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First exams 2025

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Post-Transcriptional Modification (HL) (HL IB Biology)

Revision Note

Emma

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Emma

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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

Splicing of pre-mRNA 1, downloadable AS Level & A Level Biology revision notes Splicing of pre-mRNA 2, downloadable AS Level & A Level Biology revision notes

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

Alternative Splicing, downloadable AS & A Level Biology revision notes

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

Examiner Tip

It is important you learn the terms pre-mRNA and mRNA, their location and whether they include introns as well as exons. A handy way to distinguish between introns and exons is to remember that EXons are EXpressed.

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Emma

Author: Emma

Expertise: Biology

Prior to working at SME, Emma was a Biology teacher for 5 years. During those years she taught three different GCSE exam boards and two A-Level exam boards, gaining a wide range of teaching expertise in the subject. Emma particularly enjoys learning about ecology and conservation. Emma is passionate about making her students achieve the highest possible grades in their exams by creating amazing revision resources!