Allyson_8

"Most Challenging Words" 1.Nitrogenous 2.Pyrimidines 3.replicating 4.mutational 5.Photomicrographs 6.Helix 7.Helically 8.Complementary 9.Deciphering 10.Purines Passage: The Structure of DNA 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.

__**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

1. DNA carry information, must contain for replicating itself, and be chemically stable. 2. Watson and Crick gathered DNA because to develop a model of DNA structures. 3.Purines are Adenine and Pyrimidines are Cytosine thats how they are differ and they are similar because their both DNA'S.