Gene Expression 2: RNA Translation

Gene Expression 2: RNA Translation and Genetic Code

• Learning Objectives (ILOs): By the end of the lecture, students will be able to:
  • Discuss the rules of the genetic code.
  • Correlate the function of different RNAs in translation.
  • Describe the process of translation.
  • Interpret the role of translation and post-translational modification in health and disease.

What is Translation?

• Translation is the process of converting the nucleotide sequence of mRNA (codons) into an amino acid sequence of a protein to synthesize proteins.
• Codons are sequences of three nucleotides.
• The genetic code is the collection of codons.
• Protein biosynthesis is called translation because it involves translating information from the four-letter language and structure of nucleic acid into the 20-letter language and structure of proteins.

Requirements of Translation

• mRNA: Carries genetic information.
• tRNA: Adapter molecule; recognizes an amino acid and its corresponding codon. Each amino acid has a specific tRNA.
• Ribosomes: Molecular machine coordinating interactions between mRNA, tRNA, enzymes, and protein factors.

Genetic Code (Figure 1)

• The genetic code is the relationship between the nucleotide sequence in DNA or mRNA and the amino acids in a polypeptide chain.
• Each amino acid can be specified by more than one codon.
• AUG is the start codon (methionine).
• There are three stop codons: UAA, UAG, UGA.
• The genetic information along mRNA is read from 5' to 3' direction.

Characteristics of the Genetic Code

• Degenerate: Multiple codons can code for the same amino acid, except for tryptophan and methionine.
• Unambiguous: Each codon specifies only one amino acid.
• Non-overlapping: The code is read from a fixed starting point in a continuous sequence of bases, three at a time.
• Commaless: No punctuation between codons.
• Universal: The same code words are used in all organisms (prokaryotes and eukaryotes).

Protein Biosynthesis Stages

• Initiation: The 80S eukaryotic ribosome dissociates into 40S and 60S subunits. eIF-3 and eIF-1 bind to the 40S subunit preventing re-association. GTP and eIF-2 bind, along with mRNA and methionine-tRNA (recognizing the AUG start codon). The ribosomal subunits re-associate, factors are released, and GTP is hydrolyzed.
• Elongation: There are three tRNA binding sites (A, P, and E) on the ribosome. Aminoacyl-tRNA binds to the A site, a peptide bond forms between the amino acids on the A and P sites. The ribosome moves along the mRNA, and the tRNA carrying the growing polypeptide chain moves from the A to the P site. The empty tRNA moves to the E site and is released.
• Termination: A stop codon (UAA, UAG, or UGA) appears in the A site. No tRNA recognizes these codons. Release factors (eRFs) bind to the A site, triggering the hydrolysis of the bond between the polypeptide chain and the tRNA in the P site. The ribosome subunits dissociate, releasing the completed polypeptide chain.

tRNA Charging (Figure 3)

• tRNA charging is the attachment of a specific amino acid to the 3' hydroxyl adenosine terminus of tRNA catalyzed by aminoacyl-tRNA synthetases. ATP is required.

Additional Notes

• Protein Maturation: Proteins are often modified after translation (post-translational modification) to become fully functional. Modifications include protein folding, proteolysis, phosphorylation, glycosylation, and others.
• Clinical Implications: Many antibiotics target bacterial protein synthesis, while some toxins inhibit eukaryotic translation, leading to various diseases.

This quiz focuses on RNA translation and the genetic code. You'll learn about the rules of the genetic code, the function of different RNAs in the translation process, and the significance of translation in health and disease. Dive into the process that converts mRNA into functional proteins.