your DNA = 2 X 1011 Km One human being contains ≈ 1014 cells One human cell contains 2 meters of DNA

12. Supercoiling Relaxed and supercoiled plasmid DNAs

13. Bacterial DNA is a compact nucleoid

14. Nucleosome is the structural unit of chromatin Chromatin spilling out of lysed nuclei Digestion of chromatin with micrococcal nuclease. Nucleosomes are 10A0 beads.

15. Beads on a String—10 nm Fiber

16. 10 nm filament; nucleosomes protein purification H1 H3 H2A histones (= 1g per g DNA) DNA H2B H4 • Basic (arg, lys); • + charges bind • to - phosphates • on DNA

17. Separate DNA from protein proteins DNA H1 200 nt fragments 2H3 2H2A the histone octamer 2H2B 2H4 deoxyribonuclease I (DNase I) digestion nucleosomes Core Particle • Conclude: histonesin a • nucleosome protect 200 nt • from DNase I attack.

18. A Nucleosome has 200 bp DNA and core histones bases 800 600 400 200

19. Organization of histone octamer

20. Path of nucleosomes in the chromatin fiber 10 nm fiber 30 nm fiber Chromosome movie

21. Folding of 30 nm fiber Loops of DNA attached to a nuclear scaffold. Numerous loops of DNA attached to a scaffold like structure.

22. Packaging of chromatin fiber Two chromatids (10 coils each) Net result: Each DNA molecule has been packaged into a chromosome that is 10,000-fold shorter than its extended length One coil (30 rosettes) One rosette (6 loops) One loop (75000 bp) 30 nm fiber 10 nm fiber DNA

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