L9 Translation Flashcards
What are ribosomes made up of?
Ribosomes are large, abundant RNP particles, consisting of two unequal subunits.
Ribosome consists of two RNP subunits, what are they and what are their functions?
Small subunit: responsible for decoding the mRNA sequence.
Large subunit: responsible for forming peptide bonds between amino acids.
Explain ribosome synthesis in eukaryotes.
Ribosome synthesis is a complex and energy-intensive process. It involves:
- Transcription and early processing of rRNA occurs in the nucleolus
- Later processing and assembly occurs in the nucleoplasm and cytoplasm
- Export from the nucleus: Functionally active ribosomes are generated after export
What is the function of ribosomes?
Ribosomes are the cellular machinery responsible for protein synthesis. They translate the genetic information encoded in mRNA into amino acid sequences, which are then assembled into proteins.
How many tRNAs are bound to the ribosome during translation elongation?
Two tRNAs are bound to the ribosome at any given time during translation elongation.
What are the three steps of translation elongation?
- Aminoacyl-tRNA binding
- Peptide bond formation
- Translocation of the ribosome along the mRNA
What factor delivers aminoacyl-tRNA to the ribosome? (A site)
Elongation factor EF1A (EF-Tu in prokaryotes)
What factor is required for translocation?
Elongation factor EF2 (EF-G in prokaryotes)
What are EF1A and EF2 ?
EF1A and EF2 are GTPases
Two GTP molecules are hydrolysed per elongation cycle
How are initiator methionyl-tRNAs different from elongator methionyl-tRNAs?
- Initiator methionyl -tRNA recognises the AUG start codon and is associated with eukaryotic initiation factor 2 (elF2, IF2 in prokaryotes)
- Elongator methionyl - tRNAs bind to internal AUG codons and are associated with EF1A
What is the role of eIF2?
eIF2 is a GTPase that facilitates the binding of initiator methionyl-tRNA to the start codon.
What selects the initiation codon in prokaryotic cells?
16S/rRNA (a small ribosomal subunit and the mRNA)
What ribosomal component interacts with the Shine-Dalgarno sequence in prokaryotes, and why is this interaction important?
The Shine-Dalgarno (SD) sequence within prokaryotic mRNAs is recognized by base-pairing with nucleotides at the 3’ end of the 16S rRNA. This interaction is crucial for the initiation of translation in prokaryotes. The SD sequence, located upstream of the start codon (AUG), helps position the ribosome correctly on the mRNA.
How does the initiation complex assemble at the 5’ end of mRNA in eukaryotes?
The initiator tRNA (bound to eIF2), together with the small ribosomal subunit, is assembled at the 5’ end of the mRNA through interaction with the cap-binding complex (CBC).
Describe the process of pre initiation complex during translation initiation
The pre-initiation complex scans along the mRNA, using the helicase activity of CBC, until it finds an AUG codon within an appropriate sequence context: the Kozak sequence.
What happens after the start codon is selected?
After selection of the start codon, the large ribosomal subunit is recruited.
What recognises stop codons?
Stop codons are recognised by protein termination (or release) factors, not tRNAs.
What are the two main steps in translation termination?
- Initial binding of eukaryotic release factor 1 (eRF1, RF1 or RF2 in prokaryotes) triggers peptide hydrolysis.
- Subsequently, eRF3 (RF3 in prokaryotes) allows release of RF1/RF2 from the ribosome. RF3 is a GTPase.
How many tNRA binding sites are there within the ribosomes?
Three non overlapping tRNA binding sites (A,P and E) are found at the subunit interface
A and P are the binding sites that has the two amino acids where a peptide bond is formed
What are the two distinct methionyl tRNAs?
- Elongator methionyl- tRNA
- Initiator methionyl - tRNA
How many tRNA binding sites are there on a ribosome, what tRNAs bind there and when?
1) A- site (Aminoacyl site): Entry point for incoming aminoacyl-tRNAs
2) P- site (Peptidyl site): holds tRNA that forms peptide bond with amino acid in A site
3) E- Site (exit site): Holds tRNA that has just give up its amino acid, ready to be released
Which GTPases function in the process of translation?
1) elF2 (Eukaryotic initiation factor 2) - binds to tRNA to the small ribosomal subunit
2) EF1A (Eukaryotic elongation factor 1 alpha) - Binds aminoacyl- tRNAs and delivers them to the A site of ribosome
3) EF2 (Eukaryotic elongation factor 2) - Translocates the ribosome along the mRNA by one codon
4) eRF3 (Eukaryotic release factor 3) - Binds to the release factor RF1, which recognises stop codons on the mRNA
Compare the process of translation initiation in prokaryotes and eukaryotes.
Prokaryotes:
- Simple
- Transcription and translation occur simultaneously
- Shine- Dalgarno sequence
- Fewer initiation factors
Eukaryotes:
- More complex
- Transcription in nucleus, translation in cytoplasm
- 5’ and poly-A tail: Recognised by initation factors to help recruit ribosomes
- Scanning from mRNA from the 5’ end to find AUG
