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Overview of Translation

  • Translation is the final stage of the central dogma of molecular biology.
  • It involves converting the information in mRNA into a sequence of amino acids in a protein.

Translation in Molecular Biology

  • Key Terminology:

    • Translation: Process of converting mRNA into proteins.
    • mRNA (Messenger RNA): The intermediate molecule synthesized during transcription that carries genetic information to the ribosome.

  • The cell translates nucleic acid language (in mRNA, made of nucleotides) into protein language (made of amino acids).

The Genetic Code

  • The genetic code defines how nucleotide sequences in mRNA specify amino acids in proteins.
  • Key Definitions:
    • Codon: A three-nucleotide sequence on mRNA that codes for a specific amino acid.
    • There are 64 different codons formed from 4 nucleotides (A, G, C, U).

Determining Codons for Amino Acids

  • Early molecular biologists established that:
    • One nucleotide cannot code for an amino acid because there are only 4 nucleotides and 20 amino acids.
    • Two nucleotides yield $4^2 = 16$ combinations, insufficient for 20 amino acids.
    • Three nucleotides give $4^3 = 64$ combinations, ample to code for all 20 amino acids.

Structure of Codons

  • Codons are coded from the 5' to 3' direction; examples included in a codon table.
  • Codons can code for the same amino acid:
    • Leucine: 6 codons (UUA, UUG, CUU, CUC, CUA, CUG)
    • Methionine: Only 1 codon (AUG), which is also the start codon.

Stop Codons

  • Stop codons signal the end of translation and include:
    • UAA, UAG, UGA.

Properties of the Genetic Code

  • Continuous: No spaces between codons; codons are adjacent.
  • Non-overlapping: Each nucleotide belongs to only one codon.
  • Degenerate: Most amino acids are coded by multiple codons.
  • Universal: Minor exceptions; triplet codons have the same meaning across different organisms.

Components of Translation

  • Requires:
    • mRNA: Carries the message to synthesize proteins.
    • tRNA (Transfer RNA): Brings specific amino acids to the ribosome.
    • Ribosomes: Made of rRNA and proteins; facilitate translation.

Ribosome Structure

  • Consists of:
    • 50S Subunit (large)
    • 30S Subunit (small)
  • Sites in ribosome:
    • A Site: Aminoacyl Site (binds incoming tRNA)
    • P Site: Peptidyl Site (holds the tRNA with the growing polypeptide)
    • E Site: Exit Site (where uncharged tRNA exits the ribosome)

tRNA Function and Structure

  • tRNA molecules carry amino acids and have a unique structure called a cloverleaf due to base pairing.
  • Five Prime End: tRNA carries the amino acid at the three prime end.
  • Anticodon: A sequence of three bases that pairs with the corresponding codon on mRNA.

Codon-Anticodon Pairing

  • Involves base pairing (A-U, C-G) between codons on mRNA and anticodons on tRNA.
  • Example: Codon AUG pairs with tRNA anticodon UAC, carrying Methionine.

Stages of Translation

  1. Charging of tRNA: tRNA bonds with its specific amino acid.

    • Catalyzed by aminoacyl-tRNA synthetases.

  2. Initiation: Recognition of the start codon (AUG) by initiator tRNA (carrying Methionine or modified Methionine in bacteria).

  3. Elongation: Successive amino acids are added:

    • Initiator tRNA occupies the P site.
    • New tRNA carrying the next amino acid arrives at the A site.
    • Peptide bond forms between the amino acids; the ribosome moves (translocation).

  4. Termination: When a stop codon is reached in the A site:

    • No corresponding tRNA binds.
    • A releasing factor binds to the stop codon, releasing the polypeptide from the ribosome.

Evolution of Proteins

  • Analogy of word changes illustrates how mutations can affect protein function:
    • Most mutations are non-functional (gibberish words).
    • Some mutations may yield new functional proteins or new meanings (new words).

Conclusion

  • The translation process converts mRNA into functional proteins, culminating in the understanding of the central dogma of molecular biology: replication, transcription, and translation.