Shelby_8

1.pyrimidines 2.replicating 3.mutational 4.crystalline 5.helically 6.crystalline 7.complementary 8.Purines 9.Thymine 10.complementary **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.
 * Most Challenging Words**
 * Passage: The Structure of DNA**
 * 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.

__**Questions:**__ 1. Recall what DNA must carry, contain, and be. 2. Clarify why Watson and Crick gathered data about DNA. 3. Compare and contrast purines and pyrimidines.