Cell divisionCell division is the process of a biological cell dividing into two daughter cells. This leads to growth in multicellular organisms (the growth of tissue) and to procreation in unicellular organisms. Mitosis is the most common way of cell division. A cell which has reached certain parameters (size, volume, stored energy, environmental factors, etc.) will replicate its DNA and divide into two (usually equal) daughter cells. Both cells stay diploid or haploid, depending on which they started with. Meiosis is the division of a diploid cell into (four) haploid ones. This mechanism is often found in multicellular organisms to produce haploid gametes, which can then fuse again to form a diploid cell. Multicellular organisms replace worn-out cells through cell division. However, in some animals, cell division eventually halts, and the cell is then referred to as senescent. Senescent cells deteriorate and die, causing the body to age. Cells stop dividing because the telomeres, protective bits of DNA on the end of a chromosome, become shorter with each division and eventually can no longer protect the chromosome. Cancer cells, on the other hand, are "immortal." An enzyme called telomerase allows them to continue dividing indefinitely.