What are genes made of and how do they work? DNA What are genes made of and how do they work?

Frederick Griffith In 1928, a British scientist tried to figure out the how bacteria produce the lung disease pneumonia. He cultured 2 different strains of pneumonia bacteria – one that caused the disease and one that did not. He found that the disease-causing strain of bacteria did not release a poison because when heated, they did not produce pneumonia in the injected mice. When the heat-killed bacteria and harmless bacteria were mixed, then injected into mice, the mice developed pneumonia and many died. He called this change from one bacterial strain into another “transformation.”

Oswald Avery Repeated Griffith’s experiments with the goal of isolating what caused transformation. Avery and his colleagues made an extract (juice) from the heat-killed bacteria and treated it with a variety of enzymes that broke down proteins, lipids, carbohydrates, and other molecules… The harmless bacteria were still transformed! However, when using enzymes that broke down DNA, transformation did not occur. Discovery: DNA stores and transmits genetic information from one generation to the next!

Hershey-Chase Experiment Experimented with bacteriophages (viruses that infect and kill bacteria). Composed of a DNA core and a protein coat Attaches to surface of bacteria, injects DNA, and viral genes produce new bacteriophages that destroy the bacterium. When the bacteria splits open, hundreds of new viruses burst out.

Hershey-Chase Experiment Wanted to determine if genes were made of DNA or protein Grew viruses in cultures with radioactive isotope “markers” Phosphorus-32 attached to DNA, and Sulfur-35 attached to protein… whichever one was found in the bacteria would have been injected by the virus. Nearly all radioactivity in bacteria was from Phosphorus-32. Discovery: Genetic material was DNA – not protein!

Structure of DNA Long molecule Made of units called nucleotides Phosphate, deoxyribose (5-C sugar), base Backbone of sugar and phosphate 4 bases: Adenine, Guanine, Cytosine, Thymine 2 varieties: Purines (A,G): 2 rings; Pyrimidines (T,C): 1 ring

Structure of DNA Chargaff’s Rules X-Ray Evidence Found that percentages of C and G were nearly the same in samples of DNA, as were percentages of A and T. X-Ray Evidence Rosalind Franklin aimed an x-ray beam through a concentrated DNA sample to make a pattern on film. Pattern gave clues about helical structure, 2 strands, and nitrogenous bases near the center.

Structure of DNA Double Helix James Watson and Francis Crick constructed a 3D model of DNA. They used information from Rosalind Franklin’s X-ray diffraction, and published their results. It was a double helix (twisted ladder) with 2 strands wound around each other. They discovered the Hydrogen bonds between bases that held the strands together. Discovered base pairing: C pairs with G – 3 H bonds A pairs with T – 2 H bonds