DNA Replication and the Genetic Code

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Semi-Conservative Replication

  • 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.
  • This code is known as degenerate.
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