Figure 2.

T-loop formation as a factor for telomere length regulation. The 3′ overhang invades the double-stranded telomeric DNA and, by base-pairing with the C-strand, displaces the G-strand, which causes the end of the telomere to be tucked in. Double-stranded DNA-binding proteins of shelterin, TRF1 and TRF2, mediate and fold DNA to the t-loop. The amount of shelterin that is bound to telomeres is dependent on the t-loop length. (a) A short t-loop with less abundant shelterin has a lower probability that shelterin covers and hides the 3′ overhang of telomeric DNA. Telomerase can extend telomeric DNA. (b) The long t-loop is occupied by more shelterin complexes, so the probability that the 3′ overhang is covered and hidden is higher. Telomerase access to the 3′ end single-stranded DNA is blocked and no DNA extension occurs.