UNIT 5: Translation, Genetic Code, and Operon Concept

Exam Focus: Be able to describe the three phases of translation. For the **Genetic Code**, know the definition and key characteristics (universal, degenerate). The **Lac Operon** model is a frequently tested topic—know the roles of the regulator, promoter, operator, and structural genes, and how the inducer (allolactose) works.

Table of Contents

1. Translation
   • Mechanism of initiation
   • Elongation of polypeptides
   • Termination of polypeptides
   • Posttranslational modifications of proteins
2. Genetic code and its characteristics
3. Operon concept
   • Inducible (Lac operon)
   • Repressible system (Trp operon)

1. Translation

Translation is the process by which genetic information encoded in mRNA is used to synthesize a polypeptide chain (protein), carried out by the **ribosome** and facilitated by tRNA molecules.

Mechanism of initiation

In prokaryotes, initiation involves:

• Binding of the small ribosomal subunit to the mRNA at the **Shine-Dalgarno sequence**.
• Binding of the initiator tRNA (carrying N-formylmethionine) to the start codon (**AUG**) in the P site.
• Association of the large ribosomal subunit, forming the complete ribosome.

Elongation of polypeptides

The ribosome moves along the mRNA (translocation) in a 5'-to-3' direction, adding amino acids sequentially.

• Aminoacyl-tRNA enters the **A site**.
• **Peptidyl transferase** (an rRNA-catalyzed reaction) forms a peptide bond between the amino acid in the A site and the growing polypeptide in the P site.
• The ribosome translocates, shifting the tRNAs so the growing polypeptide is now in the P site, and the A site is free for the next aminoacyl-tRNA.

Termination of polypeptides

Occurs when the ribosome encounters one of the three **stop codons** (UAA, UAG, or UGA) in the A site.

• **Release factors** recognize the stop codon and bind to the A site.
• Release factors catalyze the hydrolysis of the bond linking the polypeptide chain to the tRNA in the P site.
• The ribosome dissociates from the mRNA and the polypeptide is released.

Posttranslational modifications of proteins

Chemical modifications that occur after the polypeptide chain is synthesized, necessary for the protein to become functional or target its final destination.

• **Examples:** Folding (chaperones); Cleavage (removal of signal peptides or pro-sequences); Chemical modifications (phosphorylation, glycosylation, acetylation, disulfide bond formation).

2. Genetic code and its characteristics

The set of rules by which information encoded in mRNA sequences (codons) is translated into amino acid sequences.

• **Codon:** Three adjacent bases in mRNA that specify one amino acid.
• **Characteristics:**
  • **Triplet:** Each codon consists of three bases.
  • **Universal:** Nearly the same in almost all organisms.
  • **Degenerate (Redundant):** Most amino acids are coded by more than one codon.
  • **Non-overlapping and Commaless:** Codons are read sequentially without gaps or overlap.

3. Operon concept

A functional unit of genetic material in bacteria, consisting of a cluster of genes under the control of a single regulatory signal (promoter). It allows for coordinated regulation of genes involved in a common metabolic pathway.

Inducible (Lac operon)

The Lac operon controls the metabolism of lactose in E. coli. It is an **inducible system**, meaning the genes are normally **OFF** and must be turned **ON** by the presence of a substrate (the inducer).

• **Repressor:** The LacI repressor protein is normally bound to the **Operator** region, blocking RNA polymerase from transcribing the structural genes (Lac Z, Y, A).
• **Inducer Action:** When lactose is present, it is converted to **allolactose** (the inducer). Allolactose binds to the repressor, changing its shape so it detaches from the operator.
• **Result:** Transcription of the lactose metabolism genes occurs.

Repressible system (Trp operon)

The Trp operon controls the synthesis of the amino acid tryptophan. It is a **repressible system**, meaning the genes are normally **ON** and must be turned **OFF** by the presence of the end-product (the corepressor).

• **Default State:** In the absence of tryptophan, the repressor is inactive, and the structural genes are transcribed.
• **Corepressor Action:** When tryptophan is abundant, it acts as a **corepressor** by binding to the Trp repressor protein, activating it.
• **Result:** The activated repressor binds to the operator, blocking transcription of the tryptophan synthesis genes.