Anastassiya_8

//__**Most challenging words**__// //__**1.**__// Replicating. //__**2.**__// Deciphering. //__**3.**__// Double helix. //__**4.**__// Adenine. //__**5.**__// Guanine. //__**6.**__// Cytosine. //__**7.**__// Thymine. //__**8.**__// Pyrimidines. //__**9.**__// Strand. //__**10.**__// Complementary.

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 s cientists 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.
 * DNA must carry information from cell to cell, for replicating itself. It chemically stable, relatively unchanging.**
 * Watson and Crick gathered data about DNA to develop a model of DNA structure.**
 * Purines are Guanine and Adenine; pyrimidines are Thymine and Cytosine.**