22 Telomerase

Abstract

The view of evolution that asserts that the genetic material was first in the form of RNA raises the question of how the transition to DNA genomes occurred. In the current world, although most of the DNA genome of a eukaryote is replicated by copying the preexisting parental DNA, portions have also resulted from copying RNA into DNA. One of these genomic components is telomeric DNA, which requires for its maintenance the continued action of a ribonucleoprotein enzyme, telomerase. Telomerase contains an essential RNA component. The telomeric DNA sequence is specified by and generated from copying a template sequence within the telomerase RNA moiety. In this chapter, I discuss the properties of telomerase and how they may illuminate the evolutionary transition from RNA-based to DNA-based genomes.

Telomeres were originally defined functionally as the natural ends of eukaryotic chromosomes, without which a chromosome is unstable (for review, see Blackburn and Szostak 1984; Zakian 1989; Blackburn 1991). Telomeric DNA in the form of very simple, tandemly repeated sequences is a conserved feature of widely diverged eukaryotes. However, there are some interesting apparent exceptions in certain species, which are discussed below. Representative examples of the simple type of telomeric repeated sequences are shown in Table 1. A terminal stretch of this simple-sequence DNA, ranging in length from less than fifty base pairs in some ciliated protozoans, through a few hundred base pairs in yeast and some other lower eukaryotes, to thousands of base pairs in human cells, appears to be sufficient to maintain...