Dahyun week14

telomere
A telomere is a region of repetitive nucleotide sequences associated with specialized proteins at the ends of linear chromosomes. Telomeres are a widespread genetic feature most commonly found in eukaryotes. In most, if not all species possessing them, they protect the terminal regions of chromosomal DNA from progressive degradation and ensure the integrity of linear chromosomes by preventing DNA repair systems from mistaking the very ends of the DNA strand for a double-strand break.
 

Telomeres are necessary for cell division, and they shorten with each division. When telomeres become too short, further cell division becomes impossible. Consequently, tissues age, and aging accelerates. Telomeres can be regenerated by an enzyme called telomerase. Telomerase is a ribonucleoprotein enzyme that extends the ends of DNA by adding repetitive nucleotide sequences. It "replenishes" the telomere "caps" without requiring ATP. In most multicellular eukaryotic organisms, telomerase is active only in certain types of stem cells, such as germ cells, embryonic stem cells, and some white blood cells.

The reason telomeres are necessary for cell division is due to the limitations of the DNA replication process. DNA must be replicated during cell division, a process carried out by DNA polymerase. This enzyme can only add new nucleotides in the 5' to 3' direction. While the leading strand is continuously replicated, the lagging strand is replicated in fragments (Okazaki fragments). At the end of replication, the RNA primer at the very end of the lagging strand is removed and must be replaced with DNA. However, at this last portion, there is no 3' end for the DNA polymerase to add new DNA nucleotides. Thus, after the primer is removed, there is no way to completely fill the gap with DNA, leading to a slight loss of DNA at the molecule's end after replication is complete.

Telomeres mitigate this issue. They are non-coding, repetitive DNA sequences located at the ends of chromosomes, containing non-essential genetic information. Thus, their loss does not directly affect gene function. However, as telomeres shorten with each cell division, and once they become too short, the cell can no longer divide, leading to cellular aging or death. Therefore, the length of telomeres is a crucial determinant of a cell's division capability and lifespan.
ref: https://www.youtube.com/watch?v=U0fRAr-ZHCo