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bio9 - shawn3387
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09-27-2006, 04:13 PM
Which of the following molecules is found in a nucleoside?
a. A pyrophosphate group b. A 1′base linked to a pentose sugar c. A 5′-phosphate group linked to a pentose sugar d. A 3′-phosphate group linked to a pentose sugar e. A terminal triphosphate
09-27-2006, 04:27 PM
. The answer is b. (Murray, pp 412“434. Scriver, pp 3“45. Sack, pp 3“29.
Wilson, pp 99“121.) A nucleoside consists of a purine or pyrimidine base linked to a pentose sugar. The 1′carbon of the pentose is linked to the nitrogen of the base. In DNA, 2′-deoxyribose sugars are used; in RNA, ribosesugars are used. Nucleotides are phosphate esters of nucleosides with one to three phosphate groups, such as adenosine monophosphate (AMP), adenosine diphosphate (ADP), or adenosine triphosphate (ATP). The nitrogenous bases are adenine, thymine, guanine, and cytosine in DNA, with thymine replaced by uridine in RNA. Nucleotide polymers are chains of nucleotides with single phosphate groups, joined by bonds between the 3′-hydroxyl of the preceding pentose and the 5′-phosphate of the next pentose. Polymerization requires high-energy nucleotide triphosphate precursors that liberate pyrophosphate (broken down to phosphate) during joining. The polymerization reaction is given specificity by complementary RNA or DNA templates and rapidity by enzyme catalysts called polymerases |
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