Protein Synthesis

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Genetic Code


It is a Triplet Code: A sequence of 3 nucelotide bases codes for one amino acid 

It Redundant: There are 64 possible codes but only 20 amino acids atr found in protiens. More than one triplet can code for an amino acid

It is Puntuated: There are 3 triplet codes that do not code for amino acids. In mRNA, they are called 'stop' codons and mark the end of a portion to be translated

It is Universal: In all know organisms, the same triplet codes for the same amino acid

It is non-overlapping: Each triplet code is read seperatly  

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Transcription is the production of mRNA from a DNA templete


  • DNA helicase breaks the hydrogen bonds between the base pairs, unwinding the double helix, exposing unpaired bases on the templete strand
  • RNA polymerase links to the template starnd of DNA, inserting mRNA nucleotides, according to the base pair rules and forms bonds between them down the sugar phosphate back bone
  • Beyond the end of the gene there is a stop sequence, where RNA polymerase leaves DNA
  • A copy of the base sequence of the templete strand of DNA has been made and this is known as pre mRNA 
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Translation is the production of a polypeptide from the mRNA stand in ribosomes using tRNA to bring the correct amino acid in the sequence


  • A molecule of mRNA binds with a ribosome 
  • 2 codons are exposed 
  • A tRNA with the complimentary anticodon and specific amino acid is activated using ATP and attracted to the complementary codon by hydrogen bonds
  • A second tRNA carrying a different amino acid binds to the next exposed codon
  • A peptide bond is formed between the 2 amino acids
  • The first tRNA molecule releases its amnio acid and leaves the ribosome and its attachment site vacant. It returns to the cytoplasm to bind with another copy of its specific amino acid
  • The process is repeated until the ribosomes reaches a 'stop codon' and the primary structure of the polypeptdie is formed
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