Genetic Code and Translation

Key Concepts in Translation and Protein Synthesis

  • Translation Overview

    • Process of decoding mRNA to synthesize proteins at the ribosome.
    • Involves reading genetic code and assembling amino acids into polypeptides.

  • Transcription Review

    • Occurs in the nucleus; involves creating mRNA from a DNA template.
    • Promoter: The regulatory region of the gene where RNA polymerase binds to initiate transcription.
    • Coding region vs. Regulatory region:
    • Coding Region: Encodes the sequence for protein synthesis.
    • Regulatory Region: Controls timing and amount of gene expression.
    • Termination Sequence: Signals RNA polymerase to stop transcription.

  • mRNA Modifications (in Eukaryotes)

    • 5' Cap: Modified guanine added to the 5' end of the mRNA.
    • Poly-A Tail: A string of adenine nucleotides added to the 3' end.
    • Intron Removal:
    • Introns (non-coding regions) are removed, and exons (coding regions) are spliced together to form mature mRNA.
    • Alternative splicing allows for multiple protein variants from a single gene, increasing protein diversity.

  • Investing Energy in Introns

    • Large portion of DNA consists of introns (approx. 25% of the genome).
    • Bacteria lack introns, making them more efficient in energy use for transcription and translation.

  • The Genetic Code

    • Comprised of codons: groups of three nucleotides that code for amino acids.
    • 64 Codons total: 61 code for amino acids, 3 are stop codons.
    • Start Codon: AUG (methionine) signifies the start of translation.

  • Experiment to Decipher Genetic Code

    • Early experiments involved mixing ribosomes, tRNAs, amino acids, and specific mRNAs in vitro.
    • Using UUU mRNA proved to code for phenylalanine; identified through trial with ribosomes and tRNAs.
    • Significant findings included synonymous codons (multiple codons representing a single amino acid) and stop codons with no corresponding tRNAs.

Ribosomes and tRNA Functionality

  • Ribosomes

    • Complex of rRNA and proteins; consist of a small subunit and large subunit.
    • Comprise three functional sites: E (exit), P (peptidyl), and A (aminoacyl).

  • tRNA Role

    • Each tRNA carries a specific amino acid and contains an anticodon that pairs with the mRNA codon.
    • tRNA Activation: Involves attaching the correct amino acid to its respective tRNA.

Steps of Translation

  1. Preparation of tRNAs

    • Proper amino acids attached to tRNAs by specific enzymes.

  2. Ribosome Assembly

    • The small and large ribosomal subunits assemble with mRNA and initiator tRNA (carrying methionine) to start translation.

  3. Peptide Chain Elongation

    • The next aminoacyl tRNA binds to the A site.
    • Peptide Bond Formation: New amino acid added to the growing chain.
    • Process repeats: Bind, Bond, Move strategy:
      • Bind next tRNA, bond the new amino acid, and move the ribosome down the mRNA (translocation).

  4. Termination of Translation

    • Encountering a stop codon (e.g., UAA, UAG, UGA) halts peptide elongation.
    • Release Factor binds, catalyzing the release of the newly formed polypeptide chain.

  5. Post-Translational Modifications

    • Proteins may undergo modifications (e.g., glycosylation) post-translation to finalize their structure and functionality.

Conclusion: Understanding Protein Synthesis

  • Importantly, translations occur in both prokaryotic and eukaryotic cells, albeit with differences in ribosomal structure and initiation mechanisms.
  • Mastery of translating mRNA sequences, understanding codons, and knowing the roles of tRNA and ribosomes are crucial for grasping the core concepts in molecular biology and genetics.
  • Knowing the genetic code and its applications, particularly regarding start and stop codons, is essential for the exams.