Proteins Translation 4c'

Summary

This lecture delves into the intricacies of translation and the genetic code, focusing on the three stages of translation: initiation, elongation, and termination. It distinguishes between prokaryotic and eukaryotic initiation, highlighting the role of the Shine-Dalgarno sequence in prokaryotes and the scanning model in eukaryotes. The lecture also covers the essential components of the translation machinery, such as ribosomes, tRNAs, and various initiation and elongation factors. It further explains the process of peptide bond formation and the role of release factors in terminating translation. Additionally, it touches on the impact of antibiotics that target bacterial translation without affecting eukaryotic cells.

Takeaways

• 🔬 Translation is a three-step process: initiation, elongation, and termination, which is crucial for protein synthesis.
• 🌐 In prokaryotes, translation initiation involves the Shine-Dalgarno sequence, which helps the small ribosomal subunit bind to the mRNA at the correct site.
• 🔑 The initiator tRNA carries a modified methionine (N-formyl-methionine) in prokaryotes, which is the first amino acid added during translation.
• 🧬 Eukaryotic initiation differs as it lacks the Shine-Dalgarno sequence and instead uses a scanning mechanism to find the start codon, often the first AUG.
• 🌟 Both prokaryotic and eukaryotic translation initiations require initiation factors and GTP for energy.
• 🔄 Elongation is a cyclic process involving tRNA binding, peptide bond formation, and translocation, which is facilitated by elongation factors and GTP.
• 🏁 Termination occurs when a ribosome encounters a stop codon, and release factors help in detaching the completed polypeptide chain from the tRNA.
• 🌱 Eukaryotes have a single release factor, eRF, compared to multiple release factors in prokaryotes, highlighting a key difference in the termination process.
• 📚 Translation can occur in the cytoplasm or on the endoplasmic reticulum, influenced by signal peptides that direct the ribosome and mRNA to the ER for proteins destined for secretion or organelles.
• 💊 Antibiotics often target the differences in translation mechanisms between prokaryotes and eukaryotes, inhibiting bacterial protein synthesis without affecting human cells.

Q & A

What are the three steps of translation?

The three steps of translation are initiation, elongation, and termination.

What is the role of the Shine-Dalgarno sequence in translation initiation?

The Shine-Dalgarno sequence is a purine-rich sequence upstream from the AUG initiation codon in procaryotes that helps align the mRNA and small ribosomal subunit at the correct location for translation initiation.

How does the initiation of translation differ between procaryotes and eukaryotes?

In procaryotes, translation initiation involves the Shine-Dalgarno sequence and binding directly to the ribosomal binding site. In eukaryotes, initiation involves binding of the 40S subunit to the 5' cap of the mRNA with the help of eukaryotic initiation factors, followed by scanning for the first AUG codon.

What is the function of the initiator tRNA?

The initiator tRNA carries the first amino acid, methionine (or its modified form, formylmethionine in procaryotes), to the ribosome during translation initiation.

What is the role of GTP in translation initiation?

GTP provides the energy required for the binding of the small ribosomal subunit to the mRNA and for the association and dissociation of initiation factors during the initiation process.

How does elongation factor Tu (EF-Tu) contribute to translation elongation?

Elongation factor Tu (EF-Tu) in procaryotes helps deliver aminoacyl-tRNAs to the ribosome by facilitating their binding to the A site of the ribosome.

What is the significance of the peptidyl transferase center in translation?

The peptidyl transferase center, which is part of the 23S rRNA in the large ribosomal subunit, catalyzes the formation of peptide bonds between amino acids during translation elongation.

How does translocation occur during translation elongation?

Translocation is facilitated by elongation factors (EF-G in procaryotes and EF2 in eukaryotes) and GTP, which move the ribosome one codon along the mRNA, shifting the peptidyl-tRNA from the A site to the P site and the empty tRNA from the P site to the E site.

What are the termination codons, and how does translation termination occur?

The termination codons are UAA, UAG, and UGA, which do not code for any amino acids. During translation termination, release factors bind to these codons in the A site, stimulating peptidyl transferase to cleave the bond between the polypeptide chain and the tRNA, releasing the completed protein.

How can multiple ribosomes translate a single mRNA molecule simultaneously?

Multiple ribosomes can bind to a single mRNA molecule and translate it simultaneously, forming a polyribosome (or polysome) complex, where each ribosome is translating a different section of the mRNA into a polypeptide chain.

How do antibiotics target bacterial translation without affecting eukaryotic translation?

Antibiotics target specific components or processes of bacterial translation that differ from those in eukaryotes, such as binding to the 30S ribosomal subunit in bacteria, which has a different structure than the 40S subunit in eukaryotes, thus inhibiting bacterial protein synthesis without affecting eukaryotic cells.