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If the cap fits, wear it: an overview of telomeric structures over evolution - PubMed

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If the cap fits, wear it: an overview of telomeric structures over evolution

Nick Fulcher et al. Cell Mol Life Sci. 2014 Mar.

Abstract

Genome organization into linear chromosomes likely represents an important evolutionary innovation that has permitted the development of the sexual life cycle; this process has consequently advanced nuclear expansion and increased complexity of eukaryotic genomes. Chromosome linearity, however, poses a major challenge to the internal cellular machinery. The need to efficiently recognize and repair DNA double-strand breaks that occur as a consequence of DNA damage presents a constant threat to native chromosome ends known as telomeres. In this review, we present a comparative survey of various solutions to the end protection problem, maintaining an emphasis on DNA structure. This begins with telomeric structures derived from a subset of prokaryotes, mitochondria, and viruses, and will progress into the typical telomere structure exhibited by higher organisms containing TTAGG-like tandem sequences. We next examine non-canonical telomeres from Drosophila melanogaster, which comprise arrays of retrotransposons. Finally, we discuss telomeric structures in evolution and possible switches between canonical and non-canonical solutions to chromosome end protection.

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Figures

Fig. 1
Fig. 1

Telomeric structures from prokaryotes, mitochondria, and viruses. The presence of covalently closed hairpin structures at terminal regions has been observed within B. burgdorferi, A. tumefaciens, and the Vaccinia virus (VACV) (a). 5′ overhangs of the human Ad2/5 adenovirus are bound by a terminal protein (TP) (b). Mitochondrial telomeres from the green algae C. reinhardtii comprise long inverted terminal repeat regions with a 3′ overhang (c) whereas C. parapsilosis exhibits tandem repeats with 5′ overhangs (d)

Fig. 2
Fig. 2

Canonical telomeres exhibiting TTAGG-like sequences. The shelterin complex is responsible for sequestration of the G-overhang into duplex double-stranded telomeric DNA (a). Once hidden, the G-overhang is no longer susceptible to DNA repair processes or the actions of telomerase. A similar complex is also found in fission yeast with the conserved functions of Pot1 (b). Budding yeast telomeres are protected by the CST (Cdc13/Stn1/Ten1) complex which associates with the G-overhang (c). C. elegans telomeres contain 5′ overhangs that are bound by CeOB2 (d). Blunt-ended telomeres present at a subset of A. thaliana chromosome ends are protected by the Ku heterodimer (e)

Fig. 3
Fig. 3

Telomeric retrotransposons in Drosophila melanogaster. Telomeric DNA from D. melanogaster comprises tandem repeats of retrotransposons HeT-A, TAHRE, and TART (a). Telomeric retrotransposons are capped by the terminin complex (b, figure adapted from Raffa et al. [159]). Telomeric retrotransposition results in telomere elongation (c). Transcripts are exported from the nucleus where GAG and RT proteins are translated. These proteins are then responsible for localization of transcripts to chromosome termini where they contribute to telomere elongation

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