Telomeres of polytene chromosomes in a ciliated protozoan terminate in duplex DNA loops - PubMed (original) (raw)
Telomeres of polytene chromosomes in a ciliated protozoan terminate in duplex DNA loops
K G Murti et al. Proc Natl Acad Sci U S A. 1999.
Abstract
The end of a telomeric DNA sequence isolated from a polytene chromosome of a hypotrichous ciliate folds back and hybridizes with downstream telomeric sequence to form a t loop that is stable in the absence of protein and DNA cross-linking. The single-stranded, telomeric DNA sequence at the end of a macronuclear molecule does not form a t loop but, instead, is complexed with a heterodimeric, telomere-binding protein. Thus, two mechanisms for capping the ends of DNA molecules are used in the same cell.
Figures
Figure 1
Electron micrograph of a collection of duplex loops at the molecular ends of polytene chromosome DNA. The arrow points to a single-stranded stretch at the junction of a loop. (Bar = 1 μm.)
Figure 2
Electron micrograph of polytene chromosomes from a developing macronucleus of O. fallax. T, telomere. (Bar = 0.1 μm.)
Figure 3
Electron micrograph of DNA prepared from vegetative macronuclei of O. fallax. (Bar = 1 μm.)
Figure 4
Models of two different mechanisms for capping telomeric repeats in the micronucleus and macronucleus. (A) Formation of a t loop by a fold-back of the telomeric DNA in a micronuclear chromosome. The single-stranded, G-rich 3′ overhang hybridizes with the C-rich chain in a proximal segment of telomere, displacing the original G-rich chain. Redrawn from Griffith et al. (5). (B) Formation of a terminal complex between heterodimeric, telomere-binding protein and the single-stranded, G-rich 3′ overhang of the telomere in a macronuclear DNA molecule. Redrawn from Froelich-Ammon et al. (18).
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