Telomeres are repeating DNA sequences at the end of chromosomes.Telomeres prevent chromosomes from fusing with other chromosomes and also stop chromosomes from deterioration by the loss of base pair sequences. A telomere can include up to 15,000 base pairs and whenever a cell divides the telomere is reduced. This is called erosion. Think of telomeres as a protective guardrail on a chromosome. A little bit of the chromosome is lost with every division. Telomeres are just "trash" DNA sequences that are lost during division instead of the essential and important DNA on the chromosome. Eventually telomeres becomes too short (reaching a 'critical length') and the chromosome can no longer replicate or divide causing it to die. This process is called apoptosis.