(A) Stem cells can divide asymmetrically to produce a new stem cell and a non-stem cell that can differentiate to replace other types of cells that are lost from the tissue because of cell death (top). Stem cells can also divide symmetrically, producing two daughter stem cells (symmetric self-renewal, middle) or two non-stem cells (symmetric differentiation, bottom). (B) Telomeres are the regions of DNA that cap and protect the ends of linear chromosomes. Each time a cell divides the telomeres get shorter. When telomeres become too short, cell division stops.