Term of the Day

Telomere Clock: A theory concerning cellular aging and death. The telomere clock is presumed to determine the maximum number of times a given cell can divide. Telomeres are strings of nucleotides at the very ends of chromosomes, presumably to keep the chromosome from fraying or fusing with other chromosomes. In humans, the telomere base sequence TTAGGG repeats itself over a thousand times. The sequence, itself, does not code for anything, but every time the cell divides, up to 100 bases are lost from the telomere, which gets shorter. The theory is that when telomeres shorten to a critical point, then a signal to cease further mitosis is given. Credence to this theory was given by the 1994 discovery of the telomerase enzyme, which protects telomeres from degrading. Interestingly, telomerase is found in almost all cancer cells (which undergo uncontrollable mitosis), but not in other cell types.