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23 Telomerase

Elizabeth H. Blackburn


According to the view of evolution in which the genetic material was first in the form of RNA, a transition to the present-day DNA genomes occurred. In the current world, although most of the DNA genome of a eukaryote is replicated by copying of the preexisting parental DNA strands by DNA-dependent DNA polymerase, portions have also resulted from copying RNA into DNA. One of these genomic components is the telomeric DNA, which requires for its continued maintenance the action of a ribonucleoprotein (RNP) enzyme, telomerase. The telomeric DNA sequence is specified by a template sequence within the telomerase RNA moiety, which is copied by reverse transcriptase action of the telomerase RNP. In this chapter, I discuss the properties of telomerase and suggest how they may shed light on two evolutionary transitions: from RNA-based to DNA-based genomes and from RNA-based enzymes to protein enzymes.

Telomeres were originally defined functionally as the natural ends of eukaryotic chromosomes, without which a chromosome is unstable (for review, see Blackburn 1984for review, see Blackburn 1991a,b, 1994; Zakian 1995). Telomeric DNA in the form of very simple, tandemly repeated sequences is a conserved feature throughout the eukaryotes, with some interesting exceptions in certain species (Table 1). A terminal stretch of this simple-sequence DNA, ranging in total length from under fifty base pairs in some ciliated protozoans, through a few hundred base pairs in yeasts and several other lower eukaryotes, to thousands of base pairs in mammalian cells, appears to supply sufficient cis-acting information to maintain a stable telomere. As...

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