chapter 14.3 AQA Biology

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Translating the mRNA into a sequence of amino acid

Although the basic structure of tRNA is always the same, the anticodon loop varies. There are at least 60 variants, each of which corresponds to a codon of three bases on the mRNA. At the end of the tRNA molecule, there is a point of attachment for an amino acid. Therefor each amino acid has its own tRNA molecule with its own anticodon of bases.

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Synthesising the polypeptide

  • mRNA leaves the nucleus through nuclear pores
  • a ribosome becomes attached to the starting codon (AUG) on one end of the mRNA molecule
  • the tRNA molecule with the complimentary anticodon sequence (UAC) moves rto the ribosome and pairs up with the sequence on the mRNA. This tRNA carries an amino acid.
  • a tRNA molecule with a complimentary anticodon (UCG) pairs with the next codon on the mRNA. This tRNA carries another amino acid.
  • the ribosome moves along the mRNA bringing together two tRNA molecules at any one time each pairing up with the corresponding codons on the mRNA
  • using an enzyme and ATP the two amino acids on the tRNA are joined by a peptide bond
  • the ribosome moves to the 3rd codon in the sequence on the mRNA, thereby linking a 3rd amino acid. as this happens the first tRNA is released from its amino acid and is free to collect another one from the pool. 
  • the process continues this way until a complete chain is built up and the ribosome reaches a stop codon. at this point the ribosome, mRNA and the last tRNA molecule all separate that the chain is complete
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a summary of synthesis

  • DNA triplets determine the codons on mRNA
  • these codons determine the order in which the tRNA molecules line up
  • they, in turn determine the sequence of amino acids in the polypeptide
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assembling a protein

sometimes a single polypeptide chain is a functional protein. Often a number of polypeptides are linked together to give a functional protein (quarternary structure). To acheive this, the polypeptide is coiled or folded (secondary structure), which is then folded again into the tertiary structure, and different polypeptide chains and non protein groups are linked to form the quarternay structure.

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