post transcriptional regulation in eukaryotes

phenotypic plasticity

• From a single genotype, multiple phenotypes can be released 

mRNA processing

• 5' capping 

  • Protects mRNA from degradation and helps ribosomes recognise the start of translation 

  • Inverted guanine that has been methylated 

  • Bonded to first nucleotide on the mRNA 

    • 5' to 5' bond 

• Polyadenylation 

  • At the 3' end 

  • Stabilises the mRNA and aids in its export from the nucleus 

mRNA polyA tail

• Increases length of mRNA 

• Added via polymerase 

• Protects mRNA from degradation 

  • Longer tail = longer protected 

• Helps interact with proteins involved in recruitment of the ribosome 

• Aids efficient translation 

mRNA degradation

• Instantly degraded 

  • Multiple pathways 

• Attack at 3' end by 3' exonucleases 

  • Act to erode the polyA tail, which destabilises the mRNA 

• Attack at 5' end by 5' exonucleases 

  • Remove 5' cap and degrade mRNA 

why release mRNA into a hostile environment

• With structures in place, a cell is able to control how long a mRNA is in the cytoplasm and how much transcription occurs 

  • 5' cap 

    • And associated proteins 

  • PolyA tail 

transcriptome

• The array of mRNA transcripts produced in the cell 

• Can see how gene expression changes over time across different states 

RNA splicing

• Removal of segments of transcript 

• The spliceosome consists of 5 small ribonuclear proteins (snRNPs) 

alternative RNA splicing

• C value paradox 

  • The number of genes does not correlate with organismal complexity 

• Splice variants (can expand the proteome) 

  • A single gene can produce multiple proteins through alternative splicing 

  • A pre-mRNA undergoes splicing to remove non-coding introns 

  • Some exons can also be spliced, leading to different mRNA transcripts from a single gene 

  • This allows greater transcriptional complexity from a single genome

• RNA editing 

  • Through deamination 

  • A-> I (G equivalent in DNA) 

  • C -> U (T equivalent in DNA) 

what is microRNA (miRNA)

• A class of small, non-coding RNAs (roughly 22 nucleotides long) 

• Bind to sequences in the mRNAs 

• Used to inhibit translation into protein 

• Perfect complementarity = mRNA degraded 

• Partial complementarity = transcription blocked 

• SiRNA (small interfering RNA) guides the RISC complex to the mRNA 

  • By generating siRNA (double stranded) gene expression can be lowered in vivo 

  • This can help to understand the roles of a gene 

  • Can also be used to reduce expression of target genes in diseases 

production of miRNA

• Transcription (RNA polymerase produces pri-miRNA) 

• Drosha-DGCR8 (processes pri-miRNA to pre-miRNA in the nucleus) 

• Exportion-5 (Pre-miRNA is transported into the cytoplasm) 

• Dicer (cleaves pre-miRNA into mature miRNA) 

• RISC (mature mi-RNA is loaded into the RNA induced silencing complex) 

mRNA binding proteins and translational control

• Translational control is a regulatory mechanism that determines the efficiency and timing of translation of mRNAs into proteins 

  • 5' untranslated region contains elements that influence translation initiation 

  • 3' untranslated region contains regulatory sequences that can impact translation efficiency and mRNA stability 

• Initiation factors are proteins that facilitate the assembly of the ribosome machinery at the start codon of the mRNA