D1.2.15 Post-transcriptional modification in eukaryotic cells (HL) Notes

Post-Transcriptional Modification Turns pre-mRNA Into Mature mRNA

1. In eukaryotic cells, the initial RNA transcript, called pre-mRNA, undergoes several modifications to become mature mRNA.
2. These modifications occur in the nucleus to ensure the mRNA is stable, functional, and ready for translation in the cytoplasm.

Key Steps in Post-Transcriptional Modification

1. Removal of Introns and Splicing of Exons
   1. Eukaryotic genes contain exons (coding regions) and introns (non-coding regions).
   2. Introns are removed from the pre-mRNA by a complex called the spliceosome.
   3. The remaining exons are joined together (spliced) to form a continuous coding sequence.
   4. This produces mature mRNA containing only the coding regions.
2. Addition of a 5' Cap
   1. A modified guanine nucleotide is added to the 5' end of the pre-mRNA.
   2. The 5' cap:
      1. Protects the mRNA from degradation by enzymes.
      2. Facilitates ribosome binding during translation.
      3. Helps the mRNA exit the nucleus.
3. Addition of a 3' Poly-A Tail
   1. A sequence of 100–200 adenine nucleotides is added to the 3' end of the pre-mRNA.
   2. The 3' poly-A tail:
      1. Stabilizes the mRNA by preventing degradation.
      2. Assists in the export of mRNA from the nucleus.
      3. Enhances translation efficiency.

Why Are These Modifications Important?

1. Stability: The 5' cap and 3' poly-A tail protect mRNA from degradation, ensuring it survives long enough to be translated.
2. Accuracy: Splicing ensures that only coding regions (exons) are included in the final mRNA, preventing errors in protein synthesis.
3. Transport: These modifications help the mature mRNA exit the nucleus and reach the ribosome for translation.