OCR A Level Biology

Revision Notes

6.1.5 Gene Control: Post-Transcriptional Modification

Post-Transcriptional Modification

  • Within eukaryotic genes, there are both coding and non-coding sequences of DNA
    • The coding sequences are called exons and these are the sequences that will eventually be translated into the amino acids that will form the final polypeptide
    • The non-coding sequences are called introns and are not translated (they do not code for any amino acids)
  • When transcription of a gene occurs, both the exons and introns are transcribed
  • This means the messenger RNA (mRNA) molecule formed also contains exons and introns
    • This RNA molecule is often referred to as primary mRNA or pre-mRNA
  • As the introns are not to be translated, they must be removed from the pre-mRNA molecule
  • The exons are then all fused together to form a continuous mRNA molecule called mature mRNA that is ready to be translated
  • This process is sometimes called ‘splicing’ and is part of the process of post-transcriptional modification (referring to the modification of the RNA molecule after transcription but before translation occurs)
  • Splicing ensures that only the coding sections of mRNA are used to form proteins by translation (if any introns were included in the mature mRNA, the resulting protein would not be formed properly and may not function as it should)

Control at the post-translational level

  • After polypeptides are formed by translation, they undergo modifications in the Golgi apparatus or in the cytosol
  • Some polypeptides may then require activation by cyclic AMP (also known as cAMP)
    • cAMP is derived from ATP and is formed by the action of the enzyme adenyl cyclase
  • One important role carried out by cAMP is the activation of protein kinases
    • In eukaryotic cells, cAMP activates protein kinase A (also known as PKA)
    • PKA is an inactive precursor enzyme
    • Once it is activated, it can activate other proteins (e.g. other enzymes)
  • For example, when muscle cells require energy, an enzyme called glycogen phosphorylase releases glucose from glycogen
  • This enzyme is activated by cAMP, which changes the shape of the enzyme to expose its active site
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