7. How the eukaryotic transcription activators and repressors regulate transcription itation?(10 points) Activators: (1)recruit the transcriptional machinery to the gene(2.5).(2)recruit nucleosome modifiers that help the transcription machinery bind at the promoter Repressors: activator's binding site, a repressor inhibit binding of the activator. 1 b. Inhibition--- A repressor binds to a site besides an activator occluding its activating region. c. Direct repression---A repressor binds to a site upstream of a gene, by interacting with the transcriptional machinery at the promotor in some way, inhibits transcriptional inhibition d. Indirect repression--- recruit nucleosome modifiers that alter nucleosomes to inhibit transcription. This is the most common mechanism of the repressor inhibition of the transcription in eukaryotes 8. Please describe how the transcription activators Sw15 and SBF work synergistically to have the yeast Ho gene expressed only in mother cells and at certain point in the cell cycle, resulting in the budding division feature of yeast S cerevisiae.(10 points SW15: acts only in the mother cell(1)and binds to multiple sites unaided some distance from the promoter of the HO gene(1) SBF: acts only at the correct stage of the cell cycle(1)and cannot bind its sites unaided near to the promoter of the HO gene(1), Thus, when both activators are present and active, the action of sw15 opens the multiple binding sites of SBF and enables SBF to bind. SBF, in turn, recruits the transcriptional machinery by directly binding Mediator and activates expression of the HO gene 5 Therefore, through the cell-type specific activation of sw15 and the cell cycle-specific activation of SBF, and the synergistic interaction of these two activators, the yeast HO gene expressed only in mother cells and at certain point in the cell cycle 9. What is RNA interference, and what are the molecular mechanisms that lead to the observed phenotype of RNA interference? (10 points) RNA interference (RNAi) refers to a phenomenon that double-stranded RNA inhibits expression of genes homologous to that dsRna Mechanism of RNAi: the dsRNa is first chopped to siRna by dicer(2), and the SiRNA is incorporated into the effector complex called RISC(RNA interference silencing complex)(2 ). Then the siRISC complex will find its homologous mRNAs to cause the degradation or translational inhibition of the target mRNAs (2). The siRISC complex can also interact with its homologous DNA sequence in the promoter7. How the eukaryotic transcription activators and repressors regulate transcription initiation? (10 points) Activators: (1) recruit the transcriptional machinery to the gene (2.5’). (2) recruit nucleosome modifiers that help the transcription machinery bind at the promoter (2.5’). Repressors: a. Competition--- By binding to a site on DNA that overlaps the activator’s binding site, a repressor inhibit binding of the activator. 1’ b. Inhibition--- A repressor binds to a site besides an activator, occluding its activating region. 1’ c. Direct repression--- A repressor binds to a site upstream of a gene, by interacting with the transcriptional machinery at the promotor in some way, inhibits transcriptional inhibition. 1’ d. Indirect repression--- recruit nucleosome modifiers that alter nucleosomes to inhibit transcription. This is the most common mechanism of the repressor inhibition of the transcription in eukaryotes 2’ 8. Please describe how the transcription activators SWI5 and SBF work synergistically to have the yeast HO gene expressed only in mother cells and at certain point in the cell cycle, resulting in the budding division feature of yeast S. cerevisiae. (10 points) SWI5: acts only in the mother cell (1’) and binds to multiple sites unaided some distance from the promoter of the HO gene (1’). SBF: acts only at the correct stage of the cell cycle (1’) and cannot bind its sites unaided near to the promoter of the HO gene (1’), Thus, when both activators are present and active, the action of SWI5 opens the multiple binding sites of SBF and enables SBF to bind. SBF, in turn, recruits the transcriptional machinery by directly binding Mediator and activates expression of the HO gene. 5’ Therefore, through the cell-type specific activation of SWI5 and the cell cycle-specific activation of SBF, and the synergistic interaction of these two activators, the yeast HO gene expressed only in mother cells and at certain point in the cell cycle. 1’ 9. What is RNA interference, and what are the molecular mechanisms that lead to the observed phenotype of RNA interference? (10 points) RNA interference (RNAi) refers to a phenomenon that double-stranded RNA inhibits expression of genes homologous to that dsRNA. 3’ Mechanism of RNAi: the dsRNA is first chopped to siRNA by dicer (2’), and the siRNA is incorporated into the effector complex called RISC (RNA interference silencing complex) (2’). Then the siRISC complex will find its homologous mRNAs to cause the degradation or translational inhibition of the target mRNAs (2’). The siRISC complex can also interact with its homologous DNA sequence in the promoter