Dereck_8

Most Challenging Words 1. Purines 2. Pyrimidines 3. Mutational 4. Deciphering 5. crystalline 6. Heliex 7. Adenine 8. Guanine 9. Cytosine 10. Thymine

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.
 * Erwin Chargaff** analyzed the 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.
 * 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./ DNA must carry informationfrom parent cell to daughter cell/ it contained information for replicating itself./it must be chemically stable 2. Clarify why Watson and Crick gathered data about DNA. /to develop a model 3. Compare and contrast purines and pyrimidines. /they are diffrent