Human chromosomes are linear chains of DNA. Due to the mechanism how DNA can be copied it is uneasy to start the copy process from the very beginning of the chromosome. Over the series of the copy processes the chromosomes would become shorter and shorter and genes close to chromosomal ends would be copied only partially and later even omitted altogether. Some primordial eukaryote in the very past achieved ability to extend the chromosomal ends with a simple hexameric repeat (encoded by TERC gene) by reverse-transcription. By this enzyme (called TERT, we wrote about it previously and also here), the ever shortening chromosomal ends can be extended back to some safe length. In addition, the chromosome ends form some kind of loop structure which prevents exonucleases (enzymes degrading DNA) from recognition of the physical ends and prevents degradation of the chromosomal DNA. Those long stretches of repeats we call telomeres. They are not just the telomeric regions but also, they are wrapped by a number of proteins sheltering the loops hiding the chromosome ends. The telomeres have usually dozens of thousands of nucleotides in length. During each cell division they shorten by about 70 – 120 nucleotides. Once the telomeres are shortened below about 4 thousand nucleotides, the cells enter senescence – they stop dividing. In contrary, cancer cells are typically immortal and can replicate without limits. It was found that cancer cells enable the genes involved in telomere synthesis and hence can extend their telomeres which is as explained above, a pre-requisite for their replication.
Nowadays we can determine length of such telomeric repeats using various laboratory methods but mostly, we determine averaged length of telomeres at all chromosome ends in all cells in the sample. Also we can determine the sequence of such regions but only when focusing on short and isolated regions of about few hundred of nucleotides. The telomeres are typically at least 15 thousands of nucleotides long, some above 100 thousands. Other methods can determine length of the shortest instances of telomeres while ignoring the longer ones. We are trying to develop a new method to study telomere length which could give us additional information about their physiological status.
Illustration: Human chromosomes (in grey) in metaphase – after duplication. The telomeres were labeled and visualized as white spots in the figure. There are both chromatids attached to each other and hence, the two white spots next to each other. Source: Wikipedia.