There are various channels allowing DNA, RNA and ribonucleotides(rNTPs) into and out of the enzymes active center cleft 16
16 There are various channels allowing DNA, RNA and ribonucleotides (rNTPs) into and out of the enzyme’s active center cleft
Transcription by RNA a polymerase proceeds n in a series of steps ■ Initiation 中a9 Elongation ■ Termination 17
17 Transcription by RNA polymerase proceeds in a series of steps ◼ Initiation ◼ Elongation ◼ Termination RNA polymerase and the transcription cycle
Initiation Promoter: the DNA sequence that initially binds the RNa polymerase a The structure of promoter- polymerase complex undergoes structural changes to proceed transcription a 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 binding Binding(closed (closed complex complex) promoter maltin Promoter“ melting” (ope complex (open complex) initial Initial RNA transcription 19
19 Fig 12-3-initiation Binding (closed complex) Promoter “melting” (open complex) Initial transcription
Elongation a Once the RNa polymerase has synthesized a short stretch of RNA ( nt), transcription shifts into the elongation phase. a This transition requires further conformational change in polymerase that leads it to grip the template more firmly. a 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