Extension of the ends of linear chromosomes by telomerase. Telomeres at the ends of eukaryotic chromosomes are composed of a short repeating DNA sequence. Shown here is the repeating 5′-TTGGGG-3′ sequence of Tetrahymena. Telomerase extends the 3′ single-stranded DNA end with dNTPs, using its internal RNA molecule as template. The extended 3′ single strand of DNA is filled in by RNA priming and DNA synthesis. Removal of the RNA primer for this fill-in reaction still leaves a short 3′ single-stranded DNA overhang; this end is sequestered by telomere DNA–binding proteins. These proteins protect the chromosome ends from becoming substrates for the cell’s double-strand break repair machinery.