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Unit 6 1 U NIT 6 P ART A: DNA S TRUCTURE P ART B: DNA R EPLICATION P ART A: DNA S TRUCTURE The most remarkable of all the properties of living cells and organisms is their ability to reproduce themselves with nearly perfect fidelity for countless generations. This continuity of inherited traits implies constancy, over thousands or millions, of years in the structure of the molecules that contain the genetic information. This hereditary information is preserved in DNA. DNA is a long thin organic polymer in which the linear sequence of covalently linked nucleotide subunits encodes the genetic message. Two of these polymeric strands are twisted about each other to form the DNA double helix, in which each monomeric subunit in one strand pairs specifically with the complementary subunit in the opposite strand. This unit provides an overview of the structure of nucleotides and nucleic acids found in most cells. Assignment: Nelson & Cox, pp. 271 - 281, 287 - 288, 945 - 971, 976. 1. Given the structures of the four common bases (Fig. 8-2, p. 272), identify each as a purine or pyrimidine and give its common name. (Note: You will not be asked to draw the structure of the bases, given the name.)
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Unit 6 2 2. Nucleosides and Nucleotides a. Given the structure of the bases in Fig. 8-2 (p. 272), draw structures representing (Fig. 8-1, p. 271): 1) a 2'-deoxynucleoside 2) a 2'-deoxynucleotide 3) a 2'-deoxynucleoside 5'-di- or triphosphate b. Use Fig. 8-1 (p. 271) to discuss the numbering convention for the pentose ring. Which atom in the sugar is the base attached to? What kind of bond joins the sugar to the base (p. 272)? Using Fig. 8-4 (p. 273), identify the atom that is found at the 2' position in deoxyribonucleotides? - in ribonucleotides? What functional group is at the 3' position in ribo- and deoxy-ribonucleotides? To which atom in the sugar is the phosphate bound? c. Use Fig. 8-4 (p. 273) to discuss the nucleotides found in DNA. Compare and contrast these nucleotides to those found in RNA. 3. Draw a section of DNA (use the letters A, C, G, and T to represent the structure of the base). Point out the following (Fig. 8-7, p. 275): a. phosphodiester bond (or linkage; note the difference between a phosphodiester bond and a phosphanhydride bond by comparing to Fig. 1-15, p. 12) b. sugar-phosphate backbone c. 5' and 3' ends d. overall charge 4. Distinguish between an oligonucleotide and a polynucleotide (p. 276).
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