Terminal Repeats (Molecular Biology)

Telomeres consist of repeated DNA sequences, and these terminal repeats are synthesized by telomerase. A number of such distinct DNA sequences (Fig. 1) are utilized in fungi, protozoa, plants and animals. The existence of terminal repeats is important for replicating DNA at the chromosome ends and for assembling specialized nucleoprotein structures. General features of telomeric repeats include strand asymmetry in base composition in which the G-rich strand is oriented 5′ to 3′ toward the end of the chromosome. The G-rich strand can have a single-stranded terminus for at least some of the cell cycle, which in turn may facilitate the formation of unusual DNA structures. In humans, the size of the repeat array varies from cell to cell or from tissue to tissue, dependent on telomerase activity. Heterogeneity in the length of the terminal repeats is caused by variation in telomerase activity and by loss of terminal sequences due to incomplete replication.

Figure 1. Terminal repeat sequences at the telomeres of different organisms.