At the end of each Chromatid there comes a region where you would find repetitive sequences of nucleotides. What chromatid does is protect the end of chromosome from detoriating, while being fused with neighbor chromosomes.
Telomere is a name derived from Greek, Telos meaning end, and meros meaning part. Sequence in telomere in vertebrates is TTAGGG.
During a cell division, these telomere acts as disposable cushions at the end of each chromosome that protects the genes on those chromosomes from being truncated, instead after every cell division, telomere itself gets truncated or shortened. Over the time, after some repetitive cell divisions, these telomeres becomes shorter and are further restocked by an enzyme called Telomerase Reverse Transcriptase.
The DNA replication does not begin at the end of the strand, be it any end. Instead, starts at the center there are found two strands to be replicated, one is leading and the other is lagging. Because of this End replication problem, telomeres shorten in every cell division process. Telomeres shorten in part because of the ”end replication problem” that is exhibited during DNA replication in.
The problem comes when going in the direction of a lagging strand, unlike the leading strand in which case DNA can make a complementary DNA strand without any difficulty because of its leading direction during a cell division.
Ahead of the initiative point on a Chromosome, there are short sequences of RNA found which actually the acting primers, attached to the lagging strand to counteract such a shortcoming of lagging strand.
The DNA polymerase can start replication at that point and go to the end of the initiation site, resulting in the formation of Okazaki fragments, and when more RNA has attached itself to the DNA, it continues in the formation of a new one.
DNA polymerase, RNA nuclease and DNA ligase come along and fill in the gaps between the okazaki fragments and help RNA covert into a new DNA. But in order for this to happen, there has to be another DNA strand in front of the RNA primer (neighbor Chromosome). This does not happen in the place where RNA primer is attached, instead it happens at the site of a lagging strand. Followed by an ultimate destruction of the RNA by enzymes. These enzymes degrade any RNA present on the DNA strand, and in this entire process a part of telomere is lost.