THE NATUR E OF THE GENE Evidence:1. 1869 MIESCHER isolated from pus cells and new class of chemicals - “Nuclein” rich in P + CHON (later called) NUCLEIC ACIDS.2. LEVENE (1925-30) - showed DNA contained: purines (A + G) and pyrimidines (T + C)3. 1953 WATSON & CRICK - proposed MODEL OF DNA.EXPERIMENTAL BASIS OF GENE CONCEPT:² AVERY 1944 showed that a bacterial agent normally capable of causing genetic change lost itsactivity when treated with Deoxyribonuclease but not when treated with proteolytic enzymes (e.g.trypsin) therefore the agent is DNA² 1940’s MIRSKY & RIS Also BOIVIN & VENDRELY. Examined the amount of DNA / nucleusin animal tissues (mg x 10-9). Species Sperm RBC Heart Kidney Chicken 1.26 2.49 2.45 2.20 Carp 1.64 3.49This shows:(i) Amount of DNA / nucleus is CONSTANT IN ALL BODY CELLS OF A GIVEN ORGANISM.(ii) SOMATIC CELLS are DIPLOID and GERM CELLS are HAPLOID² 1952 HERSHEY & CHASE working on bacteriophages (viruses which attack bacteria) showedwhether the substance injected into bacteria by phage was PROTEIN OR DNA by using:32 35 P - incorporated into DNA: S - incorporated into protein(i) Infect non radioactive bacteria with such a phage.(ii) Agitate to remove excess viruses.(iii) Break open bacteria cells and analyse for 32P and 35S. 35 32 35Results: Remains of VIRUS contained S. While Bacteria contained P and v. little, if any, S.² 1940’s CHARGAFF found: Relative amounts of Purines and Pyrimidines in samples of DNA. Source Adenine Guanine Cytosine Thymine Beef Liver 28.8 21.0 21.1 29.0 Human Liver 30.3 19.5 19.9 30.3 T2 Bacteriophage 32.5 18.2 16.7 32.6In all samples: A+G = T+C A = T G = C² late 1940s LINUS PAULING identified Helix structure of some proteins held in by H-bonds,² ROSALIND FRANKLIN produced x-ray diffraction pictures of DNA.ALL THES E HE L P E D W A TSO N A N D CR I C K TO P RO P OS E THE DO U B L E HEL I X MO D E L OFTHE DN A MOL E C U L E I N 1953.