Definition
Telomeres are part of every DNA. They are located at the ends of chromosomes and in no case code for genes. Unlike the rest of the chromosome, telomeres do not have double-stranded DNA.
They are present as a single strand. In contrast to the rest of the DNA, they also do not exhibit a high variability in the sequence of bases, but consist of always repeating base sequences. This is important for fulfilling their function. Due to the repeating sequences, the telomeres of the chromosome curl up in such a way that they do not allow an enzyme to attack the chromosome end. With each cell cycle, a shortening of the telomeres occurs due to cell proliferation.
Anatomical subtleties of the telomeres
Each chromosome consists of two DNA strands that run in different directions, so-called antiparallel direction. On each side of a DNA strand there is a telomere at the end. Thus, depending on the cell cycle, there are either two or four telomeres per chromosome.
In total, 46 chromosomes have either 96 or 192 telomeres per cell. If the DNA strands would simply end blind, this would allow different proteins to attack the DNA. In contrast to a large part of the DNA, the telomeres do not carry any information that is important for the function of the cells.
Instead, the telomeres contain a base sequence that is always repeated. This sequence consists of six bases and has three guanine, one adenosine and two thymine. This repetitive sequence leads to the bases of a telomere forming base pairs with each other. This leads to a folding of the ends and the telomeres are no longer present as a single strand but as a ball. For cell propagation during replication, however, it is necessary for the folded telomeres to unfold.
What functions do the telomeres have?
Telomeres essentially have two tasks. First, they are important during the normal cell cycle or during the G0 phase. Within the cells there are enzymes that continuously break down the DNA.
On the one hand, this serves to ward off intruders, but on the other hand it is also undesirable. For the normal DNA of the cell nucleus this is an enormous problem and can lead to undesired events. To prevent this from happening, there is an overhang, the telomere, on one side at the end of each DNA single strand.
Because the telomere consists of base sequences that do not code for proteins, this alone is a protection for the coding DNA, since it is degraded first. Furthermore, the folding of the telomeres makes it difficult for the DNA-degrading enzymes to find a point where they can begin their degradation by rolling up the free DNA end. In addition, folded telomeres provide special protein binding sites.
These proteins are relatively large, surrounding and protecting the DNA end. On the other hand, the telomeres are important during replication, i.e. during the doubling of DNA. The enzymes responsible cannot structurally start doubling the DNA at the end of a DNA strand.
This results in a loss of base pairs with each cycle and the chromosomes shorten continuously. To prevent this from leading to an early loss of essential DNA sections, the telomeres are located at the ends. They carry no genetically important information and can survive a loss of some bases without any problems. This topic could also be interesting for you: Tasks of the cell nucleus
