Gene Expression: From DNA to Protein

Jul 9, 2024

Gene Expression: From DNA to Protein

Overview

  • Gene expression: process by which information in a gene is used to make a protein.
  • Steps covered:
    1. Transcription
    2. Genetic code and translation
    3. Translation on the ER (Endoplasmic Reticulum)
    4. Mutations

The Foundation

  • Genes are segments of DNA that code for a particular protein or RNA molecule.
  • DNA analogy: recipe book; Gene: recipe; RNA: copied recipe; Protein: dish prepared.
  • Human genome: ~3 billion base pairs, ~25,000 genes.
  • Two gene expression processes:
    1. Transcription: DNA → RNA (Occurs in nucleus, eukaryotic cells)
    2. Translation: RNA → Protein (Occurs in cytoplasm)

Transcription Details

  • Stages of Transcription:
    • Initiation: RNA polymerase binds to promoter (TATA box easier due to A-T bonds).
    • Elongation: RNA polymerase synthesizes complementary RNA strand.
    • Termination: RNA polymerase reaches end of gene, releases RNA transcript.
  • Promoter: Contains TATA box, easier to break due to 2 hydrogen bonds between A-T.
  • RNA polymerase reads DNA 3’ to 5’, synthesizes RNA 5’ to 3’.

RNA Processing in Eukaryotes

  • Primary RNA transcript is processed:
    • 5′ cap and poly-A tail added.
    • Introns spliced out, exons joined.
  • Spliceosome: Complex of proteins and snRNA that removes introns.
  • Exon shuffling: Mechanism to create new genes via re-arrangement of exons.

Types of RNA

  • mRNA: Encodes protein.
  • rRNA: Structural component of ribosome.
  • tRNA: Delivers amino acids to ribosomes during protein synthesis.

The Genetic Code

  • Codons: Sequence of three nucleotides that correspond to an amino acid.
  • Start codon: AUG (methionine).
  • Stop codons: UAG, UAA, UGA.
  • Redundancy in genetic code: Multiple codons for same amino acid.
  • Genetic code is universal: Can express genes across different species.

Translation: RNA to Protein

  • Ribosomes: Facilitate matching of codons (mRNA) to anticodons (tRNA).
    • Small subunit: Binds mRNA.
    • Large subunit: Has A (aminoacyl), P (peptidyl), E (exit) sites.
  • tRNA: Carries amino acids to ribosomes, has anticodon for base-pairing with mRNA codons.
  • Phases of Translation: Initiation, Elongation, Termination
    • Initiation: Small subunit binds start codon (AUG), initiator tRNA with methionine binds, large subunit assembles.
    • Elongation: Sequential addition of amino acids, peptide bonds formed.
    • Termination: Stop codon reached, release factor releases the new protein.

Additional Concepts

  • Polyribosome: Multiple ribosomes translate the same mRNA simultaneously.
  • Chaperonins: Assist in the proper folding of newly synthesized proteins.
  • Free vs. ER-bound ribosomes:
    • Free ribosomes: Synthesize proteins for the cytoplasm.
    • ER-bound ribosomes: Synthesize proteins for ER lumen, lysosomes, or secretion.