Unit 3: Translation

1. Genetic Code and Wobble Hypothesis

Translation is the process of decoding the mRNA (messenger RNA) into a polypeptide chain (protein). The genetic code serves as the dictionary for this translation.

Salient Features of the Genetic Code

• Triplet Nature: Three nucleotides form a codon, which specifies one amino acid.
• Universal: The code is nearly identical in all organisms, from bacteria to humans.
• Degenerate: Most amino acids are specified by more than one codon (e.g., Leucine is coded by 6 different codons).
• Non-overlapping: The mRNA is read in a continuous sequence of triplets without overlapping.
• Commaless: There are no "commas" or spaces between codons in the mRNA sequence.
• Start and Stop Codons: AUG is the universal start codon (codes for Methionine), while UAA, UAG, and UGA are stop codons that signal termination.

The Wobble Hypothesis

Definition: Proposed by Francis Crick in 1966, the Wobble Hypothesis explains how a single tRNA can recognize multiple codons for the same amino acid.

The hypothesis states that the pairing between the first two positions of the mRNA codon and the last two positions of the tRNA anticodon follows strict Watson-Crick rules. However, the pairing at the third position of the codon (the "wobble" position) is flexible. This reduces the number of tRNAs required by the cell.

2. Process of Protein Synthesis in Prokaryotes

In prokaryotes, translation is a highly efficient process that occurs in the cytoplasm and involves three main stages: Initiation, Elongation, and Termination.

I. Initiation

• The small ribosomal subunit (30S) binds to the mRNA at the Shine-Dalgarno sequence (upstream of the start codon).
• The initiator tRNA (carrying N-formylmethionine or fMet) binds to the start codon (AUG).
• The large ribosomal subunit (50S) attaches, forming the complete 70S initiation complex.
• The ribosome has three sites: A (Aminoacyl), P (Peptidyl), and E (Exit). The initiator tRNA starts in the P site.

II. Elongation

1. Codon Recognition: A new aminoacyl-tRNA enters the A site, matching the next mRNA codon.
2. Peptide Bond Formation: The enzyme peptidyl transferase catalyzes the formation of a peptide bond between the amino acid in the P site and the one in the A site.
3. Translocation: The ribosome moves one codon forward along the mRNA. The empty tRNA in the P site moves to the E site and exits, while the tRNA with the growing chain moves from the A site to the P site.

III. Termination

Translation ends when a stop codon (UAA, UAG, or UGA) enters the A site. Since there are no tRNAs for these codons, Release Factors (RFs) bind to the site, triggering the release of the completed polypeptide chain and the disassembly of the ribosome.

[Image of the stages of Translation: Initiation, Elongation, and Termination]

3. Post-Translational Modifications

Newly synthesized proteins are often non-functional and must undergo post-translational modifications (PTMs) to achieve their mature, active form.

• Folding: Chaperone proteins help the polypeptide fold into its specific 3D structure.
• Cleavage: Removal of the initiator methionine or cutting a large precursor protein into smaller active parts (e.g., Proinsulin to Insulin).
• Chemical Modifications: Addition of functional groups, such as:
  • Phosphorylation: Addition of phosphate groups (regulates activity).
  • Glycosylation: Addition of carbohydrates (common in membrane proteins).
  • Acetylation/Methylation: Common in histones to regulate gene expression.
• Disulfide Bridge Formation: Covalent bonds between Cysteine residues stabilize the 3D structure.

4. Difference Between Prokaryotic and Eukaryotic Translation

While the basic mechanism is conserved, eukaryotes have distinct features due to their cellular organization.

5. Exam Focus Enhancements