Telomeres

• Telomeres are a repetitive sequence of base pairs at the end of chromosomes that act as a cap to prevent the loss of pertinent genetic information during replication. Over the course of a cell's life, the length of the telomeres slowly decreases by 40 to 200 base pairs per replication.
  
  

Hayflick Limit

• Cellular replication continues until the telomere is shortened to 5,000 to 8,000 base pairs, at which point the cell has reached its "Hayflick limit." While these cells can live for a long time, they no longer reproduce.
  
  

Alternative Lengthening of Telomeres

• Alternative lengthening of telomeres (ALT) is a phenomenon that occurs in telomerase-negative cells. Telomerase, an enzyme that exists naturally in fetal cells and stem cells, continually refreshes the telomere length, regardless of the number of replications, rendering these cells immortal, or always able to replicate. ALT cells, however, do not contain telomerase. ALT cells have inconsistent telomere lengths ranging from extremely short to extremely long. Despite their varying telomere lengths, ALT cells remain immortal by homogenous recombination with other chromosomes in the cell.