21.   Every time a cell divides, some of this tail is used up.

22.   Hence each time a cell divides, its chromosomes get a little bit shorter.

23.   Hours after fertilization, the cell divides.

24.   If the drug could promote cell death at a rate that equaled the rate at which the cancer cells divide, then it could keep tumors from growing.

25.   If they allowed the cells to divide, they would accept the genes.

26.   In normal cells, the telomerase are shortened slightly each time the cell divides.

27.   In embryonic stem cells, too, the telomerase gene is active, and the cells can divide indefinitely as long as they remain embryonic.

28.   In the remainder of the cells, telomerase acts like a kitchen timer counting how many times a cell divides.

29.   It was after World War I that doctors first noticed that mustard gas seemed to be most deadly in organs, like skin, where cells divided rapidly.

30.   Kept at body temperature in a company factory, the skin cells divide at a phenomenal rate.