Model Answer
0 min readIntroduction
Protein synthesis, also known as translation, is the process by which cells build proteins. It is a fundamental biological process essential for all life forms. While the basic principles remain conserved, significant differences exist between protein synthesis in prokaryotic and eukaryotic cells, largely due to the compartmentalization of eukaryotic cells. The process involves ribosomes, messenger RNA (mRNA), transfer RNA (tRNA), and various protein factors. Understanding these differences is crucial for comprehending cellular function and dysfunction, particularly in the context of disease and drug development. This answer will provide an illustrated account of protein synthesis in eukaryotic cells, compare it with prokaryotic protein synthesis, and detail the mechanisms of protein translocation into the endoplasmic reticulum (ER) lumen.
Protein Synthesis in Eukaryotic Cells
Eukaryotic protein synthesis is a complex process occurring in the cytoplasm and, for proteins destined for secretion or membrane integration, also involves the endoplasmic reticulum (ER). It can be divided into three main stages: initiation, elongation, and termination.
Initiation
Initiation is a highly regulated process. It begins with the binding of the small ribosomal subunit (40S) to the mRNA, guided by the 5' cap. Initiation factors (eIFs) play a crucial role in this step. A specific initiator tRNA carrying methionine (Met-tRNAi) binds to the start codon (AUG). Finally, the large ribosomal subunit (60S) joins the complex, forming the functional 80S ribosome.
Elongation
Elongation involves the sequential addition of amino acids to the growing polypeptide chain. tRNAs, each carrying a specific amino acid, enter the A-site of the ribosome, guided by the mRNA codon. Peptidyl transferase, an enzymatic activity of the ribosome, catalyzes the formation of a peptide bond between the amino acids. The ribosome then translocates along the mRNA, moving the tRNA in the A-site to the P-site, and the tRNA in the P-site to the E-site, where it is ejected. This cycle repeats until a stop codon is reached.
Termination
Termination occurs when the ribosome encounters a stop codon (UAA, UAG, or UGA) in the mRNA. Release factors bind to the stop codon, causing the polypeptide chain to be released from the ribosome. The ribosome then dissociates into its subunits.
Protein Synthesis in Prokaryotic Cells
Prokaryotic protein synthesis is similar to eukaryotic protein synthesis but has several key differences. It occurs entirely in the cytoplasm, as prokaryotes lack a nucleus and other membrane-bound organelles.
Initiation
In prokaryotes, the small ribosomal subunit (30S) binds directly to the mRNA via the Shine-Dalgarno sequence, a ribosome-binding site upstream of the start codon. Formylmethionine-tRNAfMet is the initiator tRNA. Initiation factors are fewer and simpler than in eukaryotes.
Elongation & Termination
Elongation and termination are broadly similar to eukaryotes, but utilize different elongation factors and release factors. Prokaryotic ribosomes are 70S (composed of 30S and 50S subunits), smaller than their eukaryotic counterparts.
Comparison of Prokaryotic and Eukaryotic Protein Synthesis
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| Location | Cytoplasm | Cytoplasm & ER |
| Ribosome Size | 70S | 80S |
| Initiator tRNA | Formylmethionine-tRNAfMet | Methionine-tRNAi |
| mRNA Binding | Shine-Dalgarno sequence | 5' Cap |
| Initiation Factors | Fewer & simpler | More complex (eIFs) |
| Coupling with Translation | Transcription and translation are coupled | Transcription and translation are spatially separated |
Protein Translocation into the ER Lumen
Proteins destined for secretion, membrane integration, or localization within organelles like the ER, Golgi, or lysosomes are synthesized on ribosomes that become associated with the ER membrane. This process involves several key components:
Signal Sequence
Most secretory proteins contain a signal sequence, a short stretch of hydrophobic amino acids at the N-terminus. This sequence directs the ribosome to the ER membrane.
Signal Recognition Particle (SRP)
The SRP binds to the signal sequence as it emerges from the ribosome. This pauses translation and guides the ribosome-mRNA-SRP complex to the ER membrane.
Translocon
The SRP binds to the SRP receptor on the ER membrane. The ribosome then transfers to the translocon, a protein channel that forms a pore through the ER membrane. The signal sequence opens the translocon pore. As translation resumes, the polypeptide chain is threaded through the translocon and into the ER lumen. The signal sequence is typically cleaved off by signal peptidase within the ER lumen.
Conclusion
In conclusion, protein synthesis is a highly conserved yet distinctly different process in prokaryotic and eukaryotic cells. Eukaryotic protein synthesis is more complex, involving compartmentalization and a greater number of regulatory factors. The translocation of proteins into the ER lumen is a crucial step for proteins destined for secretion or membrane integration, relying on the coordinated action of signal sequences, SRP, and the translocon. Understanding these processes is fundamental to comprehending cellular biology and its implications for health and disease.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.