Transcription by RNA 是 polymerase proceeds Q in a series of steps ■工 nitiation ■ Elongation ■ Termination <0
17 Transcription by RNA polymerase proceeds in a series of steps ◼ Initiation ◼ Elongation ◼ Termination RNA polymerase and the transcription cycle
Initiation a Promoter: the DNA sequence that initially binds the rNa polymerase The structure of promoter olymerase complex undergoes structural changes to proceed transcription DNA at the transcription site unwinds and a“ bubble” forms a Direction of RNA synthesis occurs in a 5'-3' direction (3 -end growing)
18 Initiation ◼ Promoter: the DNA sequence that initially binds the RNA polymerase ◼ The structure of promoterpolymerase complex undergoes structural changes to proceed transcription ◼ DNA at the transcription site unwinds and a “bubble” forms ◼ Direction of RNA synthesis occurs in a 5’-3’ direction (3’-end growing)
RNA polymerase Fig 12-3-initiation upstream downstream DNA DNA promoter bindu Binding(closed (closed complex complex) promoter melting Promoter“ melting” (open complex) (open complex) initial Initial transcription RNA transcription 19
19 Fig 12-3-initiation Binding (closed complex) Promoter “melting” (open complex) Initial transcription
Elongation Once the RNA polymerase has synthesized a short stretch of RNA (N 10 nt), transcription shifts into the elongation phase. This transition requires further conformational change in polymerase that leads it to grip the template more firmly. Functions: synthesis RNA unwinds the dna in front re-anneals it behind, dissociates the growing RNA chain
20 Elongation ◼ Once the RNA polymerase has synthesized a short stretch of RNA (~ 10 nt), transcription shifts into the elongation phase. ◼ This transition requires further conformational change in polymerase that leads it to grip the template more firmly. ◼ Functions: synthesis RNA, unwinds the DNA in front, re-anneals it behind, dissociates the growing RNA chain
Termination After the polymerase transcribes the length of the gene (or genes), it will stop and release the rna transcript. ■工 n some cellls, termination occurs at the specific and well. defined DNA sequences called terminators. some cellis lack such termination sequences. 21
21 Termination ◼ After the polymerase transcribes the length of the gene (or genes), it will stop and release the RNA transcript. ◼ In some cells, termination occurs at the specific and welldefined DNA sequences called terminators. Some cells lack such termination sequences