TM

Week 1 S - Eukaryotic Protein Synthesis

Introduction to Eukaryotic Protein Synthesis

  • Eukaryotic Protein Synthesis: A complex process involving the translation of mRNA into proteins, facilitated by the ribosome.

  • Nobel Prize in Chemistry (2009): Awarded for the determination of the structure and function of the ribosome to Thomas A. Steitz, Venkatraman Ramakrishnan, and Ada E. Yonath.

The Genetic Code and Translation

  • Chapter Reference: Genetics, A Conceptual Approach by Benjamin A. Pierce, covering gene expression.

  • Translation: The process by which ribosomes synthesize proteins using mRNA as a template.

Levels of Gene Expression Regulation

  • Genome Organization:

    • Amplification or rearrangement of DNA segments.

    • Chromatin structure changes (decondensation/condensation).

    • DNA methylation impacts gene expression.

  • Transcription Regulation: Control over the synthesis of RNA from DNA.

  • RNA Processing: Modifications and nuclear export of RNA products.

  • Translation Regulation: Control of protein synthesis.

  • Post-translational Modifications: Includes folding, assembly, cleavage, and organelle import/secretion.

  • Degradation: Regulation of mRNA and protein lifespan.

Learning Objectives

  • Contrast Prokaryotic vs Eukaryotic Translation: Key differences in mechanisms.

  • Eukaryotic Translation Steps:

    • Initiation: Beginning of translation.

    • Elongation: Extension of the amino acid chain.

    • Termination: Ending of translation.

  • Exon Structure Importance: Ensuring correct reading frame and identifying potential errors.

Gene Expression in Bacteria vs Eukaryotes

  • Bacterial Gene Expression: Coupled transcription and translation.

  • Eukaryotic Gene Expression: Multistage process involving transcription in the nucleus and translation in the cytoplasm.

  • Transcription Process: Involves DNA to mRNA synthesis, splicing of introns, and transport of mRNA out of the nucleus.

Key Components of Translation

  • Translation Process:

    • Ribosome: Structure that synthesizes proteins using mRNA.

    • tRNA: Carries amino acids; has anticodon to match mRNA codons.

    • Peptide Bonds: Covalent link between amino acids in a protein chain.

  • Codon: Triplet of nucleotides coding for an amino acid or a termination signal.

Translation Components and Factors

  • Essential Components:

    • Ribosome.

    • mRNA.

    • tRNA.

    • Translation Factors (Initiation, Elongation, and Release Factors).

  • Ribosomal Structure:

    • Composed of RNA and proteins, dissociates into large and small subunits.

    • Eukaryotic ribosomes are larger than prokaryotic ones.

tRNA Binding Sites in Ribosome

  • Binding Sites:

    • A Site: Accepts incoming aminoacyl-tRNA.

    • P Site: Holds the tRNA carrying the growing polypeptide.

    • E Site: Exit site for deacylated tRNA.

  • Peptidyl-tRNA: Carrier of the growing polypeptide chain.

Amino Acid Addition Process

  • Peptide Bond Formation: The polypeptide is transferred from peptidyl-tRNA at the P site to the incoming aminoacyl-tRNA at the A site.

  • Translocation: Movement of the ribosome along the mRNA template.

  • Process Summary: tRNA enters A site → Peptide bond formation → Translocation → Exit of deacylated tRNA at E site.

Mitochondrial and Chloroplast Ribosomes

  • Different from cytoplasmic ribosomes in protein and RNA composition.

  • Reflects evolutionary differences in protein synthesis machinery.

mRNA Structure in Eukaryotes

  • 5' Cap and Poly-(A) Tail: Essential for stability, initiation, and export from the nucleus.

  • Exons and Introns: Splicing modifies pre-mRNA to produce mature mRNA.

Codon and Anticodon

  • Triplet Code: Comprises 64 codons, with 61 coding for amino acids and 3 as termination signals.

  • Wobble Hypothesis: Third position in the codon allows for variability in tRNA binding.

Open Reading Frame (ORF)

  • Sequence coding for proteins starting with initiation codon (AUG) and ending with a termination codon (UAA, UAG, UGA).

Aminoacyl-tRNA Synthetase Function

  • Enzymes responsible for linking specific amino acids to their corresponding tRNAs in an ATP-dependent manner.

Eukaryotic Translation Factors

  • Functional Homologies: Comparison between prokaryotic and eukaryotic translation factors.

  • Initiation Factors: Assist in the assembly and function of the translation machinery.

  • Elongation Factors: Facilitate peptide chain elongation during translation.

  • Release Factors: Recognize stop codons and facilitate termination of translation.

Stages of Translation

  1. Initiation: Assembly of ribosomal subunits, mRNA, and initiator tRNA.

  2. Elongation: Addition of amino acids to the growing chain.

  3. Termination: Recognition of stop codons and disassembly of translation components.

Details of Translation Initiation

  • 43S Complex Formation: Initiation factors bind to the small ribosomal subunit.

  • CAP-binding: Essential for mRNA stabilization and recognition.

  • Start Codon Scanning: Ribosomal subunit scans mRNA for AUG to begin translation.

Final Aspects of Translation

  • Ribosome Recycling: Uses ATP hydrolysis to dissociate tRNA, mRNA, and ribosomal subunits post-termination.

  • Summary of Translation Process:

    • Involves many factors to ensure accurate and efficient protein synthesis.

    • Metabolically expensive as it relies on ATP/GTP for energy.

Conclusion

  • Eukaryotic translation is a highly regulated process involving complex interactions between various factors ensuring proper protein synthesis.