Maggie_8

1. nitrogenous 2. purines 3. pyrimidines 4. replicating 5. mutational 6. deciphering 7. photomicrographs 8. Guanine 9. Cytosine 10. Thymine
 * "Most Challenging Words."**

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 / 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. DNA must carry information on one gene to another.

2. Clarify why Watson and Crick gathered data about DNA. Watson and Crick gathered data about DNA cause, tp make an attempt to develop a model.

3. Compare and contrast purines and pyrimidines. Purines are Adenine and Guanine. Pyrimidines are Cytosine and Thymine.