tailieunhanh - Biochemistry, 4th Edition P96

Biochemistry, 4th Edition P96. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | How Is Transcription Regulated in Prokaryotes 913 b d FIGURE The rho factor mechanism of transcription termination. Rho factor a attaches to a recognition site on mRNA and b moves along it behind RNA polymerase. c When RNA polymerase pauses at the termination site rho factor unwinds the DNA RNA hybrid in the transcription bubble d releasing the nascent mRNA. Transcriptional control region f A Q P 1 2 3 DNA I I I f f Qperator Promotor Structural genes 1 2 3 FIGURE The general organization of consist of transcriptional control regions and a set of related structural genes all organized in a contiguous linear array along the transcriptional control regions are the promoter and the operator which lie next to or overlap each other upstream from the structural genes they control. Operators may lie at various positions relative to the promoter either upstream or downstream. Expression of the operon is determined by access of RNA polymerase to the promoter and occupancy of the operator by regulatory proteins influences this access. Induction activates transcription from the promoter repression prevents it. transcribed determines whether transcription takes place. This sequence is termed the operator Figure . The operator is located next to a promoter. Interaction of a regulatory protein with the operator controls transcription of the gene cluster by controlling access of RNA polymerase to the Such co-expressed gene clusters together with the operator and promoter sequences that control their transcription are called operons. Transcription of Operons Is Controlled by Induction and Repression In prokaryotes gene expression is often responsive to small molecules serving as signals of the nutritional or environmental conditions confronting the cell. Increased synthesis of enzymes in response to the presence of a particular substrate is termed 3Although this is the paradigm for prokaryotic gene regulation it must be