'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.