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Biology AS CIE

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  1. 1-cell-structure
    10 主题
  2. 2-biological-molecules
    19 主题
  3. 3-enzymes
    13 主题
  4. 4-cell-membranes-and-transport
    16 主题
  5. 5-the-mitotic-cell-cycle
    8 主题
  6. 6-nucleic-acids-and-protein-synthesis
    9 主题
  7. 7-transport-in-plants
    11 主题
  8. 8-transport-in-mammals
    16 主题
  9. 9-gas-exchange
    6 主题
  10. 10-infectious-diseases
    6 主题
  11. 11-immunity
    10 主题
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Exam code:9700

The universal genetic code

  • A gene is a sequence of nucleotide bases in a DNA molecule that codes for the production of a specific sequence of amino acids

    • These amino acids in turn make up a specific polypeptide (protein)

  • The DNA nucleotide base code found within a gene is a three-letter, or triplet, code

  • Each sequence of three bases (in other words each triplet of bases) codes for one amino acid 

    • There are 20 different amino acids that cells use to make up different proteins

  • For example:

    • CAG codes for the amino acid valine

    • TTC codes for the amino acid lysine

    • GAC codes for the amino acid leucine

    • CCG codes for the amino acid glycine

  • Some of these triplets of bases code for start and stop signals

    • These signals tell the cell where individual genes start and stop

    • This ensures the cell:

      • Reads the DNA correctly (the code is non-overlapping)

      • Can produce the correct sequences of amino acids (and therefore the correct protein molecules) that it requires to function properly

  • The genetic code is universal, meaning that almost every organism uses the same code

  • This means that the same triplets code for the same amino acids in all living things

    • This means that genetic information is transferable between species

Diagram of DNA transcription showing a DNA molecule with labelled triplet codes for valine and methionine, depicting 5' to 3' mRNA synthesis.
A DNA molecule with the triplet codes for two amino acids (methionine and valine)

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