Protein synthesis Flashcards
Describe the structure of mRNA at the start of translation
5' 7-methylguanosine cap 5' UTR (untranslated region) Start codon (AUG) Stop codon 3' UTR Poly-A tail
What is the most common start codon, and what does it code for?
AUG
Methionine
How many codons are there for how many amino acids?
What can DNA be described as as a result?
64 for 20
Degenerate
What are the 3 stop codons?
UAA
UAG
UGA
What direction is mRNA translated?
5’ to 3’
What is the entry site for a ribosome?
7-methylguanosine cap
What is the difference between ribosomes in eukaryotes/prokaryotes?
Eu: 80s ribosomes, 49 proteins
Pro: 70s ribosomes, 33 proteins
What does the difference in eu/pro ribosomes allow the use of?
Antibiotics secreted from bacteria/fungi which only act on prokaryotes
They give a selective advantage.
Name the three stages of translation
- Initiation
- Elongation
- Termination
Where does an amino acid bind to tRNA?
3’
Which enzyme ‘picks up’ an amino acid, and binds it to tRNA?
How does it work?
aminoacyl tRNA synthetase - specific to each amino acid.
Amino joined to enzyme using ATP. Forms E-AMP-Aminoacid
tRNA removes enzyme and AMP
What is the first stage of initiation?
40s + 60s subunits dissociate into cytoplasm looking for mRNA
What happens when the 40s subunit locates mRNA?
Preinitiation complex assembles.
What does the preinitiation complex consist of?
Met-tRNA, eIF’s 40s subunit
Met as its always the first AminoA
What is eIF?
An initiation factor
Group of proteins
What happens to the preinitation complex?
The 60s subunit binds.
A full ribosome is formed.
What do eIFs do? Name them (3)
eIF 4E + 4G bind to 5’ cap
eIF2 contains GTP which is hydrolysed to bind 60s when AUG is found
Name the 2 sites of a ribosome in elongation
P site - on the left (peptidal)
A site on the right of the P site (amino acyl)
Describe elongation (3 steps)
- Next tRNA binds to a site
- Peptide bond formed between 2 amino acids on P and A sites by peptidyl transferase.
- Translocation of tRNA from A to P, (previous P dissociates)
Repeat
How quick is elongation?
15 codons/second
Describe termination (3 steps)
- Stop codon recognised
- Release factors bind to empty A site.
- Protein dissociates, ribosome dissociates
What is a polyribosome?
Several ribosomes working on a single strand of mRNA.
Protein synthesis takes place in the cytoplasm with which exception?
Mitochondrial protein synthesis.
What is the name for the first 20-24 amino acids?
Signal sequence
What does the signal sequence bind to?
Signal reception particle on ER
What happens when a signal sequence is recognised by a SRP?
Translation halts, SS binds to SRP, translation resumes
Protein channel forms in ER, and protein enters.
What happens when the preproprotein has entered the ER?
The signal sequence is cleaved by signal peptidase creating a proprotein.
What will be present is the protein is destined to become a transmembrane protein?
Extra hydrophobic sequence at C terminus which integrates into membrane
What allows the signal sequence to easily travel across membranes?
It is hydrophobic. (Lipophilic)
Name the ways a proteins tertiary structure can be modified!
There are 6/7
Disulphide bond formation Proteolytic cleavage Glycosylation (addition of carbohydrate) Phosphorylation (addition of phosphate) Prenylation, Acylation (addition of lipid groups) Hydroxylation
