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Eukaryotic
DNA
Replication -
In eukaryotes, the process of DNA replication is the same as that of the bacterial/prokaryotic DNA replication with some minor modifications. In eukaryotes, the DNA molecules are larger than in prokaryotes and are not circular; there are also usually multiple sites for the initiation of replication.
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Thus, each eukaryotic chromosome is composed of many replicating units or repliconsstretches of DNA with a single origin of replication. In comparison, the E. coli chromosome forms only a single replication fork. In eukaryotes, these replicating forks, which are numerous all along the DNA, form "bubbles" in the DNA during replication. The replication fork forms at a specific point called autonomously replicating sequences (ARS).The ARS contains a somewhat degenerate 11-bp sequences called the origin replication element (ORE). The ORE is located adjacent to an 80-bp AT rich sequence that is easy to unwind.
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In yeast, ORE is called as DUE (DNA unwinding element). Multiple origins allow eukaryotes to replicate their larger quantities' of DNA in a relatively short time, even though eukaryotic DNA replication is considerably slowed by the presence of histone proteins associated with the DNA to form chromatin.
Eukaryotes have clamp loader complex, similar to β subunit of pol of prokaryotes, and a six-unit clamp called the proliferating cell nuclear antigen. The RNA primers are removed during Okazaki fragment completion by mechanisms similar to those in Prokaryotes. In eukaryotes, RNase enzymes remove the RNA primers in okazaki fragments; a repair polymerase fills gaps and a DNA ligase forms the final seal.
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Helicases, topoisomerase and single strand binding proteins play roles similar to that in prokaryotes. All the enzymatic processes are generally the same in prokaryotes and eukaryotes. DNA replication developed in prokaryotes, and was refined as prokaryotes evolved into eukaryotes. The completion of the replication of linear eukaryotic chromosome involves the formation of specialized structures at the tips of chromosomes.
Termination of replication in eukaryotes is different from that of prokaryotes. Eukaryotes have linear chromosome, and once the first primer is removed from the strand, there is no known way to fill in the gap, since DNA cannot be extended in the 3' -5 ' direction and there is no 3 ' end upstream as there would be in a circle. If this were actually the situation, the DNA strands would get shorter every time they replicated and genes would be lost forever.
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To avoid this condition, the cell has devised a system. The ends of chromosomes do not have genes and instead, they are composed of many repeats of short, GC rich sequences. The exact sequence of the repeat in a telomere is species-specific.
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These repeats are added to the 3' end of DNA, not by Semiconservative replication, but by an enzyme called telomerase. This enzyme has small RNA of 159-200 bp length which act as template. The telomerase adds many repeated copies of its characteristic sequence to the 3' ends of chromosome.
Priming for synthesis of the opposite strand can then occur within these telomeres. Interestingly, somatic cells lack telomerase while the germ cell retains the enzyme. Clearly, a picture of the "replication apparatus" of eukaryotic organisms is beginning to emerge, but still there are many things which need to be explored.
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