.
Shown here are the reactions involved in synthesizing the repeating G-rich sequences that form the ends of the chromosomes (telomeres) of diverse eucaryotic organisms. The 3′ end of the parental DNA strand is extended by RNA-templated DNA synthesis; this allows the incomplete daughter DNA strand that is paired with it to be extended in its 5′ direction. This incomplete, lagging strand is presumed to be completed by DNA polymerase α, which carries a DNA primase as one of its subunits (see Figure 5-28
). The telomere sequence illustrated is that of the ciliate Tetrahymena, in which these reactions were first discovered. The telomere repeats are GGGTTG in the ciliate Tetrahymena, GGGTTA in humans, and G1–3A in the yeast S. cerevisiae.
). The telomere sequence illustrated is that of the ciliate Tetrahymena, in which these reactions were first discovered. The telomere repeats are GGGTTG in the ciliate Tetrahymena, GGGTTA in humans, and G1–3A in the yeast S. cerevisiae.