Translation (AQA A Level Biology): Revision Note

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, which 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

The process of translation

• This stage of protein synthesis occurs in the cytoplasm of the cell

• After a transcribed mRNA molecule leaves the nucleus, it attaches to a ribosome

• Within the cytoplasm, there are free molecules of tRNA (transfer RNA)

• tRNA has an anticodon (a triplet of unpaired bases) at one end and a site for a specific amino acid at the other

  • There are at least 20 types, each with a unique anticodon and corresponding amino acid

• The tRNA molecules bind with their specific amino acids (found within the cytoplasm) and bring them to the mRNA molecule on the ribosome

• The 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; the amino acid chain coded for by the mRNA molecule is complete

• This amino acid chain then forms the final polypeptide

Steps of translation