102 quiz 5 KEY - MCB102 Quiz#5 11/15 Question 1(4 points 1a...

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MCB102 Quiz #5 11/15 Question 1 (4 points) 1a. Which base is shown below? Is it a purine or pyrimidine? Guanine, purine (2 points) -1 for wrong base -1 for pyrimidine 1b. How do magnesium ions stabilize the structure of DNA? (Maximum length: 1 sentence) They form ionic interactions with the negatively charged phosphate groups of the DNA backbone. (2 points) -0.5 if say ionic interactions but don’t mention negative charge -0.5 if don’t mention phosphate groups -0.25 if just say phosphodiester/sugar-phosphate backbone without specifying phosphate groups Question 2 (4 points) 2a. Give one reason why DNA is maintained in a supercoiled state inside a cell. It reduces tangling, compacts DNA so it can fit in a cell and aids in unwinding DNA for replication and transcription. (2 points) -1 for vaguely described alternative answers 2b. The linking number of a closed, circular DNA molecule changes from 50 to 49 after incubation with a purified enzyme. What is the enzyme? (Maximum length: 1 sentence) Topoisomerase I (2 points) -1.5 if only say I or type I and give incorrect enzyme name (-0.25 to -1 for spelling errors) -0.5 if don’t say I or type I -1 if say gyrase (it’s a type II topoisomerase)
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