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Section: Genetics » Expression of Gene » Transcription in Prokaryotes and Eukaryotes
 
 
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  'Inchworm model' for elongation of transcript
 
     
 
Content
⇒ Expression of Gene : Protein Synthesis 2.  Transcription in Prokaryotes and Eukaryotes
⇒ Transcription in prokaryotes 
⇒ Single RNA polymerase in E. coli
⇒ Promoter sites for initiation of transcription in prokaryotes
⇒ Initiation and elongation of RNA synthesis in prokaryotes
⇒ 'Inchworm model' for elongation of transcript
⇒ Elongation arrest vs termination of transcription
⇒ Termination and antitermination of mRNA synthesis in prokaryotes
⇒ Transcription in eukaryotes 
⇒ Multiple RNA polymerases in eukaryotes
⇒ Promoter, enhancer and silencer sites for initiation of transcription in eukaryotes
⇒ Transcription factors and initiation of RNA synthesis in eukaryotes
⇒ Formation of preinitiation (transcription) complex with RNA polymerase II (Pol II)
⇒ Structure and role of TFIID and other transcription factors (TBP, TAFs)
⇒ TFIIB domains for interaction with TFIID/TATA complex
⇒ Phosphorylation of CTD of a subunit of Pol II
⇒ Formation of pre-initiation complex with Pol I and Pol III
⇒ Separate DNA binding and transcription activation domains
⇒ Transcription factors and elongation of RNA chains in eukaryotes
⇒ Chromatin structure and transcription
⇒ Transcription in mitochondria
⇒ Transcription of vertebrate mtDNA
⇒ Transcription of yeast and plant mtDNA
⇒ Transcription in chloroplasts


'Inchworm model' for elongation of transcript
In the year 1993, M.J. Chamberlin proposed that the transcript elongation involves inchworm like translocation of RNAP (RNA polymerase) along the DNA template. RNAP is assumed to have one site for DNA binding and two sites for RNA binding (the 'leading product site' and the 'lagging product site'). While the 'leading product site' gets fiHed by about 10 newly added nucleotides (nt) in each successive cycle of elongation, the 'lagging product site' occupies the preceding segment of nascent RNA earlier synthesized. RNAP translocates forward by about 10 nt, when the leading site is full, so that tfie leading site in emptied again, thus permitting the next cycle of elongation. Stability to the elongation complex is provided by the interaction of RNA with the lagging site of RNAP. The above 'inchworm model' is based on experimental evidence and is ' 'diagrammatically represented in Figure 32.6.
  
Inchworm model of elongation showing (a) elongation cycle ; (b) transcription arrest and (c) termination.
Fig. 32.6. Inchworm model of elongation showing (a) elongation cycle ; (b) transcription arrest and (c) termination.

 
     






     
     
 
 
     
 
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