Biology Lecture 3: Translation Process

Lecture 3: Translation

• Translation is the process where mRNA is decoded by ribosomes to produce an amino acid chain (polypeptide) that folds into an active protein.
• Translation occurs in the 5' to 3' direction.
• The process starts with DNA in the chromosome, and utilizes mRNA to pass the genetic information.

Components Needed for Translation

• Ribosome: The protein synthesis factory, composed of 40S and 60S subunits.
  • Large subunit (60S): contains the enzymatic activity (ribozyme) made of 5S, 5.8S and 28S rRNA and 47 proteins.
  • Small subunit (40S): responsible for reading mRNA and monitoring complementarity between codons and anticodons, made of 18S rRNA and 32 proteins.
• mRNA: Contains genetic information in the form of codons, arranged in a linear, non-overlapping manner.
  • mRNA consists of 64 codons (4³).
  • mRNA has a 5' methylated cap and a 3' poly-A tail.
  • AUG codon is the initiation codon.
  • UAA, UAG, UGA are termination codons.
• tRNA: Carriers of amino acids.
  • Small RNA molecules (73-93 nucleotides) folded into 3D structures.
  • Each tRNA carries a unique amino acid.
  • Contains an anticodon that binds to the complementary codon on mRNA.
  • Contains:
    • Amino acid arm (3' end) that attaches to specific amino acids (CCA).
    • 5' end rich in poly guanylate (pG).
    • Anticodon arm.
    • D arm has dihydrouridine(D) residues.
    • TψC arm has unusual bases, ribothymidine(T) and pseudouridine(Ψ).

Ribosome Sites

• Aminoacyl-site (A site): Where "charged" tRNA resides (carrying an amino acid)
• Peptidyl-site (P site): Where the polypeptide chain resides
• Exit-site (E site): Where "uncharged" tRNA leaves the ribosome

mRNA: Genetic Information

• Composed of numerous codons arranged in a linear, non-overlapping manner.
• Each codon codes for a specific amino acid (AA).
• There are 64 codons (4³).
• AUG specifies methionine (Met), the initiation codon.
• UAA, UAG, and UGA are termination codons.
• Translation proceeds in the 5' to 3' direction of the mRNA.

Genetic Code Degeneracy

• The genetic code is degenerate; multiple codons can code for the same amino acid.

Coding Sequence: Linear & Non-overlapping

• The coding sequence of mRNA is linear and non-overlapping. This arrangement creates a direct, one-to-one correspondence between codons and the amino acid sequence they specify.

Translation Stages:

• Stage I - Activation of amino acids: Amino acids are activated by attaching to tRNA molecules. A specific aminoacyl-tRNA synthetase is needed to conjugate the correct tRNA with the correct amino acid.

• Stage II - Initiation: The ribosome binds to the mRNA, initiator tRNA and the start codon.

  • The 40S ribosome subunit binds to eukaryotic initiation factor 3 (eIF3).
  • eIF2 and guanosine triphosphate (GTP) bind to methionyl-tRNA.
  • The 43S complex then initiates forming the rest of the complex.
  • The 60S ribosome subunit joins the 40S subunit.

• Stage III - Elongation: New amino acids are added to the growing polypeptide chain.

  • Decoding: An aminoacyl-tRNA binds to the A site, elongation factors (eEF1A and GTP) are involved. GTP is hydrolyzed.
  • Peptide bond formation: A peptide bond forms between the amino acids in the P and A sites. Peptidyl transferase is part of the 60S ribosome subunit.
  • Translocation: The ribosome moves along the mRNA, and the polypeptide chain translocates. eEF2 and GTP are involved. An uncharged tRNA moves from the P site to the E site. The tRNA in the A site now moves to the P site, and the ribosome moves to the next codon.

• Stage IV - Termination: A stop codon enters the A site and polypeptide release factors (eRF1 and eRF3) are involved. GTP hydrolysis occurs. Ribosomes dissociates into subunits. tRNA disassociates from the P site.

• Stage V - Folding & Post-translational Modifications: Folding of protein into its 3D form, chaperons, heat shock proteins. Disulfide isomerases stabilize structure - disulfide bonds. Post-translational modifications: glycosylation, phosphorylation, acetylation, etc add functional groups.

Polyribosomes/Polyomes:

• Multiple ribosomes can read a single mRNA molecule at the same time, leading to multiple identical protein molecules being synthesized from a single mRNA.

Clinical Correlations

• Ribosomopathies: Genetic disorders related to defects in ribosomal proteins/rRNA
• Diseases related to misfolded proteins: Misfolded proteins can form insoluble aggregates (amyloids). Amyloids accumulate in organs/extracellular spaces, causing pathology. Examples include Alzheimer's disease and Creutzfeldt-Jakob disease (CJD). Proteins misfolding can cause CJD (e.g. Prion protein).

The "Wobble" Hypothesis

• Flexibility in the pairing of the third base of a codon with the corresponding anticodon of tRNA can accommodate changes in the base and still yield the correct amino acid. This flexibility in base-pairing allows for more than one tRNA to recognize the same codon.
  • This is particularly important for efficiency and for the overall accuracy of the translation process.