Layne_8


 * "Most Challenging Words."**
 * 1. Mutations**
 * 2. Deciphering**
 * 3. Photomicrograph**
 * 4. Pyrimidines**
 * 5. Cytosine**
 * 6. Adenine**
 * 7. Diffraction**
 * 8. Crystalline**
 * 9. Thymine**
 * 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). **Pyrimidine**s/ 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. **DNA must carry information from parent cell to daughter cell. And It must contain information for replicating itself and It must be chemically stable, relatively unchanging** 2. Clarify why Watson and Crick gathered data about DNA. **Watson and Crick gathered all available data in an attempt to develop a model of DNA structures**. 3. Compare and contrast purines and pyrimidines. **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**