telomeres

In every cell, genetic material is contained within the double-stranded molecules of DNA called chromosomes. Every time a cell divides, the chromosomes are copied so that each new cell gets a copy of this genetic material.  However, the process isn’t perfect – a little bit of the end of the chromosome is lost each time so the copy is not a perfect reproduction. To prevent important genetic material from being lost, the cell protects itself with structures called telomeres located at the ends of the chromosomes. Telomeres are repeating DNA sequences and proteins that act like a guard so that when the chromosome is copied, the important material is protected and only the telomere is shortened. Since telomere shortening happens every time the cell divides, telomeres act like biological clocks for the cell. When the chromosome has been copied enough times that the telomere has been completely worn away, any future copies of the chromosome may be missing important genetic material and the cell may cease to function normally.