7.2.4 Post-transcriptional Modification

• In all kingdoms of life, gene expression can be regulated after an mRNA transcript has been produced
• Post-transcriptional modification of mRNA
  • Helps prevent degradation
    • mRNA is single stranded and therefore, inherently unstable
  • Increases the efficiency of protein synthesis
  • In eukaryotes, expands the complexity of the proteome
• Prokaryotic mRNA does not require any significant post-transcriptional modification as translation can occur immediately which prevents degradation of the mRNA
• In eukaryotes, transcription and translation occur in separate parts of the cell, allowing for significant post-transcriptional modification to occur
• In eukaryotes, the immediate product of an mRNA transcript is called pre-mRNA which needs to be modified to form mature mRNA
• Three post-transcriptional events must occur
  1. A methylated cap is added to the 5' end to protect against degradation by exonucleases
  2. A poly-A tail (long chain of adenine nucleotides) is added to the 3' end for further protection and to help the transcript exit the nucleus
  3. Non-coding sequences are removed through mRNA splicing

• Eukaryotic genes contain both coding and non-coding sequences of DNA
  • Coding sequences are called exons
  • Non-coding sequences are called introns
• During transcription the whole gene is transcribed including all introns and exons
  • Introns are not translated as they do not code for amino acids and need to be removed
• Before the pre-mRNA exits the nucleus, splicing occurs, during which
  • Introns (non-coding sections) are removed
  • Exons (coding sections) are joined together
  • The resulting mature mRNA molecule contains only exons and exits the nucleus before joining a ribosome for translation

The RNA molecule (known as pre-mRNA) produced from the transcription of a gene contains introns that must be removed (to form mature mRNA) before translation can occur

Alternative splicing

• The exons (coding regions) of genes can be spliced in many different ways to produce different mature mRNA molecules through alternative splicing
• A particular exon may or may not be incorporated into the final mature mRNA
• Polypeptides translated from alternatively spliced mRNAs may differ in their amino acid sequence, structure and function
• This means that a single eukaryotic gene can code for multiple proteins
• This is part of the reason why the proteome is much bigger than the genome

Image showing the alternative splicing of a gene to produce two different proteins