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Unformatted text preview: 1 PRACTICE EXAM #2 ANSWERKEY Question 1. (1) Answer: The trp repressor/tryptophan combination cannot fully repress transcription when the tryptophan levels are high. An RNA molecule, called the leader mRNA, that ends at a position (the attenuator site) in the trpL region is produced although at a relatively low level under these conditions. (2) Answer: The LacI/lactose combination cannot fully activation transcription when glucose is also present in the medium. Under these conditions, -galactosidase level is low in the cell, suggesting that derepression at the operator site is not enough to turn on the lac operon. (3) Answer: Most eukaryotic genes are transcribed more than once in the cell. Having the so-called DAB complex left behind at the promoter regions is thought to facilitate the next round of transcription (or re-initiation) by saving the first few obligatory steps required to set up the pre- initiation complex (PIC) complex. It is known that the DAB complex attracts several other generalinitiation complex (PIC) complex....
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