Genetic Code and Translation

Translation Overview

  • Translation synthesizes polypeptides from mRNA.

Proteins and Polypeptides

  • Proteins: Macromolecules made from one or more polypeptides.

  • Polypeptides: Chains of amino acids linked by peptide bonds.

Amino Acids

  • Monomers of polypeptides.

  • 20 different amino acids used, with unique properties determined by R groups (hydrophobic, polar, acidic, basic).

Polypeptide Formation

  • Peptide bonds: Formed via dehydration synthesis (requires ATP, GTP).

  • Ends of polypeptides:

    • Amino terminus (N-terminus): Unbonded amino group.

    • Carboxyl terminus (C-terminus): Unbonded carboxyl group.

Protein Structure

  • Levels of structure:

    • Primary: Sequence of amino acids.

    • Secondary: Helices and sheets.

    • Tertiary: 3D folding due to covalent and ionic interactions.

    • Quaternary: Multiple polypeptides combined.

Functions of Proteins

  • Enzymatic activity (e.g., glycolytic enzymes).

  • Structural roles (e.g., actin, collagen).

  • Regulation / signaling (e.g., receptors).

Genetic Code

  • Relationship between nucleic acid sequences and amino acids.

  • Codon: Sequence of three bases; specifies amino acids or stops.

  • mRNA and DNA sequences correspond in coding.

Characteristics of the Genetic Code

  • Universal: Common among organisms.

  • Triplet: Codons are three bases long.

  • Degeneration: Multiple codons can specify the same amino acid.

  • Includes start (AUG) and stop codons, with no gaps or overlaps in coding.

Open Reading Frame (ORF)

  • Continuous sequence from start codon to stop codon.

mRNA Structure

  • mRNA includes coding (protein-coding) and non-coding regions (5’ UTR, 3’ UTR).

Translation Mechanism

  • Involves mRNA, tRNA, and ribosome.

  • tRNA carries amino acids and pairs with mRNA codons.

Aminoacyl-tRNA

  • tRNA that is charged with an amino acid.

  • Charging is done by aminoacyl-tRNA synthase.

Ribosome

  • Site of protein synthesis; ribsosome consists of rRNA and proteins.

  • Composed of large and small subunits with A, P, and E sites.

Phases of Translation

  1. Initiation: Small subunit binds mRNA; initiator tRNA enters P site.

  2. Elongation: Repeated cycles of tRNA entry, peptide bond formation, ribosome shifting.

  3. Termination: Stop codon reached; release factors bind and release polypeptide.

Polycistronic versus Monocistronic

  • Polycistronic (bacteria): Multiple coding sequences in one mRNA.

  • Monocistronic (eukaryotes): One coding sequence per mRNA.

Eukaryotic Translation Features

  • Ribosome scanning for start codon.

  • Involvement of poly(A) tail and separate transcription/translation compartments.