Question

Where are telomeres found on a chromosome? What is their function? What is the function of telomerase?

Answer 1

Telomeres are repeat sequences present on ends of chromosomes. The chromosome will form a duplex loop at the telomere, thereby giving a T-loop structure. Telomere has tandem repeat hexameric sequences of TTAGGG. Telomeres protect the chromosomes from degradation. Telomeres also prevent fusion chromosomes with other chromosomes during cell division. Thus, they help to maintain genomic stability. Telomeres shorten during cell division as the replication does not replicate telomere repeat at the 3’ end. Telomeres are mainly responsible for senescence.

Telomerase is the ribonucleoprotein enzyme that extends the 3’ end of telomere. The telomerase enzyme has a reverse transcriptase TERT and a telomerase RNA TR. This telomerase RNA is used as a template for elongation of DNA. Telomerase functioning also requires auxiliary components while other proteins help in maturation/degradation of telomere complex. Initially there is base pair formation between TERC and 3’ end of G rich overhand present in telomere. This occurs in the active site of the enzyme. The 3’ end of the telomere is then elongated by addition of new nucleotides. There is translocation of the telomerase to new 3’ overhang. This initiates the new process of telomere DNA synthesis. Thus, Telomerase is required to replicate the 3’ end of telomeres. The 5’ end of telomeres can be extended by DNA polymerase.