41.   The physical basis of the Hayflick rule is a section of DNA known as the telomere, which gets shorter each time the cell divides.

42.   The telomeres get shorter each time a cell divides, as if marking off the number of times a cell can divide before lapsing into senescence.

43.   Their progeny cells divide rapidly and can differentiate into both neurons and the support cells that are found copiously throughout the brain.

44.   This enzyme keeps the DNA in cells from deteriorating, as it normally does, when the cell divides.

45.   They seem normal but, as the cells divide, genetic abnormalities appear unpredictably among future generations.

46.   Tumor cells divide rapidly, burning energy in the process.

47.   When cells divide, they must make copies of the chromosomes for each of the new cells.

48.   But each time a cell divides, the telomeres get shorter.

49.   Cancerous areas show reduced potential because cancer cells divide more often than normal, Crowe said.

50.   Each time the cells divide, the protective end of the chromosome, called the telomere, is shortened.