A telomere is a region of highly repetitive DNA at the end of a linear chromosome that functions as a disposable buffer.
Every time linear chromosomes are replicated during late S phase, the DNA polymerase complex is incapable of replicating all the way to the end of the chromosome; if it were not for telomeres, this would quickly result in the loss of vital genetic information, which is needed to sustain a cell's activities.
Every time a cell with linear chromosomes divides, it will lose a small piece of one of its strands of DNA.
This process has been referred to by James Watson and Alexei Olovnikov as the "end replication problem." It is believed that telomeres have a function in the ageing process.
Telomerase is a "ribonucleoprotein complex" composed of a protein component and an RNA primer sequence which acts to protect the terminal ends of chromosomes.
This is because during replication, DNA polymerase can only synthesize DNA in a 5'to 3'direction and can only do so by adding polynucleotides to an RNA primer that has already been placed at various points along the length of the DNA.
These RNA strands must later be replaced with DNA.
At the terminal of the DNA strand, the RNA primer is laid but DNA polymerase cannot extend beyond it.
This RNA primer will not later be replaced by DNA, and therefore cannot be translated into gene products or replicated later.
Without telomeres at the end of DNA, this genetic sequence would be deleted and the chromosome would grow shorter and shorter in subsequent replications.
The telomere prevents this problem by employing a different mechanism to synthesize DNA at this point, thereby preserving the sequence at the terminal of the chromosome.