What are Telomeres ?

TELOMERES are specialized DNA-protein complexes that cap the ends of the chromosomes. Their function is to help with replication of genetic material when cells divide and to protect the ends of the chromosome ends from degradation. The telomere region of each human chromosome is made up of three areas:

(T₂AG₃)_n sequence:

The DNA sequence that is found at the end of each human chromosome. This region, shown in yellow in the figure above, is composed of a repeated sequence of nucleotides, the building blocks of DNA. The repeated nucleotide sequence is TTAGGG, which makes up 3-20 kb (kilobases) of DNA at the end of the chromosome. This region of the telomere has been implicated in cancer and aging processes.
Subtelomeric Repeat Region or Telomere associated repeats (TAR):

The DNA sequence that is adjacent and centromeric to the (T₂AG₃)_n sequence. This region, shown in the above figure as blue and white diagonal lines, is also made up of repetitive pieces of DNA. These sequences are shared among many different subsets of telomeres.
Subtelomere unique sequence DNA:

This region of DNA is unique to each chromosome and is located centromeric to the TAR region about 100-300 kb from the end of the chromosome. Many studies suggest that the unique sequence regions adjacent to human telomeres have the highest concentration of genes of any chromosomal regions in the genome. Addition or subtraction of genetic material in the subtelomeric region can lead to significant problems with growth and development. The region labeled in red is the region that is targeted when clinical testing for subtelomeric rearrangements is performed.