Telomeres are extensions on linear chromosome ends. They consist of a tandem array of repeated G/T rich sequences, (TTGGGG)n in Tetrahymena, (TTAGGG)n in mammals.
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The repeats are generated by the enzyme telomerase, which uses an internal RNA template and reverse transcriptase activity to extend the 3' end independently of the normal DNA template. The G/T repeat extension is partly replicated by DNA polymerase, giving C/A repeats on the complementary strand. The remainder forms a single-stranded overhang extending from the 3' end, about 100 nt in mammals. The single stranded overhang can fold into a characteristic G-quadruplex . |
Yeast genes EST1-3 (ever shorter telomeres) are essential for telomerase activity, Est2p being the reverse transcriptase component. TLC1 encodes the RNA used as an internal template by Est2p. Est1p, Est3p and Cdc13p hold the telomerase in place on the 3'-overhang. Telomerase is regulated to control the length of repetitive sequence DNA added to the chromosome end.
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The telomere binding protein Rap1p binds in multiple copies to the double stranded region of the telomere, and forms part of the telomere length sensing mechanism. The chromosomal end of the Rap1p cluster carries the silencing factorsfactors Sir 2/3/4p, whereas the distal end carries proteins Rif1/Rif2. The end of the double stranded region is marked by a pair of proteins Ku70/Ku80, which link to Cdc13 on the single-stranded overhang. |
The telomere appears to exist in "closed" and "open" states, with only the open state able to recruit the Cdc13/Est2p telomerase complex. The "open" state is induced by temperature rise and by DNA replication, and the primase component of DNA polymerase appears to be part of the functional telomerase complex. Regulation involves a mechanism to "count" the number of Rif1/Rif2 (Rap1 interacting factor), and loss of either component results in abnormally long telomere T/G repeats. Rif inhibits Tel1p, a protein kinase that maintains telomerase activity ().
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In multicellular eukaryotes, somatic cells do not exhibit telomerase activity, and shorten their telomeres with increasing passage number. In these cells, the 3' single-stranded overhang loops back and base pairs to the complementary A/C in a structure called the t-loop. Sequestering the single stranded overhang seems to exclude the telomerase. Cells become senescent after a critical number of generations, the so-called Hayflick limit, when the telomeres become shortened below the minimum for cell survival. |