Telomeres, the specialized DNA protein structures located at the end of eukaryotic chromosomes, consist of small, tandemly repeated DNA sequences. Numerous telomere sequences have been identified that display very few sequence variations, even between phylogenetically divergent organisms such as Tetrahymena (sequence: TTGGGG) and humans (sequence: TTAGGG).
Because DNA polymerase is unable to replicate the very ends of linear DNA, it was suggested that chromosomal ends progressively shorten with each replication cycle (called the “end-replication problem”). This phenomenon, which has been demonstrated in vitro and in vivo, seems to be linked to the limited proliferative capacity of normal somatic cells (“mitotic clock”). Since germ-line cells, stem cells, and tumor cells all exhibit a prolonged or even infinite life span, it was suggested that these cells must possess a particular mechanism for maintaining telomere length.
Maintaining stable telomere length is associated with the activation of telomerase. This enzyme is a ribonucleoprotein that compensates for the loss of telomeric DNA by adding repeat sequences to the chromosome ends, using its intrinsic RNA component as a template for DNA synthesis.
Telomeres play an essential role in the stable maintenance of eukaryotic chromosomes within a cell by specifically binding to structural proteins. These proteins cap the ends of linear chromosomes, preventing nucleolytic degradation, end-to-end fusion, irregular recombination, and other events that are normally lethal to a cell.
Analysis of telomere length in research samples of human peripheral blood mononuclear cells reveals that telomere length decreases with increased age in the donor, reflecting the replicative history of those cells. In several disorders (e.g., Down′s syndrome, ataxia telangiectasia, and HIV infection), accelerated telomere loss has been described, suggesting the reduction in telomere length may be related to the immune dysfunction associated with these disorders. This kit is intended to increase scientific knowledge about these relationships.
Assay time: Approximately 18 hours
Sample material: Cell cultures and other biological samples
Nonradioactive chemiluminescent assay to determine telomere length.
The kit utilizes Southern analysis of terminal restriction fragments (TRF) that are obtained by the digestion of genomic DNA using frequently cutting restriction enzymes.
Step 1: Digestion of genomic DNA
Purified genomic DNA is digested by an optimized mixture of frequently cutting restriction enzymes. The enzymes have been selected in such a way that telomeric DNA and sub-telomeric DNA is not cut. This is due to the special sequence characteristics of the repeats. Non-telomeric DNA is digested to low molecular-weight fragments.
Step 2: Gel electrophoresis and Southern blotting
Following DNA digestion, the DNA fragments are separated by gel electrophoresis, then transferred to a nylon membrane by Southern blotting.
Step 3: Hybridization and chemiluminescence detection
The blotted DNA fragments are hybridized to a digoxigenin (DIG)-labeled probe that is specific for telomeric repeats, then incubated with a DIG-specific antibody covalently coupled to alkaline phosphatase. Finally, the immobilized telomere probe is visualized by a highly sensitive chemiluminescent substrate for alkaline phosphatase, CDP-Star. The average TRF length can be determined by comparing the signals to a molecular-weight standard.