English: Telomeres at the ends of chromosomes contain many repeats of a short DNA sequence. (a) When the last Okazaki fragment on the lagging strand is completed, a short region of DNA remains unduplicated at the chromosome tip. (b) To avoid chromosome shortening, the enzyme telomerase extends the template strand by synthesizing additional copies of the telomere repeat sequence. Telomerase does not need a DNA template to perform this reaction because it contains its own template (made of RNA). (c) The extended DNA template is used by the DNA polymerase machinery to synthesize a new Okazaki fragment. (d) Primers are removed, and the internal primer is replaced with DNA as usual. The last primer cannot be replaced with DNA because not enough template strand is present. A short stretch of single-stranded DNA therefore remains at the end of the chromosome. In this example, the length of the chromosome has increased slightly during DNA synthesis; normally, telomeres are maintained at a constant length by complex regulatory mechanisms. In some cells, telomerase is absent and telomeres gradually shorten with each cell division.^[1]