- 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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- 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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- 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.
- This code is known as degenerate.
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