DNA Replication Start to Finish

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DNA replication is an anabolic reaction (things are being put together rather than being taken apart).

  • The fact that each strand is complementary to the other is a key factor in DNA replication. This process is semi-conservative meaning that we end up with two new strands, but each is composed of half the original strand.
    • Step 1
      • Topoisomerase and Gyrase relax the supercoiled DNA
    • Step 2
      • Helicase ‘unzips’ the strand in the middle between the hydrogen bonds of the base pairs
    • Step 3
      • Primase adds RNA primers to the free bases
      • Starts on the 3′ (three prime) end of the leading strand and randomly on the lagging strand.
    • Step 4
      • DNA Polymerase III (yellow in above photo) attaches to the 3′ end of the DNA
      • The DNA is read 3′ to 5′
      • New DNA is made continually on the leading strand 5′ to 3′
      • New DNA is made in pieces called Okazaki Fragments
        • this only occurs on the 3′ end of the lagging strand’s RNA primers.
    • Step 5
      • Primase I (orange in above photo) removes RNA primers from lagging strand and replaces with the appropriate base
    • Step 6
      • Ligase fills in the holes of the lagging strand’s new leading backbone between Okazaki fragments.
  • DNA is unwound in some bacteria in two directions by the topoisomerase (title photo is a picture of such). This is possible because of their circular chromosome.
  • DNA is methylated; methylation is important in gene expression, initiation of DNA replication, protection against viruses, and the repair of DNA.
  • In most bacterial cells, the opposite strands are attracted to opposite poles of the organism, when the cell splits each daughter cell has a copy of the DNA.
  • This process is close to perfect, only 1 in 10^10 (10,000,000,000) bases are placed in error.

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