7.3 Translation Flashcards
What are ribosomes made of?
Ribosomes are made of protein (for stability) and ribosomal RNA (for catalytic activity)
What two subunits do ribosomes consist of?
They consist of a large and small subunit
What does the small subunit consist of?
The small subunit contains an mRNA binding site
What does the large subunit consist of?
The large subunit contains three tRNA binding sites – an aminoacyl (A) site, a peptidyl (P) site and an exit (E) site
Where can ribosomes be found?
Ribosomes can be found either freely floating in the cytosol or bound to the rough ER (in eukaryotes)
What number of regions does tRNA have?
tRNA molecules fold into a cloverleaf structure with four key regions:
What are the 4 regions of tRNA?
The acceptor stem
The anticodon
The T arm
The D arm
What is the role of the acceptor stem? tRNA
The acceptor stem (3’-CCA) carries an amino acid
What is the role of the anticodon? tRNA?
The anticodon associates with the mRNA codon (via complementary base pairing)
What is the role of the T arm? trna
The T arm associates with the ribosome (via the E, P and A binding sites)
What is the role of D arm? trna
The D arm associates with the tRNA activating enzyme (responsible for adding the amino acid to the acceptor stem)
What is the role of the tRNA molecule in the cytoplasm/
Each tRNA molecule binds with a specific amino acid in the cytoplasm in a reaction catalysed by a tRNA-activating enzyme
What recognises the specific amino acid? trna
Each amino acid is recognised by a specific enzyme (the enzyme may recognise multiple tRNA molecules due to degeneracy)
Where does the amino acid bind?
The binding of an amino acid to the tRNA acceptor stem occurs as a result of a two-step process:
What are the 2 steps of activation?
The enzyme binds ATP to the amino acid to form an amino acid–AMP complex linked by a high energy bond (PP released)
The amino acid is then coupled to tRNA and the AMP is released – the tRNA molecule is now “charged” and ready for use
What is the function of ATP in activation?
The function of the ATP (phosphorylation) is to create a high energy bond that is transferred to the tRNA molecule
What bond does ATP help form?
This stored energy will provide the majority of the energy required for peptide bond formation during translation
What does the first step of translation involve?
The first stage of translation involves the assembly of the three components that carry out the process (mRNA, tRNA, ribosome)
Where does the small ribosomal subunit bind?
The small ribosomal subunit binds to the 5’-end of the mRNA and moves along it until it reaches the start codon (AUG)
What happens once the small subunit is in place?
Next, the appropriate tRNA molecule bind to the codon via its anticodon (according to complementary base pairing)
When does the large subunit bind?
Finally, the large ribosomal subunit aligns itself to the tRNA molecule at the P site and forms a complex with the small subunit
What is the first step of elongation?
A second tRNA molecule pairs with the next codon in the ribosomal A site
How do amino acids bond?
The amino acid in the P site is covalently attached via a peptide bond (condensation reaction) to the amino acid in the A site
What happens once the amino acids have binded?
The tRNA in the P site is now deacylated (no amino acid), while the tRNA in the A site carries the peptide chain
What is the first step of translocation?
The ribosome moves along the mRNA strand by one codon position (in a 5’ → 3’ direction)
What happens to the deacylated trna?
The deacylated tRNA moves into the E site and is released, while the tRNA carrying the peptide chain moves to the P site
What is the general cycle of translation?
Another tRNA molecule attaches to the next codon in the now unoccupied A site and the process is repeated
What is the role of termination?
The final stage of translation involves the disassembly of the components and the release of a polypeptide chain
What initiates the beginning of termination?
Elongation and translocation continue in a repeating cycle until the ribosome reaches a stop codon
What is the role of the stop codons?
These codons do not recruit a tRNA molecule, but instead recruit a release factor that signals for translation to stop
What is the final step of translation?
The polypeptide is released and the ribosome disassembles back into its two independent subunits
How are ribosomes separated from the genetic material?
In eukaryotes, the ribosomes are separated from the genetic material (DNA and RNA) by the nucleus
Where must the mRNA be transported?
After transcription, the mRNA must be transported from the nucleus (via nuclear pores) prior to translation by the ribosome
What does the transport of mRNA involve?
This transport requires modification to the RNA construct (e.g. 5’-methyl capping and 3’-polyadenylation)
What is the big difference between translation in pro and eukaryotes?
Prokaryotes lack compartmentalised structures (like the nucleus) and so transcription and translation need not be separated
Are transcription and translation separated in prokaryotes?
Ribosomes may begin translating the mRNA molecule while it is still being transcribed from the DNA template
Why can translation occur at the same time as transcription in prokaryotes?
This is possible because both transcription and translation occur in a 5’ → 3’ direction
What is a polysome?
A polysome (or a polyribosome) is a group of two or more ribosomes translating an mRNA sequence simultaneously
How do polysomes look like?
The polysomes will appear as beads on a string (each ‘bead’ represents a ribosome ; the ‘string’ is the mRNA strand)
How are polysomes formed in prokaryotes?
In prokaryotes, the polysomes may form while the mRNA is still being transcribed from the DNA template
Which ribosomes will have longer polypeptide chains?
Ribosomes located at the 3’-end of the polysome cluster will have longer polypeptide chains that those at the 5’-end
Where are all proteins produced by eukaryotic synthesised?
All proteins produced by eukaryotic cells are initially synthesised by ribosomes found freely circulating within the cytosol
What determines whether the ribosomes are free in the cytosol or bound to the ER?
If the protein is targeted for intracellular use within the cytosol, the ribosome remains free and unattached
If the protein is targeted for secretion, membrane fixation or use in lysosomes, the ribosome becomes bound to the ER
What determines protein destination?
Protein destination is determined by the presence or absence of an initial signal sequence on a nascent polypeptide chain
What does the presence of a signal sequence result in?
The presence of this signal sequence results in the recruitment of a signal recognition particle (SRP), which halts translation
Where do the SPR-ribosomes locate?
The SRP-ribosome complex then docks at a receptor located on the ER membrane (forming rough ER)
What then causes translation to be re-initiated?
Translation is re-initiated and the polypeptide chain continues to grow via a transport channel into the lumen of the ER
Where will the synthesised protein then be transported?
The synthesised protein will then be transported via a vesicle to the Golgi complex (for secretion) or the lysosome
What happens to proteins synthesised for membrane fixation?
Proteins targeted for membrane fixation (e.g. integral proteins) get embedded into the ER membrane
What happens to the signal sequence at the end?
The signal sequence is cleaved and the SRP recycled once the polypeptide is completely synthesised within the ER
What is the primary sequence?
The first level of structural organisation in a protein is the order / sequence of amino acids which comprise the polypeptide chain
How is the primary structure formed?
The primary structure is formed by covalent peptide bonds between the amine and carboxyl groups of adjacent amino acids
Why is primary structure so important?
Primary structure controls all subsequent levels of protein organisation because it determines the nature of the interactions between R groups of different amino acids
What is secondary structure?
The secondary structure is the way a polypeptide folds in a repeating arrangement to form α-helices and β-pleated sheets
How is secondary structure formed?
This folding is a result of hydrogen bonding between the amine and carboxyl groups of non-adjacent amino acids
What secondary structure will proteins that do not for alpha helices or beta pleated sheets have?
Sequences that do not form either an alpha helix or beta-pleated sheet will exist as a random coil
What is the role of the secondary structure?
Secondary structure provides the polypeptide chain with a level of mechanical stability (due to the presence of hydrogen bonds)
What is the tertiary structure?
The tertiary structure is the way the polypeptide chain coils and turns to form a complex molecular shape (i.e. the 3D shape)
What creates the tertiary structure?
It is caused by interactions between R groups; including H-bonds, disulfide bridges, ionic bonds and hydrophobic interactions
What is particularly important in the tertiary structure?
Relative amino acid positions are important (e.g. non-polar amino acids usually avoid exposure to aqueous solutions)
Why may tertiary structure be important?
Tertiary structure may be important for the function of the protein (e.g. specificity of active site in enzymes)
What is the quaternary structure?
Multiple polypeptides or prosthetic groups may interact to form a single, larger, biologically active protein (quaternary structure)
What is a prosthetic group?
A prosthetic group is an inorganic compound involved in protein structure or function (e.g. the heme group in haemoglobin)
What is a conjugated protein?
A protein containing a prosthetic group is called a conjugated protein
What may hold the tertiary structure together?
Quaternary structures may be held together by a variety of bonds (similar to tertiary structure)
