Double helix has to unwind and separate into two separate strands.
Hydrogen bonds between complementary bases must be broken.
Free DNA nucleotides then pair up with their complementary bases, which have been exposed as the strands separate.
Hydrogen bonds formed.
New nucleotides join to their adjacent nucleotides with phosphodiester bonds.
2 new molecules of DNA are formed.
Each has one old strand of DNA and one new strand of DNA.
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Roles of Enzymes in Replication
Helix is unwound and separated by DNA helicase, travelling along the sugar phosphate backbone, catalysing the reactions that break the hydrogen bonds between complementary base pairs following a replication fork.
DNA polymerase joins the nucleotides to the strands of DNA, catalysing the formation of phosphodiester bonds between the nucleotides.
DNA ligase joins the Okazaki fragment on the 3' to 5' lagging strand.
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The Whole Process
DNA helicase causes the strands to separate and unwind.
DNA helicase completes the separation of the strand. Meanwhile, free nucleotides that have been activated are attracted to their complementary base pairs.
Once the activated nucleotides are lined up, the leading 5' to 3' strand is joined together by DNA polymerase.
The lagging strand is built up in Okazaki fragments in a 3' to 5' direction by DNA ligase.
Finally all the nucleotides are joined to form a complete polynucleotide chain. Two identical molecules of DNA are formed.
Semi-conservative replication.
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Replication Errors
An incorrect sequence may occur in the newly-copied strand.
These errors are random and known as mutations.
They can lead to changes in the sequence of bases.
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The Triplet Code
The instructions that DNA carries are contained in the sequence of bases along the chain of nucleotides that make up the two strands of DNA.
The code in these bases is known as the Triplet Code.
It is a sequence of three bases (codon).
Each codon codes for an amino acid.
A section of DNA that contains a complete sequence of codons is called a gene.
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Degenerate Code
4 bases means there are 64 different base triplets or codons possible (4x4x4).
There is one start codon called methionine. This always comes at the beginning of the gene and it signals the start of a sequence that codes for a protein.
There are three stop codons that do not code for any amino acids - they simply just signal the end of the sequence.
Having a single start codon ensures the codons are read in frame. This ensures the DNA is non-overlapping.
There are only 20 regularly occuring amino acids so many amino acids can be coded for by more than one codon.
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