III.

Centromeres and Telomeres

The chromosomes of nearly all eukaryotic life forms contain two important structures: centromeres and telomeres. During cell division, the centromere—visible through a microscope as a knotlike structure—connects to an apparatus called the spindle. The spindle contains fibers that move the centromeres around, causing the rest of each chromosome to follow. This process ensures that each chromosome moves to its proper place during mitosis, when a cell divides to give rise to two cells, and during meiosis, the process of cell division that gives rise to eggs or sperm.

Telomeres are specialized sequences of DNA that are found at the tips of chromosomes. Telomeres serve as a kind of cap that prevents the ends of chromosomes from attaching to the ends of other chromosomes. Scientists suspect that telomeres may influence the activity of nearby genes and may play a role in determining the life span of a cell.