Cell division

Cell 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.

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