As you know from the previous page, RNA primers are used in the lagging strand to allow for DNA polymerase III to add DNA nucleotides to the lagging strand. These RNA primers are then removed by DNA polymerase I which replaces each RNA nucleotide with DNA nucleotides and finally are joint together by the enzyme DNA ligase.
However, at the ends of chromosome, there is no existing nucleotide chain that can be elongated. Therefore, DNA polymerase cannot fill the gap with the required DNA nucleotides and results in a newly synthesized strand to be shortened. As another of round of replication starts, the lagging strand produces a shorter template, which results in shorter daughter cells. To avoid this, eukaryotic chromosomes contain a buffer of highly repetitive noncoding DNA which protects the genes near the end of the chromosome. The region of noncoding DNA is called TELOMERE. A telomere consists of short DNA sequence that is repeated multiple times. In humans, the telomere repeat is 5'-TTAGGG-3'.
The length of the telomeres can be maintained by the action of an enzyme called TELOMERASE, which adds DNA to the ends of chromosomes. The telomerase is similar to that of DNA polymerase in which is elongates the available 3' end of the top strand. Telomerase carries its own template in the form of single-stranded RNA molecule.
Telomerase adds a telomere repeat to the 3' end of the DNA using RNA as a template. Then it shifts toward the end of the chromosome and adds another and another. Over several repeats added to the top strand, it is primed and used as a template as usual. When RNA primer is remover, there will be a single-stranded region at the end of the chromosome as before.
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