DNA160-page14 - telomeres. It's the telomere region that...

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DNA: Structure and Replication - 14 Before we leave the subject of DNA replication, we have one more issue: the end of the DNA molecule and telomeres. Telomeres We've just learned that DNA is synthesized in the 5' ± 3" direction from the 3' ± 5" template, initiated with the RNA primer region. When there is an available 3' open binding site, as there is with the Okazaki fragments, the RNA primer segment is removed and replaced by DNA nucleotides added to the 3' end. Although the initial primer section can be removed from the leading strand, the end is the 5" phosphate so no DNA nucleotides can be added. DNA polymerase cannot "finish" the 5' ends of the "daughter" DNA strands. After each replication, the DNA molecule gets shorter. Why don't we lose valuable genetic information with each DNA replication? Telomeres! The ends of eukaryotic chromosomes have special nucleotide sequences called
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Unformatted text preview: telomeres. It's the telomere region that gets shorter with each DNA replication. Most cells can divide about 30 times before they run out of telomeres and can no longer replicate DNA without losing codable genes. Telomere Ends of Chromosomes Mouse Telomeres (Orange Tips) Some tissues have telomerases, special enzymes that catalyze new telomere code to chromosomes. In humans, telomerases are normally found only in tissues that produce gametes, but that ensures that gamete chromosomes have long telomeres. Shortening telomeres in tissues may be a factor in aging and lifespan. Regrettably telomerases seem to be abundant in some cancer cells, so that rapid reproduction does not result in shortening of telomeres and cell death from lack of needed genes....
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