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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.
- Breaking Down What Makes Life Up (Intro to DNA)
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