DNA in eukaryotes (organisms whose cells all have a nucleus, as opposed to prokaryotes, which do not have one) consists of both coding and noncoding sites. The former are much fewer than the latter. However, it is not strange at all, because a multicellular organism is an extremely complex structure, and its regulation requires a lot of data. Telomeres perform one of such regulatory functions: they determine the age of the cell. And here it is worth clarifying: this age is not the time that the cell has existed.

The telomere itself is an extremely simple structure. It is the end sections of chromosomes, which contain a repeating nucleotide pattern. In most animals it looks like TTAGGG (the letters designate the nucleotides: thymine, adenine, guanine, respectively). Such sequences, as already written above, do not code anything. Their role can be called sacrificial. With each new cell division its genetic material is copied. A whole complex of enzymes is responsible for this: they unwind, stabilize and reproduce DNA strands, as well as form primers. The central role here is played by DNA polymerase, which synthesizes a new nucleic acid chain using the existing one as a matrix.

DNA polymerase has one peculiarity: it is not able to synthesize the DNA strand from the very end. This is a kind of “not a bug, but a chip”: due to this feature of the enzyme, DNA strands are shortened by a certain length during each division. It would seem that cells obtained by this process will be genetically incomplete, because important information may be lost. But thanks to the existence of telomeres, this loss does not occur until a certain point: the DNA end sequences meekly decrease in size, allowing the genetic data that they encircle to persist.

The best succinct definition for telomeres would be “reverse counting the number of cell divisions.” Each cell can go through approximately 50 divisions: after this the telomerase protection is exhausted and this signals the beginning of apoptosis – programmed cell death. The number of 50 divisions is called the “Heiflick limit” – in honor of Leonard Heiflick, who discovered this division limit. Heiflick and his colleague Paul Moorhead set up a simple and illustrative experiment. Scientists mixed fibroblasts from men and women in equal parts; male cells had already gone through 40 cycles of division, and female cells – only ten. Purely male fibroblasts played the role of a control group.

When the cells in the control culture stopped dividing, Heiflick and Moorhead discovered that the mixed culture was no longer mixed: all the male fibroblasts in it had died, only the female cells remained. Based on this, Heiflik drew his conclusion about the life limit of human cells.