The elongation reaction proceeds as follows: The template of hTERC directs the sequential addition of nucleotides to the 3' telomeric DNA end. Following addition of a nucleotide, the template and catalytic site must move relative to one another within the telomerase RNP to place the appropriate template residue in the active site. As base-pairing and nucleotide addition occur at one end of the template, base pair melting occurs at the other (Collins and Greider 1993; Wang and Blackburn, 1997; Hammond and Cech 1998; Benjamin et al. 2000; Forstemann and Lingner 2005). This un-pairing is thought to reduce the energy used for mediating the subsequent translocation step. Nucleotide addition can occur up until the template boundary which in hTERC is defined by a helix called P1b (Chen and Greider 2003).