Telomeres are repetitive sequence structures at the ends of chromosomes. They consist of the double stranded DNA repeats followed by the short single stranded DNA. In humans and other verterbrates the telomeric sequence is composed of tandem of TTAGGG repeats. With each cells division telomeres shorten by up to 200 base pairs. Telomerase is an enzyme responsible for continuous cell growth and is repressed in most somatic cells, except proliferating progenitor cells, but in more than 85% of cancer cells telomerase expression is observed. Tumour cells with metastatic potential may demonstrate a high telomerase activity, allowing cells to escape from the inhibition of cell proliferation due to shortened telomeres. Determination of telomerase expres- sion was performed with the use of PCR ELISA in samples isolated from bovine leukaemia virus (BLV) infected cows. Telomerase activity was found in almost all investigated samples. The relative telomerase activity (RTA) was higher in infected cows than in healthy animals and the differences were statistically significant (α=0.05). In blood lymphocytes of BLV-infected cows the mean values of telomerase expression determined in real-time PCR were 3534.12 copies, in the healthy group there were 1010.10 copies and these differences were also statistically significant. For telomere length evaluation the Telomere PNA/FITC FISH and Telomere PNA/FITC FISH for flow cytometry were used. The mean fluorescence intensity of telomere sequences calculated on the surface of interphase nuclei of leukaemic blood lymphocytes was lower than that in the control animals and the difference was statistically significant. The mean length of telomeres in BLV- infected and healthy cows was 31.63 ± 12.62 and 38.4 ± 4.03, (p=0.112), respectively.

Here we report the consequences of telomere erosion in Arabidopsis thaliana, studied by examining seed and pollen production and the course of male meiosis through the last five generations (G5-G9) of telomerase-deficient Arabidopsis mutants. We used a previously described mutant line in which telomerase activity was abolished by T-DNA insertion into the TERT gene encoding telomerase reverse transcriptase. Reduced fertility accompanied by morphological abnormalities occurred in G6, which produced on average 35 seeds per silique (vs. 43 in wild type) and worsened in G7 (30 seeds) and G8 (14 seeds), as did the morphological abnormalities. The last generation of tert mutants (G9) did not form reproductive organs. Analysis of meiosis indicated that the main cause of reduced fertility in the late generation tert mutants of Arabidopsis was the numerous chromosomal end-to-end fusions which led to massive genome rearrangements in meiocytes. Fusion of meiotic chromosomes began in G5 and increased in each of the next generations. Unpaired chromosomes (univalents) were observed in G7 and G8. The study highlights some differences in the meiotic consequences of telomere shortening between plant and animal systems.