Splicing & Exporting RNA & Reading the Genetic Code

Here's a breakdown of those learning objectives, referencing the relevant sections in the provided note:

  • Splicing:

    • When/where it occurs: Splicing occurs in the nucleus of eukaryotic cells during the processing of pre-mRNA into mature mRNA.

    • Why correct regulation is important: Proper splicing is crucial for producing functional proteins. Errors can lead to non-functional proteins and disease. See sections on "Splicing Mutations," "Splicing Errors," and "Splicing Errors in Disease."

    • Steps in mRNA splicing & splice site selection: The spliceosome, composed of snRNAs and proteins, recognizes splice sites (5' splice site, 3' splice site, and branch point) and removes introns. snRNPs (U1, U2, U4, U5, U6) play key roles. See sections "The Splicing Process," "snRNPs and the Spliceosome," and "Spliceosome Action Sequence."

    • Impact of splicing errors: Errors can cause exon skipping, use of cryptic splice sites, altered amino acid sequences, mRNA degradation, and accumulation of unspliced intermediates. See sections "Splicing Errors," "Splicing Errors in Disease," and "Consequences of Errors in Splicing."

  • mRNA Export:

    • Selection for export & fate of RNA left behind: Mature mRNAs are selectively exported from the nucleus to the cytoplasm via nuclear transport receptors. The default fate of RNA remaining in the nucleus is degradation by RNA exonucleases. See sections "RNA Export," "Degradation of RNA," and "Selective Nuclear Export of mRNA."

  • rRNA S values:

    • What S values refer to: S values (Svedberg units) are a measure of sedimentation rate during centrifugation, reflecting a particle's size and shape. See section "rRNA and Ribosome Subunits."

  • rRNA and tRNA genes:

    • Why repetitive copies exist: Multiple copies of rRNA and tRNA genes are present to meet the high demand for ribosomes and tRNAs needed for protein synthesis. See section "rRNA and Ribosome Subunits."

  • Functions of rRNA, tRNA, snoRNA:

    • rRNA: Forms the structural and catalytic core of ribosomes, essential for protein synthesis. Transcribed by RNA polymerase I (except 5S rRNA by RNA polymerase III) in the nucleolus. See sections: "Ribosomes and rRNA", "Transcription of rRNAs", and "rRNA Transcription and Processing"

    • tRNA: Adaptor molecules that decode mRNA and deliver the correct amino acids to the ribosome. Transcribed by RNA polymerase III. See sections: "tRNA and the Genetic Code", "Transcription & Processing of tRNA", and "Structure & Function of tRNAs"

    • snoRNA: Involved in the processing and modification of rRNA. Transcribed by RNA polymerase II. See sections: "Chemical Modification of rRNAs", and "rRNA Transcription and Processing"

    • Transcription and processing locations: rRNA and snoRNA are transcribed and processed primarily in the nucleolus. tRNA is transcribed elsewhere in the nucleus.

  • Genetic Code:

    • How it's read (frame & non-overlapping): The genetic code is read in non-overlapping triplets (codons). The reading frame is the consecutive sequence of codons that is translated. See sections "tRNA and the Genetic Code", "Reading the Genetic Code Correctly", and "Overlapping vs Non-Overlapping Genetic Code."

    • tRNA structure & contribution: tRNAs have an anticodon loop that base-pairs with the mRNA codon, ensuring correct amino acid delivery. See sections "tRNA and the Genetic Code", "tRNA structure", and "tRNAs and Base Pairing"

    • Codon degeneracy & wobble: Some amino acids are encoded by multiple codons, often differing only in the third (wobble) base. This is due to wobble base-pairing between the tRNA anticodon and mRNA codon. See sections "The Genetic Code: Codons", and "tRNAs: Wobble Base-Pairing"

  • Mutations:

    • Types & Impact:

      • Silent: No change in amino acid sequence.

      • Nonsense: Introduces a premature stop codon.

      • Frameshift: Alters the reading frame, leading to a completely different amino acid sequence.

      • Missense: Results in a different amino acid being incorporated.

      • Translocations/Insertions/Deletions: Can disrupt gene function and/or lead to frameshift mutations.

    • See section "Genetic Code Mutations"

  • Aminoacyl-tRNA synthetase:

    • Function & accuracy: Catalyzes the attachment of the correct amino acid to its corresponding tRNA. Has proofreading mechanisms to ensure accuracy. See sections "tRNA Charging", "'Charging' tRNA with an amino acid", "Proofreading the charging of tRNAs", and "Ensuring Accuracy of Translation"

    • Comparison of numbers: There are generally 20 different aminoacyl-tRNA synthetases (one per amino acid), fewer tRNAs than codons, and 64 codons in total. See section "tRNA Charging"

  • Eukaryotic Ribosomes:

    • Structure & production: Made of a large (60S) and small (40S) subunit, composed of rRNA and proteins. Produced in the nucleolus. Translation occurs in the cytoplasm. See sections "Ribosomes and rRNA", "Transcription of rRNAs", and "Ribosome Production and Assembly"

  • Ribozymes:

    • Definition & Examples: RNA molecules with catalytic activity. Ribosomes are examples of ribozymes. See section "Ribosome Structure revisited"