TRF analysis of mutant plants. Terminal telomere fragments were obtained by digestion of DNA isolated from representative plants in second, fourth, and terminal generations (G2, G4, G6) by Tru1I restriction endonuclease, separated on 1% agarose gel, Southern transferred, and visualized by 32P labelled C3TA3 probe. (A) Wild type and nbs1 mutants had telomere lengths 5000 to 2000 bp. Gradual shortening of tert mutants (line 69–2) is accompanied by more dramatic telomere shortening in double homozygotes tert/nbs1, (line K12) to as short as 600 bp in G6 (arrowhead). The differences in the intensity of signals reflect slight variations in the amount of DNA loaded in each lane. (B) The comparison of TRF fragments of plants G4 and G5. Gradual decline in the length of the fragments of double homozygotes tert/nbs1 (line L2) from 1600–700 bp to 1300–700 bp is evident between consecutive generations. The ranges of the lengths are markedly shorter than for tert (line 69–1) displaying 2500–1400 bp and 2000–1000 bp respectively for generations G4 and G5. Arrows point to the interstitial telomere sequences in (A) and (B).