Overview of Gene Expression and mRNA Processing

Overview of mRNA Processing

  • mRNA processing is an essential step in gene expression, occurring after transcription and before translation.

Overview of Eukaryotic Gene Expression

  • Expression of genes in eukaryotes includes several steps:

    • DNA is transcribed into Pre-mRNA.

    • RNA Processing occurs, resulting in the formation of mRNA.

    • Translation of mRNA into Proteins.

Translation

  • Definition of Translation:

    • The process of protein synthesis from mRNA by ribosomes.

    • Ribosomes act as the molecular machines that read the mRNA and assemble the corresponding amino acids into a polypeptide chain.

Beadle and Tatum Experiment Overview

  • Historical Context:

    • Work conducted with the fungus Neurospora.

Methods:

  1. A culture of Neurospora was irradiated using X-rays to induce mutations.

  2. Individual spores were transferred to tubes containing a complete growth medium that allows for optimal growth.

  3. Spores from each culture were then transferred to tubes containing minimal medium, which loses certain nutrients and tests ability to synthesize them.

  4. Results:

    • Mutant Neurospora grew only when supplemented with arginine. This indicates defects in arginine synthesis.

    • Fungi with auxotrophic mutations would not grow on minimal medium unless supplemented with specific amino acids (e.g., Tryptophan, Glycine, Alanine, etc.).

Arginine Pathway

Methods:

  • Spores of auxotrophic mutants requiring arginine are placed on minimal medium, both with and without supplements.

Results:

  • The growth response of different groups of mutants was recorded based on supplementation:

    • Group I:

      • Can grow on minimal medium supplemented with ornithine, citrulline, or arginine.

      • Indicates blockage at the step before ornithine synthesis.

    • Group II:

      • Can grow on medium supplemented with arginine or citrulline but not ornithine, indicating blockage before citrulline and arginine synthesis.

    • Group III:

      • Can only grow on medium with arginine, indicating blockage directly at arginine synthesis.

Summary of the Experiment

  • Beadle and Tatum posited that mutations affecting a specific biochemical step occurred at a single locus encoding a particular enzyme, resulting in the one gene, one enzyme hypothesis and later the one gene, one polypeptide hypothesis.

Structure & Function of Proteins

  • Proteins consist of amino acids characterized by their R groups:

    • Nonpolar, aliphatic R groups.

    • Aromatic R groups.

    • Positively charged R groups.

    • Polar, uncharged R groups.

    • Negatively charged R groups.

  • The primary structure of a protein is its amino acid sequence, influencing secondary structures (like alpha helices) and tertiary structures (overall 3D conformation).

    • Several polypeptides can assemble into a quaternary structure.

Peptide Bond Formation:

  • Involves nucleophilic attack by the amino-group of one amino acid on the carbonyl group of another, leading to the loss of water.

  • N terminus and C terminus define the ends of a peptide chain, which is critical for structure and function.

RNA Processing in Eukaryotes

  • Poly(A) Tail and 5' Cap Formation:

    • This includes the addition of the poly(A) tail and the 5' cap to mRNA, which is essential for stability and translation initiation.

Export of mRNA to Cytoplasm

  • Mature mRNA is exported from the nucleus to the cytoplasm where it can be translated.

    • Export mechanisms include:

      • hnRNP proteins, SR proteins, and nuclear proteins that ensure the correct processing and export of mRNA molecules.

Translation and the Role of Ribosomes

  • Ribosome Composition:

    • In eukaryotes, ribosomes are classified as 80S consisting of large (60S) and small subunits (40S) containing various rRNA and proteins.

    • In prokaryotes, ribosomes are smaller (70S) consisting of a large (50S) and small subunit (30S).

  • The Translation Process:

    • Initiation:

      • mRNA binds to small ribosomal subunit; initiation factors facilitate the binding of the initiator tRNA to the start codon (AUG).

    • Elongation:

      • Involves entry of charged tRNAs into the A site and formation of peptide bonds catalyzed by peptidyl transferase.

    • Termination:

      • Occurs when the ribosome encounters a stop codon, leading to the release of completed polypeptide chains.

Genetic Code

  • The code is triplet-based, universal, non-overlapping, and degenerate. This means that:

    • One codon corresponds to one amino acid, but one amino acid can be coded by multiple codons.

Codon-Anticodon Pairing

  • The third position of the codon allows for some variability, known as the wobble hypothesis.

  • This flexibility allows certain tRNAs to recognize multiple codons that code for the same amino acid.

Polyribosomes

  • Multiple ribosomes can translate the same mRNA simultaneously, increasing the efficiency of translation and protein synthesis.