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DNA Replication . When?. S (synthesis) phase of cell cycle Creates copy of DNA and two copies are held together by centromere. Thousands of times per second throughout the body. Where? . Always in the nucleus of eukaryotic cells. All cells that are not in G0 phase (no longer dividing).

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When?

  • S (synthesis) phase of cell cycle

  • Creates copy of DNA and two copies are held together by centromere.

  • Thousands of times per second throughout the body.


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Where?

  • Always in the nucleus of eukaryotic cells.

  • All cells that are not in G0 phase (no longer dividing).


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Why?

  • When cells divide through meiosis or mitosis, new cells need their own DNA.


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How?

  • Two different mechanisms: lagging strand and leading strand, but always in the 5’ to 3’ direction.

  • Eukaryotic chromosomes average about 150 million nucleotides, so multiple replication forks are needed to finish the job.

  • Semi-conservative


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Key enzymes

  • Topoisomerases unknot the DNA.

  • Helicase unzips the DNA

  • RNA primase adds RNA primer

  • DNA polymerase adds nucleotides, repairs mismatch pairs, and removes the RNA primer

  • DNA ligase seals the okazaki fragments together by creating phosphodiester bonds.


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Proofreading

  • 1/1,000,000,000 nucleotides end up being incorrectly copied, however the original number is closer to 1/10,000.

  • During replication, DNA Polymerase finds mismatched pairs and fixes them.

  • There are many other proteins that also proofread the mistakes.


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Telomeres

  • The 5’ ends of daughter strands can never be fully completed due to the 5’ to 3’ direction.

  • Telomeres are DNA sections at the 5’ end of a chromosome that do not carry genes.

  • Telomerase is an enzyme with RNA that adds to the telomeres as the wear down through many rounds of replication.

  • Telomerase is only found in gamete producing cells as newborns need long telomeres.

  • Limitations on the life span of cells is based on the length of the telomeres.


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Cancer

  • Cancer cells divide very quickly and often, so telomerase activity is need to keep the telomeres from getting too small.

  • Telomerase activity is found in about 90% of all cancers.


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