An enzyme that adds a molecule of DNA to the end of a cell’s chromosomes after the cell divides. Without telomerase, a cell’s chromosomes become shorter after each cell division until their chromosomes become so short the cells lose their ability to divide. Most body cells do not have telomerase, thus they generally stop dividing after a certain number of cell divisions (called the Hayflick limit) because their chromosomes are shortened. Certain types of cells that possess telomerase can divide indefinitely (past the Hayflick limit) including embryonic stem cells and in many types of cancer cells.