Telomeres

Telomere

Telomeres are located at the two physical ends or tips of a linear eukaryotic chromosome. Telomeric DNA consists of mini satellites such as repetitive, intergenic and noncoding simple sequence DNA.

Structure
Telomere consists of specialised nucleoprotein complexes composed of telomeric DNA bound by specific proteins which is packaged into heterochromatin. The telomeric DNA consists of long stretches of multiple tandem repeats of a short nucleotide sequence with a high Guanine(G) content [5'-TTAGGG-3']. The 3'end of the G-rich strand extends 12-16 nucleotides beyond the 5'end of the complementary C-rich strand, forming a 3' single stranded overhang. Aided by specialised proteins, the 3'single stranded ends fold back to tuck itself into the duplex DNA, forming a hairpin loop.

Functions
1) Protective Function
Telomeric DNA binds specific proteins which protect the ends of a chromosome from degradation by cellular exonucleases.

2) Maintaining Stability
Telomeres maintain stability of a linear chromosome by "sealing" the ends of the linear DNA, thus preventing "DNA fraying" and by preventing chromosome fusion.

3) Prevents loss of gene and coding sequences
The simple sequence DNA ensures DNA replication occurs without the loss of coding sequences as telomere shortens with each successive round of replication.

4) Regulating Cell Division
When the telomere reaches a critically short length, the cell enters a period of replicative cell senescence. Once the cell reaches the Hayflick Limit, cell division ceases and eventually dies. Hence, the telomere length regulate a cell's life span.