IMAT Biology Lesson 4.7 | Reproduction + Inheritance | Translation

Summary

In this comprehensive biology lecture, Andy explains the central dogma of biology, focusing on protein synthesis, transcription, and translation. The process of mRNA processing, including 5' capping, polyadenylation, and splicing, is outlined, emphasizing their roles in translation. Key differences between prokaryotic and eukaryotic translation initiation are explored, such as the Shine-Dalgarno sequence in prokaryotes versus the Kozak sequence in eukaryotes. Additionally, the lecture discusses ribosomal function, translation elongation, and how rough ER is involved in protein synthesis. Finally, termination of translation and the role of release factors in protein release are highlighted.

Takeaways

• 🧬 mRNA undergoes processing before translation, including the addition of a 5' cap (7-methyl guanosine) to prevent degradation by exonucleases.
• 🧬 The 3' end of mRNA is polyadenylated with a poly-A tail (100-200 adenine bases) to protect against exonuclease activity.
• ✂️ Splicing removes introns from pre-mRNA using a spliceosome, leaving only exons for translation, which regulates gene expression.
• 🔄 In prokaryotes, transcription and translation occur simultaneously, while in eukaryotes, transcription happens in the nucleus and translation occurs in the cytoplasm.
• 📌 Prokaryotic ribosomes initiate translation by binding to the Shine-Dalgarno sequence, whereas eukaryotic ribosomes bind to the 5' cap and scan for the Kozak consensus sequence.
• ⚙️ Ribosomes consist of small and large subunits, with three key sites: A site (aminoacyl), P site (peptidyl), and E site (exit).
• 🧪 Peptidyl transferase in the large ribosomal subunit catalyzes peptide bond formation during elongation.
• ⛓️ Translation elongation involves the transfer of the growing peptide chain from the P site to the A site, GTP-driven translocation, and release of empty tRNA at the E site.
• 🏭 The rough endoplasmic reticulum forms when free ribosomes bind to mRNA with a signal peptide, guided by the signal recognition particle (SRP), enabling co-translational protein targeting.
• 🔚 Translation termination occurs when a release factor (RF) binds to the A site at a stop codon, cleaving the polypeptide from the P site, and dissociating the ribosomal subunits.
• 🧩 Signal peptidase cleaves the signal peptide from proteins in the ER lumen, allowing proper folding into secondary, tertiary, and quaternary structures.

Q & A

What is the importance of the 5' cap on mRNA in eukaryotic cells?

The 5' cap, which consists of a 7-methyl guanosine with a triple phosphate, is essential for protecting the mRNA from exonucleases that could degrade it. It also helps in the initiation of translation by preventing the degradation of mRNA in the cytoplasm.

How does the poly-A tail contribute to mRNA stability?

The poly-A tail, added to the 3' end of the mRNA, consists of 100 to 200 adenine bases. It prevents exonucleases from degrading the mRNA in the cytoplasm, helping to stabilize the mRNA and regulate its translation.

What is the role of spliceosomes in mRNA processing?

Spliceosomes are responsible for splicing out introns from the pre-mRNA. They create loops with the introns and then cut them out, leaving only the exons. This process ensures that only the coding regions of the mRNA are included in the final transcript, enabling accurate translation.

How does translation initiation differ between prokaryotes and eukaryotes?

In prokaryotes, translation begins while transcription is still ongoing, with ribosomes binding to the Shine-Dalgarno sequence upstream of the AUG start codon. In eukaryotes, transcription occurs in the nucleus, and the mRNA must be fully processed and exported to the cytoplasm before translation begins. Ribosomes in eukaryotes bind to the 5' cap and scan for the Kozak consensus sequence before translation begins.

What is the Shine-Dalgarno sequence and its role in prokaryotic translation?

The Shine-Dalgarno sequence is a specific nucleotide sequence upstream of the AUG start codon in prokaryotes. It binds to the ribosome, positioning it correctly for translation to begin. This sequence helps in the initiation of translation by aligning the ribosome with the start codon.

What is the function of the Kozak consensus sequence in eukaryotic translation?

The Kozak consensus sequence is a specific sequence in eukaryotes that helps identify the correct AUG start codon. The ribosome binds to the 5' cap, then scans the mRNA in the 5' to 3' direction until it finds the first AUG codon within the Kozak sequence, ensuring accurate translation initiation.

How do ribosomes in the cytoplasm interact with mRNA during translation?

Ribosomes in the cytoplasm bind to the mRNA's 5' cap and begin scanning for the start codon. The small ribosomal subunit binds first, and once it locates the start codon, the large subunit joins, and translation begins. The ribosome moves along the mRNA, elongating the polypeptide chain by adding amino acids based on the mRNA codons.

What is the role of peptidyl transferase during translation elongation?

Peptidyl transferase is an enzyme found in the large ribosomal subunit that catalyzes the formation of peptide bonds between the growing polypeptide chain and the incoming amino acid. It facilitates the transfer of the peptide chain from the tRNA in the P site to the tRNA in the A site during elongation.

How does the ribosome ensure accurate translation during elongation?

During elongation, the ribosome reads the mRNA in the 5' to 3' direction. It binds the appropriate tRNA with the correct amino acid to the A site, ensures proper codon-anticodon matching, and catalyzes peptide bond formation via peptidyl transferase. The ribosome then shifts, and the process repeats, elongating the polypeptide chain.

How does the endoplasmic reticulum (ER) become rough during translation?

The ER becomes rough when ribosomes, which are initially free in the cytoplasm, attach to it during translation. As soon as the nascent polypeptide chain begins to emerge from the ribosome, a signal peptide directs the ribosome to the ER. This process converts the smooth ER into rough ER by recruiting ribosomes to its surface.