Heather_8

Erwin Chargaff analyzed the nitrogenous bases in many different forms of life,/ concluding that the amount of purines does not always equal the amount of pyrimidines (as proposed by Levene). DNA must carry information/ from parent cell to daughter cell. It must contain information/ for replicating itself. It must be chemically stable,/ relatively unchanging. However, it must be capable/ of mutational change. Without mutations, there would be no process/ of evolution. Many scientists were interested in deciphering/ the structure of DNA, among them were Francis Crick, James Watson, Rosalind Franklin, and Maurice Wilkens. Watson and Crick gathered all available data/ in an attempt to develop a model of DNA structure. Franklin took X-ray diffraction photomicrographs/ of crystalline DNA extract, the key to the puzzle. The data known at the time was that DNA/ was a long molecule, and proteins were helically coiled. DNA is a double helix, with bases to the center/ (like rungs on a ladder) and sugar-phosphate units along the sides of the helix (like the sides of a twisted ladder). The strands are complementary/ (deduced by Watson and Crick from Chargaff's data, A pairs with T and C pairs with G, the pairs held together by hydrogen bonds). Purines are Adenine/ (A) and Guanine (G). Pyrimidines are Cytosine/ (C) and Thymine (T). The bases are complementary,/ with A on one side of the molecule you only get T on the other side, similarly with G and C. If we know the base sequence/ of one strand we know its complement.
 * MOST CHALLENGING WORDS**
 * 1.**nitrogenous
 * 2.**deciphering
 * 3.**photomicrographs
 * 4.**mutational
 * 5.**replicating
 * 6.**helically
 * 7.**sugar-phosphate
 * 8.**purines
 * 9.**diffraction
 * 10.**helix

1. Recall what DNA must carry, contain, and be. - Informaion. 2. Clarify why Watson and Crick gathered data about DNA. -To develope a model of DNA structure. 3. Compare and contrast purines and pyrimidines. -Purines are Adenine and Guanine while Pyrimidines are Cytosine and Thymine.