TRANSLATION Flashcards
genetic code
Code is triplet.
Code is non-overlapping. Each nucleotide
is part of only one codon and is read only once.
Code is almost universal. Most codons
have the same meaning in different organisms
(e.g., not true for mitochondria).
Code is degenerate. Met and Trp are the only exceptions.
The clover leaf form of tRNA
- The 5’-terminal phosphate group.
- The acceptor stem is a 7-bp stem made by the base pairing of the 5’-terminal nucleotide with the 3’-terminal nucleotide (which contains the CCA 3’-terminal group used to attach the amino acid).
- The CCA tail is a CCA sequence at the 3’ end of the tRNA molecule. This sequence is important for the recognition of tRNA by enzymes critical in translation. In eukaryotes, the CCA sequence is added during processing and therefore does not appear in the tRNA gene.
- The D arm is a 4 bp stem ending in a loop that often contains dihydrouridine.
- The anticodon arm is a 5-bp stem whose loop contains the anticodon.
- The T arm is a 5 bp stem containing the sequence TΨC where Ψ is a pseudouridine.
- Bases that have been modified, especially by methylation, occur in several positions outside the anticodon. The first anticodon base is sometimes modified to inosine (derived from adenine)
Amino acid activation
Amino acids are attached to their corresponding tRNAs by an ester bond to give aminoacyl-tRNAs.
This is catalyzed by aminoacyl-tRNA synthetases.
The reaction is downhill, ensuring irreversibility:
Wobbling
Two different leucine codons (CUC, CUU) can be read by the same leucine tRNA molecule, contrary to regular base-pairing rules.
Initiation prokaryotes
Shine-Delgarno (SD) site consists of 3-9 contiguous bases in the mRNA that base pair with the 3’ end of 16S rRNA
and is located optimally 5 nt upstream of the initiator codon
Initiation Eukaryotes
first AUG codon
downstream of the 5’ cap
Peptide bond formation
synthesis of the peptide bound is catalyzed by an adenin in 23S RNA
Termination
Proteins known as “release factors” recognize the stop codon (UGA, UAG, or UAA) at the A site
In E. coli RF-1 recognizes UAA and UAG, RF-2 recognizes UAA and UGA.
RF-3 binds GTP and enhances activities of RF-1 and –2.
Presence of release factors with a nonsense codon at A site transforms the peptidyl transferase into a hydrolase, which cleaves the peptidyl chain from the tRNA carrier
Hydrolysis of GTP is required for disassociation of RFs, ribosome subunit and new peptide
Translation Initiation – Eukaryotes
Eukaryotic Initiation Factors (eIF4A, eIF4B, and eIF4G) associate with the 5’ cap, the 40S subunit, and initiator tRNA
Complex moves 5’ to 3’ unwinding the mRNA until an initiation site (AUG) is discovered
Initiation factors are released and 60S subunit binds
inhibition
Inhibition of translation through phosphorylation of eIF2 Chloroamphenicol puromycin ricin dipheria
elongation
AAs are Brought to A-Site by tRNA Bound to eEF1A • GTP
EF-TU
tRna inserted at A site , GTP in pro
Ef-ts
helps releasing EF-tu and hydrolysis GTP in pro
EF-G
tRNa from asite to p site in pro
EF-1 aplha
tRna inserted at A site , GTP in eu
EF-1 beta gamma
helps releasing EF-1 aplha in eu
EF-2
tRNa from asite to p site in eu
eRF
Binds to A site catalysing clevage of a.a from tRNA, poly p relased
ribosome dissociatea
Rf1 RF2
recognized UAA, UAG, RF2. UAA, UGA
RF3
hydrolysis of GTP
FOrmation of ester bond to get aminoacyl-tRNA
aminoacyl-tRNA synthetases.
Sequence in mRNA eukaryotic between cap and start of translation
apo B
eIF2
eIF2 binds to GTP and forms a complex with Met-tRNA initiator.
- This complex (ternary complex = TC) docks on small subunit (40S) forming 43S PreInitiation Complex (PIC).
- TC scans the mRNA until the first start codon AUG.
eIF2α
When cell is under stress, eIF2α is phosphorylated. This inactivates GTP recycling protein eIF2B. Prevents formation of 43S PIC => blocks protein synthesis.
eIF4
- eIF4F = eIF4A+G+E = cap binding complex
- eIF4F binds to the mRNA G-cap (5’) by the 4E part of complex.
cistron
a section of a DNA or RNA molecule that codes for a specific polypeptide in protein synthesis.
how many tRNA needed to recognize all a.a codons
32 tRNAs
3’-UTR
A section of (mRNA) that follows the translation termination codon. It contains regulatory regions that post-transcriptionally influence gene expression.
can influence polyadenylation, translation efficiency, localization, and stability of the mRNA. IT contains both binding sites for regulatory proteins And microRNAs that degrade RNA
(elF) 4A
A helicase elF4A during translation initiation is to unwind the mRNA secondary structure in the 5’ UTR to facilitate ribosome binding
eIF4E
directing ribosomes to the cap structure of mRNAs.
rate-limiting component of the eukaryotic translation
