Synthesis of telomeric DNA by telomerase
In Physical Basis of Heredity 1. The Nucleus and the Chromosome
, we described a unique feature of the structure of the telomeric ends of eukaryotic chromosomes, characterized by repeated DNA sequences (2-10 bases long). This feature is common to all chromosomes of a species and is conserved. A special ribonucleoprotein molecule (enzymatic in nature) called telomerase
uses a special mechanism for the synthesis of DNA at these telomeric ends.
The DNA repeat sequence of telomere has one G-rich strand and the other C rich strand, the G-rich strand having a single stranded overhang. This overhang works as a primer and for its elongation uses as template the RNA component of telomerase
enzyme (Fig. 26.29). Telomerase synthesizes only the G-rich strand of telomeres. The complementary C-rich strand is perhaps synthesized (extended) by primase-polymerase mediated discontinuous synthesis, typical of semi-conservative DNA replication, for which extended G-rich strand is used as a template.
For the use of telomerase RNA as template, its sequence should be complementary to telomeric DNA repeat unit, which has been verified. For instance, in Euplotes, 5'CAAAACCCCAAAA3' is found in telomerase, which is used for synthesis of 5'GGGGTTTT3' repeats.