In addition to telomerase-mediated elongation and C-strand synthesis, other DNA processing steps are likely involved in telomere maintenance. In humans, nucleolytic activity is proposed to be involved in generating the G-rich 3' single strand overhang. In addition, differences in the structure of the overhang at telomeres that have undergone leading vs. lagging strand replication suggest that DNA processing may be different at these telomeres (Chai et al. 2006).
Electron microscopy studies of purified human telomeric DNA have provided evidence for telomeric loops, or t-loops (Griffith et al. 1999). t-loops are proposed to result from invasion of the 3' G-rich single strand overhang into the double stranded portion of the telomeric TTAGGG repeat tract. The strand displaced by invasion forms a structure called a D loop. The function of the t-loop is presumed to be the protection of the 3' telomeric end. In vitro, the double strand telomeric DNA binding protein TRF2 can increase the frequency of t-loop formation. The prevalence of the t-loops in vivo is not known.
Many proteins associate with telomeric DNA. One complex that binds telomeres is called shelterin. Shelterin is a six-protein complex composed of TRF1 and TRF2, which can bind double-stranded telomeric DNA, POT1, which can bind single-stranded telomeric DNA, and three other factors, RAP1, TIN2, and TPP1 (reviewed in de Lange 2006 "Telomeres"). Human telomeric DNA is also bound by nucleosomes (Makarov et al. 1993; Nikitina and Woodcock 2004). A number of other proteins, including some that play roles in the DNA damage response, can be found at telomeres (Zhu et al. 2000; Verdun et al. 2005).
Studies in yeast and humans indicate that the association of many proteins with telomeres is regulated through the cell cycle (Smith et al. 1993; Zhu et al. 2000; Taggart et al. 2002; Fisher et al. 2004; Takata et al. 2004; Takata et al. 2005; Verdun et al. 2005). For instance, TRF1, MRE11, POT1, ATM, and NBS1 display cell cycle regulated chromatin immunoprecipitation of telomeric DNA (Zhu et al. 2000; Verdun et al. 2005), and cytologically observable hTERT and hTERC localize to a subset of telomeres only in S-phase (Jady et al. 2006; Tomlinson et al. 2006). These data indicate that telomeres are dynamically remodeled through the cell cycle.