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DNA is a molecule made up of two strands
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twisted around each other in a double helix shape.
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Each strand is made up a sequence of four chemical bases
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represented by the letters A, C, G and T.
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The two strands are complimentary.
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This means that wherever there's a T in one strand
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there will be in an A in the opposite strand,
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and wherever there's a C
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there will be a G in the other strand.
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Each strand has a 5' end and a 3' end.
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The two strands run in opposite directions.
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This determines how each strand of DNA is replicated.
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The first step in DNA replication is to separate the two strands.
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This unzipping is done by an enzyme called helicase
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and results in the formation of a replication fork.
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The separated strands each provide a template for creating a new strand of DNA.
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An enzyme called primase starts the process.
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This enzyme makes a small piece of RNA
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called a primer.
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This marks the starting point for the construction of the new strand of DNA.
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An enzyme called DNA polymerase binds to the primer
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and will make the new strand of DNA.
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DNA polymerase can only add DNA bases in one direction,
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from the 5' end to the 3' end.
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One of the new strands of DNA, the leading strand,
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is made continuously,
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the DNA polymerase adding bases one by one in the 5' to 3' direction.
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The other strand, the lagging strand, cannot be made in this continuous way
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because it runs in the opposite direction
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the DNA polymerase can therefore only make this strand in a series of small chunks
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called Okazaki fragments.
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Each fragment is started with an RNA primer.
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DNA polymerase then adds a short row of DNA bases
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in the 5' to 3' direction.
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The next primer is then added further down the lagging strand.
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Another Okazaki fragment is then made and the process is repeated again.
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Once the new DNA has been made
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the enzyme exonuclease removes all the RNA primers from both strands of DNA.
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Another DNA polymerase enzyme then fills in the gaps that are left behind
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with DNA.
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Finally the enzyme DNA ligase seals up the fragments of DNA
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in both strands to form a continuous double strand.
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DNA replication is described as semi- conservative
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because each DNA molecule is made up of one old, conserved strand of DNA
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and one new one.
