Lecture 2.2: Translation Flashcards
What translation?
It is the process of synthesis of proteins from an mRNA template
Where does translation take place?
Ribosomes
RER or free ribosomes in cytosol
Structure of Eukaryotic ribosomes
60S and 40S subunits
Make a 80S ribosome
rRNA: where is it processed? What happens to it further along the line? (3)
rRNAs are processed and assembled into ribosome subunits in the nucleolus- the main site of “ribosome biogenesis”
In association with ribosomal proteins the rRNA further folds into 3D-structure forming a ribosome
The rRNA plays a catalytic role (peptide bonds) in translation
tRNA: structure + how many types?
Clover Structure
Anticodon to mRNA sequence triplets
Is attached to an amino acid at 3’ (all tRNAs have CCA at this end)
Cells contain about 40 different tRNAs that are acceptors for the 20 AAs
How do amino acids attach to tRNAs?
Attachment to AAs to tRNAs is mediated by a group of enzymes called “aminoacyl tRNA synthetases”
Step 1: The AA is activated by reaction with ATP to form a “aminoacyl AMP synthetase intermediate”
Step 2: The activated AA the joins the 3’ terminus of the acceptor tRNA and AMP is released
What is codon-anticodon “wobble”
Some tRNAs are constructed so they require accurate base pairing at only the first 2 possible of the codon
This they can tolerate a mismatch (“wobble”) at the 3rd position
This allows a single tRNA species to recognise more than 1 codon
Frameshift mutations
A type of mutation involving the insertion or deletion of a nucleotide
This shifts the grouping of triplets and changes the reading frame
Translation: Initiation
Begins at a specific initiation site (on the 5’ end, usually AUG, always Met)
The 5’ terminal of both pkaryotic + ekaryotic mRNAs are non-coding up until first Met
Monocistronic
Eukaryotic mRNAs are monocistronic, encoding only a single protein
Polycistronic
Prokaryotic mRNAs are frequently polycistronic- they encode multiple proteins
Translation: Initiation (Shine-Dalgarno)
Initiation codons in prokaryotic mRNAs are preceded by a specific sequence called “Shine-Dalgarno” sequence, this aligns with mRNA on ribosome
Translation: Initiation (eIFs)
Initiation of translation in eukaryotes requires at least 12 non-ribosomal proteins called eIFs (eukaryotic initiation factors)
These bind to the 40S ribosomal subunit
The ribosome then scans down the mRNA to identify the first AUG initiation codon
The 60S and 40S subunits then form a complex and begin translation
Translation: Elongation
Small subunit is framework on which tRNAs are accurately matched to mRNA codons
Large subunits catalyses peptide bond formation
Ribosome contains 4 binding sites, one for mRNA, 3 for tRNAs (A, P, E)
Translation: Elongation (How do the tRNA binding sites work?)
[tRNA containing Met is the only type that can directly bind to P site (middle)]
1) An aminoacyl-tRNA (Ala) binds to vacant A site on ribosome
2) A new peptide bond is formed between Met and Ala leaving tRNA at P site empty (uncharged)
3) The ribosome moves along to next mRNA triplet, the uncharged tRNA leaves ribosome at the E site, all tRNAs shift, thus leaving A site open for new charged tRNA
