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Questions and Answers
What is the primary advantage of coupled transcription and translation in prokaryotes?
What is a polysome and its significance in prokaryotic translation?
Where do transcription and translation occur in eukaryotic cells?
During translation initiation in prokaryotes, what role does the ribosome play?
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Which of the following is NOT a post-translational modification?
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What is the primary role of the Shine-Dalgarno sequence in prokaryotic translation?
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Which component recognizes the AUG initiation codon during translation initiation?
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During the elongation phase of translation, where does the aminoacyl tRNA first enter the ribosome?
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What is the function of peptidyl transferase during translation?
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In eukaryotic translation, how does the process of initiation differ from that in prokaryotes?
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What role does EF-G play during translation termination?
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What is the role of RF1 and RF2 in translation termination?
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After the polypeptide is released during termination, what happens to the ribosomal subunits?
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Which of the following correctly describes the composition of ribosomes in prokaryotes?
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What is the direction of mRNA translation during protein synthesis?
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Which site on the ribosome is responsible for holding the growing protein chain during translation?
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In eukaryotic cells, which RNA polymerase is responsible for transcribing tRNA genes?
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What type of interactions are crucial for the clover-leaf shape of tRNA?
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Which of the following is true about the initiation of translation in prokaryotes?
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Which type of tRNA is involved in attaching the amino acid during translation elongation?
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During translation termination, what recognition factor is typically involved?
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Which of the following structures contributes to the site for tRNA exit after translation?
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How many types of rRNA molecules are typically found in eukaryotic ribosomes?
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What is the role of eukaryotic initiator factor (eIF-4E) in translation initiation?
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During translation elongation, what occurs at the A-site of the ribosome?
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What is the purpose of the poly-A tail in eukaryotic mRNA translation?
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What triggers the termination of translation?
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Which of the following correctly describes the scanning model of translation initiation in eukaryotes?
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What is the primary location for translation in prokaryotes?
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What happens to the uncharged tRNA during the translocation phase of translation?
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Which factor helps the ribosome recognize STOP codons during translation termination?
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How is peptide bond formation catalyzed during elongation?
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Which sequence is essential for the ribosome to accurately find the start codon?
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Study Notes
Translation Initiation in Prokaryotes
- Ribosomal subunit (30S) with initiation factors (IF-1 and IF-3) binds to mRNA's AUG start codon.
- Shine-Dalgarno sequence (RBS) on mRNA guides the 30S subunit to the correct binding site.
- Initiator tRNA (tRNA.fMET) carries the first amino acid, formyl methionine (fMET), and recognizes AUG.
Translation Elongation in Prokaryotes
- mRNA is read from 5' to 3'.
- Polypeptide chain synthesis proceeds from N-terminus to C-terminus.
- Elongation factors bind to tRNA after the correct amino acid is attached.
Codon Recognition
- Aminoacyl tRNA with its specific amino acid enters the ribosome at the A-site.
- Ribosome ensures correct base pairing between the tRNA anticodon and mRNA codon.
Peptide Bond Formation
- In the P-site, adjacent amino acids form peptide bonds.
- The bond between tRNA and its amino acid is broken first, then the amino acid is attached to the adjacent amino acid in the A-site via a peptide bond.
- This process is catalyzed by peptidyl transferase.
Translocation
- The ribosome moves along the mRNA one codon at a time, shifting from the P-site to the A-site.
- Uncharged tRNA moves to the E-site.
- Requires the activity of EF-G (elongation factor).
- EF-TU-GTP complex binds to the ribosome, GTP is hydrolyzed, and translocation occurs.
Translation Termination in Prokaryotes
- Release factors (RF1 and RF2) bind to stop codons (UAA, UAG, UGA), triggering peptidyl transferase to release the polypeptide from the P-site.
- RF3-GDP binds to the ribosome triggering the release of RF from the stop codon and the ribosome.
- GDP is converted to GTP, and RF3 hydrolyzes the GTP, detaching it from the ribosome.
- Ribosome recycling factor (RRF) mimics rRNA and binds to the A-site.
- EF-G binds, leading to translocation.
- RRF moves to the P-site and the uncharged tRNA to the E-site.
- Release of tRNA, EF-G, and RRF occurs, causing the ribosome subunits to dissociate from the mRNA.
Translation Initiation in Eukaryotes
- Eukaryotic initiation factor 4E (eIF-4E) binds to the 5' cap of mRNA.
- Scanning model: 40S subunit with MET-tRNA, multiple eIFs, and GTP binds to mRNA and scans for the start codon (AUG) within the Kozak sequence.
- Upon finding AUG, the 40S subunit binds, and the 60S subunit joins, replacing eIFs except for eIF-4F. This forms the 80S initiation complex.
- Initiator MET-tRNA binds to the mRNA at the P-site of the ribosome.
- Poly-A tail binds to PABPII (poly A binding protein II), which interacts with eIF-4G at the 5' cap, facilitating a loop in the mRNA.
Translation Elongation in Eukaryotes
- mRNA is read from 5' to 3'.
- Polypeptide chain synthesis proceeds from N-terminus to C-terminus.
- Elongation factors bind to tRNA after the correct amino acid is attached.
Codon Recognition (Eukaryotes)
- Aminoacyl tRNA with its specific amino acid enters the ribosome at the A-site.
- Ribosome ensures correct base pairing between the tRNA anticodon and mRNA codon.
Peptide Bond Formation (Eukaryotes)
- In the P-site, adjacent amino acids form peptide bonds.
- The bond between tRNA and its amino acid is broken first, then the amino acid is attached to the adjacent amino acid in the A-site via a peptide bond.
- This process is catalyzed by peptidyl transferase.
Translocation (Eukaryotes)
- The ribosome moves along the mRNA one codon at a time, shifting from the P-site to the A-site.
- Uncharged tRNA moves to the E-site.
- Requires the activity of EF-G (elongation factor).
- EF-TU-GTP complex binds to the ribosome, GTP is hydrolyzed, and translocation occurs.
Translation Termination in Eukaryotes
- STOP-codon is recognized (UAG, UAA, UGA), signaling the end of translation.
- These codons do not code for any amino acid, and no tRNA has an anticodon for them.
- The ribosome recognizes the stop codon with the help of release factors (RFs).
Polysomes
- Multiple ribosomes simultaneously translate a single mRNA strand, increasing the efficiency of protein synthesis in both prokaryotes and eukaryotes.
Prokaryotic vs. Eukaryotic Translation
-
Prokaryotes:
- Translation can begin even before transcription is complete because there is no nucleus.
- Transcription and translation occur in the cytoplasm.
- Coupled Transcription and Translation: Ribosomes can bind to the 5' end of the growing mRNA and begin translating almost simultaneously with transcription.
- Polysomes: Multiple ribosomes translate a single mRNA strand.
-
Eukaryotes:
- Transcription occurs in the nucleus, and translation occurs in the cytoplasm.
- mRNA undergoes processing (capping, splicing, polyadenylation) before translation.
Post-Translational Modifications
- Enhance the functional diversity of the proteome.
- Involve covalent addition of functional groups, proteolytic cleavage of regulatory subunits, or degradation of the entire protein.
Types of Post-Translational Modifications
- Phosphorylation: Addition of phosphate groups to amino acid side chains, often regulating protein activity.
- Glycosylation: Attachment of sugars to amino acids, affecting protein folding, stability, and cell signaling.
- Ubiquitination: Tagging proteins with ubiquitin, a small polypeptide chain, for degradation by the proteasome.
The Proteasome
- Known as the "protein shredder".
- Degrades ubiquitinated proteins.
- Involves three enzymes: E1 (activating ubiquitin), E2 (ubiquitin conjugating enzyme), and E3 (ubiquitin ligase).
Ubiquitination Process
- Ubiquitin binds to E1 using ATP.
- E3 binds to the target protein and E2.
- E1 and E2 bind together.
- Ubiquitin is transferred from E1 to E2.
- Ubiquitin is transferred to the target protein.
- E1 detaches from E2 and binds to a new ubiquitin molecule.
- This process repeats, adding multiple ubiquitin molecules to the target protein, marking it for degradation by the proteasome.
Structure of tRNA
- All tRNA molecules have the sequence 5'-CCA-3' at their ends.
- They have chemically modified bases, contributing to their unique identity.
- tRNA exhibits a clover-leaf shape due to complementary base pairing within the RNA strand.
- Four loops are formed, with the second loop containing the anticodon sequence.
- Loop 1 is the D-arm, and loop 3 is the T-arm.
tRNA in Bacteria
- Not found in numerous copies in the genome.
- Transcribed by RNA polymerase.
- During transcription, pre-tRNA is produced.
tRNA in Eukaryotes
- Genes are repeated multiple times in the genome.
- Transcribed by RNA polymerase III.
- During transcription, pre-tRNA is produced, which can contain introns.
Amino Acid Attachment to tRNA
- The amino acid attaches to the 3' end of tRNA through a covalent linkage between the carboxyl group of the amino acid and the 3' or 2'-OH group of the ribose sugar in the adenine at the tRNA's 3' end.
Aminoacyl-tRNA Synthetases
- These enzymes couple the correct amino acid to its corresponding tRNA molecule.
Wobble Pairing
- Explains how a single tRNA can recognize multiple codons that differ only in their third position.
- Allows for flexibility in the base pairing between the third position of the codon and the first position of the anticodon.
Ribosomes
- Composed of approximately 35% proteins and 65% ribosomal RNA (rRNA).
- Prokaryotes have three types of rRNA, while eukaryotes have four.
- Ribosomes read mRNA from 5' to 3'.
- Eukaryotic ribosomes can be found in the cytoplasm, attached to the endoplasmic reticulum, or on the outside of the nuclear envelope.
- Composed of two subunits that come together to translate mRNA.
Ribosomal Subunits
-
Prokaryotes:
- Small subunit: 30S
- Large subunit: 50S
-
Eukaryotes:
- Small subunit: 40S
- Large subunit: 60S
Ribosomal Sites
- A-site (aminoacyl-tRNA site): Part of both the large and small subunits. Entry point for tRNA molecules with attached amino acids.
- P-site (peptidyl-tRNA site): Part of both the large and small subunits. Holds the growing polypeptide chain and its associated tRNA.
- E-site (exit site): Only part of the large subunit. Exit site for uncharged tRNA molecules and the newly synthesized polypeptide chain.
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