If the repeat sequence of telomere changes from 3’-cccaau-5’ to 3’-ucaggacg-5’ due to any mutation in TERC gene then the telomerase enzyme will be unable to attach to the 3' overhung of the telomere sequence and cannot elongate new DNA strand [telomeric part]. As a result, the telomeres of chromosomes cannot be elongated and the entire chromosome or the cell will be lost after a few rounds of division.
Telomere: They are short repeating sequences found at the end of the chromosome whose function is to save the chromosomes from degradation. But after each round of replication, the length of the telomeres becomes shorter than the previous and after a certain number of cell divisions, the telomeres become so short that they trigger the cell to stop dividing or to self-destruct (undergo apoptosis).
So, to prevent the shortening of telomeres, the TERC gene codes a telomerase enzyme which is a type of reverse transcriptase made up of two components: an RNA part that is complementary to the repeated telomeric sequence and a protein part that can add the nucleotides to the 3' end of the telomere.
After every cell division, the telomerase enzyme gets attached to the 3' end of the shortened telomere end and synthesizes the complementary DNA to the RNA part of the enzyme until the length of the telomere is restored again.
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