Post-Transcriptional Modification (DP IB Biology)

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 (introns) are removed and coding sequences (exons) are joined together

Alternative 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