pt 2

Amino Acids and Their Attachments

  • Amino Acid Attachment Site: Specific area on tRNA where amino acids are linked to the tRNA molecule.
  • tRNA: Transfers specific amino acids to the ribosome during protein synthesis.
  • Aminoacyl-tRNA Synthetase: Enzyme that catalyzes the attachment of an amino acid to its corresponding tRNA (acts like an "activated amino acid").

Ribosomes

  • Composition: Ribosomes are composed of ribosomal RNA (rRNA) and proteins.
  • Size Differences: Eukaryotic ribosomes are larger than prokaryotic ribosomes (important in medical applications).
  • Structure: Composed of a large and small subunit.
Ribosome Binding Sites
  1. A Site (Aminoacyl-tRNA Binding Site): Holds the tRNA with the next amino acid.
  2. P Site (Peptidyl-tRNA Binding Site): Holds the tRNA with the growing polypeptide.
  3. E Site (Exit Site): Exit site for tRNAs after their amino acid has been transferred.
  • Exit Tunnel: Allows the growing or completed polypeptide chain to exit the ribosome.

Stages of Translation

  1. Initiation:
  • mRNA, tRNA (with the first amino acid), and the two subunits of the ribosome must come together.
  • The small ribosomal subunit scans the mRNA for the start codon (AUG).
  • GTP is utilized for the assembly of ribosomal subunits.
  1. Elongation:
  • Proteins called elongation factors maintain the translation process.
  • Amino acids are linked via peptide bonds.
  • 2 GTP are used for codon recognition and 1 GTP for translocation of tRNA.
  1. Termination:
  • A stop codon signals the end of translation.
  • Release factors trigger the release of the polypeptide chain.
  • The ribosomal subunits dissociate after polypeptide exit.

Polyribosomes

  • Definition: Multiple ribosomes translate a single mRNA strand simultaneously.
  • Protein Types:
  • Free Ribosomes: Synthesize proteins that function in the cytosol.
  • Attached Ribosomes: Synthesize proteins destined for secretion or use in membranes.
  • Signal Peptide: A short sequence that directs the ribosome to the endoplasmic reticulum (ER).

RNA Functions and Mutations

  • RNA Versatility: Can hydrogen bond with itself and other nucleic acids, acting as a catalyst in some processes.
  • Types of Mutations:
  1. Substitution:
    • Silent Mutation: No effect on amino acid sequence.
    • Missense Mutation: Codes for the wrong amino acid.
    • Nonsense Mutation: Premature stop codon.
  2. Insertion/Deletion:
    • Causes a frameshift leading to extensive missense mutations.
    • If 3 nucleotides are inserted or deleted, it may result in an additional or missing amino acid without shifting the frame.

Processing of mRNA

  • Transcription: DNA is transcribed to form mRNA in the nucleus.
  • Post-Transcriptional Modification: Includes the addition of a 5' cap and poly-A tail, ensuring mRNA stability and transport out of the nucleus.
  • Translation Setup: Involves the initiation complex that helps ready the ribosomal subunits and mRNA for translation processes.