Translating the Genetic Code Flashcards
What is the A-site?
the ribosomal site most frequently occupied by aminoacyl-tRNA. The aminoacyl-tRNA in the A-site functions as the acceptor for the growing protein duringpeptide bondformation
What are chaperones?
proteins that assist the covalent folding or unfolding and the assembly or disassembly of other macromolecular structures
What is the E-site?
the ribosomal site harbouring decylated tRNA on transit out from the ribosome.
What is EF-Tu?
a prokaryotic elongation factorresponsible for catalyzing the binding of an aminoacyl-tRNA (aa-tRNA) to the ribosome
What is eIF2?
aeukaryotic initiation factor. It is required in the initiation oftranslation. eIF2 mediates the binding oftRNAiMetto the ribosome in aGTP-dependent manner
What is the endoplasmic reticulum?
labyrinthine membrane-bounded compartment in the cytoplasm of eukaryotic cells, where lipids are synthesised and membrane-bound proteins and secretory proteins are made
What is fMet?
a derivative of the amino acid methionine in which a formyl group has been added to the amino group. It is specifically used for initiation of protein synthesis from bacterial and organellargenes, and may be removed post-translationally.
What is the golgi apparatus?
complex organelle in eukaryotic cells, centered on a stack of flattened, membrane-enclosed spaces, in which proteins and lipids transferred from the endoplasmic reticulum are modified and sorted. It is the site of synthesis of many cell wall polysaccharides in plants and extracellular matrix glycosaminoglycans in animal cells
What is a nascent polypeptide chain?
The formingpolypeptide chainthat is attached to the 50 S subunit of a ribosome through a molecule of tRNA. The free end of thenascent polypeptidecontains the N-terminal amino acid
What is the P-site?
the ribosomal site most frequently occupied by peptidyl-tRNA, i.e. the tRNA carrying the growing peptide chain.
What is a peptide bond?
a covalent chemicalbondlinking two consecutive amino acid monomers along apeptideor protein chain
What does periplasmic mean?
The zone between the cytoplasmic membrane and the outer membrane in Gram-negative bacteria
What is a polysome?
mRNA engaged with multiple ribosomes in the act of translation
What are protein release factors?
aproteinthat allows for the termination of translation by recognizing the termination codon or stop codon in an mRNA sequence
What is a reading frame?
the phase in which nucleotides are read in sets of three to encode a protein. An mRNA molecule can be read in any one of three reading frames, only one of which will give the required protein
What is ubiquitin?
small, highly conserved protein present in all eukaryotic cells that becomes covalently attached to lysines of other proteins. Attachment of a short chain of ubiquitins to such a lysine can tag a protein for intracellular proteolytic destruction by a proteosome
True or False: mRNA is the most abundant RNA in the cell
False - rRNA and tRNA are the most abundant RNAs in the cell
Where are ribosomes located?
In the cytosol of the cell
Which subunit of the rRNA does the mRNA bind to?
16S
Which part of the mRNA binds with the ribosome in prokaryotes and eukaryotes?
Eukaryotes - 5’ cap
Prokaryotes - ribosome binding site (RBS)
What does the large subunit of the ribosome do?
Catalyses the peptide bond formation of the polypeptide
How are amino acids activated?
Amino acids are activated through their binding to the appropriate tRNA molecule with help from the appropriate tRNA synthetase
What are the steps of protein synthesis?
- Activation of AAs
- Initiation
- Elongation
- Termination
What is the aim of translation initiation?
To stabilise the mRNA on the ribosomal support and to set the correct reading frame
What are the steps of translation initiation?
- Binding mRNA to small ribosomal subunit
- Binding of the initiation tRNA (fmet tRNA) at the start codon (AUG)
- Assembly of the complete initiation complex
In prokaryotes, what assists the assembly of the rRNA complex?
Initiation factors
How is the reading frame set in prokaryotes?
A purine-rich sequence known as the Shine-Dalgarno sequence is found upstream of the start codon and is complementary to a sequence on the 16S rRNA subunit.
What is the role of eIF2?
The same as IF2, it forms a complex with the initiator fmet tRNA allowing the complex to bind with the P-site of the small rRNA subunit
What are the steps of translation elongation?
- Binding of next charged AA tRNA
- Peptide bond formation
- Translocation
- Release of the deacylated tRNA
Which site does the aminoacyl tRNA complex bind to and how does the complex dissociate?
A-site - The tRNA is in a complex with EF-Tu (GTP). The hydrolysis of GTP releases EF-Tu and GDP
True or False: The incorrect aminoacyl tRNA never binds at the A-site
False - This occurs but there is preferential dissociation of incorrect tRNA
How does translation termination occur?
Termination is signalled by the stop codons (UAA, UAG, UGA) and is recognised by protein release factors. The release factors cause the hydrolysis of the last peptidyl-tRNA bond releasing the polypetide and tRNA, allowing the ribosomal subunits to dissociate
What are the possible chemical modifications of prokaryotic polypeptides?
- removal of formyl group on fMet
- AA modifications
What are the possible chemical modifications of eukaryotic polypeptides?
- proteolytic maturation (e.g. pro-peptide removal)
- complex folding processes (chaperone-assisted, disulphide bonds)
- AA modifications (glycosylation, phosphorylation)
How does chaperone-assisted folding work?
The incorrectly or incompletely folded protein enters the protein chaperone which has hydrophobic protein-binding sites. The GroES cap is added allowing the protein to fold. The cap and the correctly folded protein is then released
What happens if a protein is not folded correctly?
It is degraded by proteosome
What does the 26S proteasome do?
Binds polyubiquitinated proteins and unfolds them. It them degrades the proteins to peptides 3-25 amino acids long
What are some examples of covalent modifications of proteins after translation?
Proteolysis (cleavage of protein)
Glycosylation (addition of sugars)
Phosphorylation (addition of phosphate groups)
