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. Author manuscript; available in PMC: 2023 Feb 14.
Published in final edited form as: Ageing Res Rev. 2016 May 20;33:52–66. doi: 10.1016/j.arr.2016.05.009

Fig. 2.

Fig. 2.

Telomere structure and mechanisms of telomere maintenance and replication are illustrated. Proteins boxed in red are defective in primary telomeropathies, and proteins boxed in blue are defective in secondary telomeropathies. Telomeres end in a looping mechanism (T-loop) whereby the G-rich single strand overhang is proposed to fold back and insert into the duplex TTAGGG repeats forming a D-loop. The shelterin complex consists of six proteins that coat the telomeres, with TRF1 and TRF2 binding duplex repeats and POT1 binding single stranded TTAGGG repeats. RTEL1 is proposed disrupt the T-loop during telomeres replication, and acts with RECQ helicases to resolve secondary structures, such as G-quadruplexes, that impede replication. CST and TRF1 are also involved in promoting proper replication. TPP1, TCAB1, and ATM kinase are involved in recruiting telomerase to the telomeres. The catalytic component of telomerase (TERT) adds TTAGGG repeats to the overhang using the integral RNA template (TR). Several proteins are involved in telomerase assembly (TCAB1, Dyskerin, Nop10, NHP2, and GAR1). The CST complex acts in C-strand fill-in following telomerase extension, and Apollo degrades the C-rich strand to generate a single strand overhang.