Jodi_8

**10. Cytosine**

 * Passage: The Structure of DNA**

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.
 * //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.


 * __Questions:__**
 * 1.** //Recall what DNA must carry, contain, and be.//
 * DNA must carry information in order to carry it from parent cell to daughter cells.**
 * DNA must contain infromation in order to replicating itself.**
 * DNA must be chemically stable yet somehow unchanging.**
 * 2**. //Clarify why Watson and Crick gathered data about DNA.//
 * The wanted to attempt to develop a model of DNA structure.**
 * 3.** //Compare and contrast purines and pyrimidines.//
 * The difference between purines and pyrimidines is purines are Adenine and Pyrimidines are Cytosine.**
 * The thing they both have in common is the bases are both complementary.**