CH 14&15 RNA Processing, Ribosome Assembly & Genetic Code – Comprehensive Notes

Course Resources & Study Tools

  • Macmillan-Learning folder under “Modules” contains a comprehensive suite of tools:

    • Adaptive quizzes for every chapter: These quizzes dynamically adjust the difficulty of questions based on your performance, pinpointing weak spots via algorithmic item selection. They cover all subsections in the textbook and lecture, even those not explicitly emphasized in lecture, allowing you to skip as needed if a topic is already mastered.

    • Flashcards: Essential for reinforcing basic definitions and ensuring simple terms don’t lead to lost points on exams.

    • PowerPoint lecture decks: Organized by week, these are the original slides presented in class, serving as a primary reference for lecture content.

    • Homework links: Direct access to assigned homework problems for practice and concept application.

  • The instructor can post missing adaptive-quiz links if students rely exclusively on the Canvas “Modules” view for access. For optimal study, it's recommended to access Macmillan directly.

  • Reminder: Students come from diverse backgrounds with different prior knowledge, leading to varying conceptual trouble spots. It's crucial to use adaptive material strategically to target your individual learning needs.

  • Juliana (TA/LA) will schedule a dedicated exam review session; students should watch their email closely for announcements regarding time and location.

Chapters 12–14 Q&A Snapshot

  • We are continuing the wrap-up of Chapter 14 (RNA processing), which covers the modifications mRNA undergoes before translation, and transitioning into Chapter 15 (Translation), which focuses on protein synthesis.

  • The plan is to finish Ch. 14 today, followed by a short introduction to Ch. 15, and then dedicate the next class entirely to an exam review session.

snRNPs (“snurps”) & Splicing Fundamentals

  • snRNP = small nuclear RNA (snRNA) + proteins: These are crucial components of the spliceosome, the molecular machine responsible for RNA splicing. Specifically, U1, U2, U4, U5, and U6 are classical spliceosomal snRNPs.

  • Function: snRNPs are responsible for identifying the precise splice sites within a pre-mRNA molecule, excising non-coding intron sequences, and ligating (joining) the coding exon sequences together to form a mature mRNA.

  • Historical context:

    • Early genomics estimated approximately 20,000ext–25,00020{,}000 ext{–}25{,}000 canonical protein-coding loci in the human genome, but biochemical studies indicated a much larger diversity, with >100{,}000 distinct proteins present in cells.

    • Multiple hypotheses emerged to explain this discrepancy: Was there a vast number of undiscovered gene types, or was it due to differential processing of the same gene?

    • The verdict confirmed that the “extra” proteins arise mainly from alternative mRNA processing (e.g., alternative splicing, alternative polyadenylation), rather than a vast number of hidden or undiscovered genes. This discovery fundamentally changed our understanding of gene expression complexity.

Alternative mRNA Processing

  • This phenomenon occurs in over 70 ext{%} of human genes, dramatically increasing the protein diversity that can be generated from a limited number of genes.

  • Mechanism: The specific pattern of alternative processing is primarily driven by cell-type, developmental stage, or cell-cycle specific expression of a precise repertoire of components:

    • Variant snRNP repertoires: Different tissues or developmental stages may express slightly different sets of snRNPs or splicing factors that influence splice site selection.

    • Splicing/co-transcriptional proteins: Regulatory proteins that interact with the spliceosome or RNA polymerase, enhancing or repressing certain splicing events.

    • Transcription factors: These proteins activate or repress the transcription of the RNA-processing genes themselves, establishing a regulatory cascade.

  • Hierarchy of regulation: A profound regulatory hierarchy exists where Transcription Factor (TF) gradients (established early in development, originating even in the fertilized egg) dictate the expression of different snRNP sets. These distinct snRNP sets, in turn, lead to specific splice decisions, ultimately controlling which protein isoforms are produced in which cells or stages.

  • Major mechanisms of alternative mRNA processing:

    1. Alternative splicing: This is the most common form, where different combinations of exons from the same pre-mRNA are included in the mature mRNA.

      • Example (3-exon pre-mRNA):

        • Isoform A: Exons 1-2-3 are joined, leading to Protein A.

        • Isoform B: Exon 2 is