AQA A Level Biology复习笔记4.2.5 Translation



  • A gene is a sequence of nucleotide bases in a DNA molecule that codes for the production of a specific sequence of amino acids, that in turn make up a specific polypeptide (protein)
  • This process of protein synthesis occurs in two stages:
    • Transcription – DNA is transcribed and an mRNA molecule is produced
    • Translation – mRNA (messenger RNA) is translated and an amino acid sequence is produced



  • This stage of protein synthesis occurs in the cytoplasm of the cell
  • After leaving the nucleus, the mRNA molecule attaches to a ribosome
  • In the cytoplasm, there are free molecules of tRNA (transfer RNA)
  • These tRNA molecules have a triplet of unpaired bases at one end (known as the anticodon) and a region where a specific amino acid can attach at the other
  • There are at least 20 different tRNA molecules, each with a specific anticodon and specific amino acid binding site
  • The tRNA molecules bind with their specific amino acids (also in the cytoplasm) and bring them to the mRNA molecule on the ribosome
  • The triplet of bases (anticodon) on each tRNA molecule pairs with a complementary triplet (codon) on the mRNA molecule
  • Two tRNA molecules fit onto the ribosome at any one time, bringing the amino acid they are each carrying side by side
  • A peptide bond is then formed between the two amino acids
    • The formation of a peptide bond between amino acids requires energy, in the form of ATP
    • The ATP needed for translation is provided by the mitochondria within the cell


  • This process continues until a ‘stop’ codon on the mRNA molecule is reached – this acts as a signal for translation to stop and at this point the amino acid chain coded for by the mRNA molecule is complete
  • This amino acid chain then forms the final polypeptide


The translation stage of protein synthesis – tRNA molecules bind with their specific amino acids





Process-of-Translation-2The translation stage of protein synthesis – an amino acid chain is formed


Exam Tip

Make sure you learn both stages of protein synthesis fully. Don’t forget – transcription occurs in the nucleus but translation occurs in the cytoplasm! Be careful – DNA polymerase is the enzyme involved in DNA replication; RNA polymerase is the enzyme involved in transcription – don’t get these confused.