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Highly Repetitive Sequences

Highly Repetitive Sequences - polymerase cannot begin at...

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Highly Repetitive Sequences 10-25% of eukaryotic DNA consists of sequences of 5 to 10 nucleotides repeated 100,000 to 1,000,000 times. This type of DNA probably does not code for proteins. A large proportion of this type of DNA is found at the tips of the chromosomes and at the centromere. Telomeres DNA polymerase is not capable of initiating the synthesis of DNA; it can only elongate a strand that has already been started. Normally, an RNA primer functions to begin the process, allowing DNA polymerase to attach and finish synthesizing the strand. A DNA polymerase molecule will then replace the RNA nucleotides with DNA nucleotides. This is not a problem for primers that are not located on the 3' end of a DNA strand because DNA polymerase extends the DNA strand that is already there. RNA Primers located on the 5' end of a DNA strand cannot be replaced because DNA
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Unformatted text preview: polymerase cannot begin at the end of a strand. It can only add to an existing strand. The new DNA strand is shorter than the template strand. As a result of the inability of DNA polymerase to initiate synthesis, the DNA molecule becomes shorter with each cell division. Human chromosomes have the sequence "TTAGGG" repeated 100 to 1500 times at each end of the DNA strand. These repetitive sequences do not contain any genetic information. Each time a cell divides, 50 to 500 of these repeats are lost, making the DNA shorter. Short telomeres may prevent a cell from dividing. The length of telomeres, therefore, may limit the number of times a cell can divide. Telomerase is an enzyme that restores the length of telomeres. This enzyme is normally not found in somatic (body) cells but is found in germ cells....
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Highly Repetitive Sequences - polymerase cannot begin at...

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