final_exam - Cover Page Genetics 380 Spring 2004 Exam III...

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Genetics 380, Spring 2004 Exam III NAME_____________________________________ 100 TOTAL 100 TOTAL 15 5 20 4 20 3 20 2 25 1 Grader Initials Points Received Possible Pts. PAGE Cover Page
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1. E. col i possesses a mechanism called the "SOS response" to cope with various types of DNA damage. More than 20 SOS genes, which are known to promote the survival of DNA-damaged cells, are induced by treatments that damage DNA (e.g., UV irradiaton). All the SOS genes share similar sequences in the regulatory regions called the "SOS box," to which LexA repressor binds to repress the transcription in the absence of DNA damage. Upon the occurrence of DNA damage, single- stranded DNA produced in the vicinity of DNA damage acts as a signal. A protein called RecA binds to the single-stranded DNA, and is activated by it. The activated RecA then promotes cleavage of LexA protein. Once cleaved, LexA no longer represses transcription. Hence, the SOS genes are allowed to be transcribed. The induced products of the SOS genes then repair the DNA lesions by various mechanisms, including recombination, excision repair and error-prone repair. Once repaired, DNA damage signal in the cell decreases and the repression of the SOS genes is restored. a. (3 pts) Draw out a linear genetic pathway for the SOS response. Be sure to include the following components: single stranded DNA, LexA, the SOS box, RecA, the SOS genes, and DNA damage. Be sure to indicate whether each genetic relationship is stimulatory ( ) or inhibitory ( ).
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