11.   But Frisen has found the cells do divide, though at too slow a rate for the earlier tests to detect.

12.   But the company was drawn to the study of stem cells because telomerase is active in embryos, in which cells rapidly divide.

13.   Each time a cell divides its telomeres get shorter, as if marking off a finite number of permissible divisions.

14.   Earlier experiments showed the cavity-lining cells did not divide, confirming their uninteresting nature.

15.   Each time the cell divides, it is liable for unknown reasons to miscopy the sequence by inserting an extra chemical letter before or after it.

16.   Eventually the telomeres lose so much DNA the cell can divide no more, and dies.

17.   Every time a cell divides, its telomeres get shorter, like the burning wick on a candle.

18.   Every time a cell divides, the telomeres become shorter.

19.   Every time a cell divides, it loses a little of its telomeres.

20.   Every time cells divide, they reproduce their chromosomes.