Topic 7.3 Translation Flashcards
1
Q
Structure of Ribosome
A
- Ribosomes are made of protein (for stability) and ribosomal RNA (for catalytic activity)
- They consist of a large and small subunit:
- The small subunit contains an mRNA binding site
- The large subunit contains three tRNA binding sites – an aminoacyl (A) site, a peptidyl (P) site and an exit (E) site
- Ribosomes can be found either freely floating in the cytosol or bound to the rough ER (in eukaryotes)
2
Q
Transfer RNA (tRNA) Structure
A
tRNA molecules fold into a cloverleaf structure with four key regions:
- The acceptor stem (3’-CCA) carries an amino acid
- The anticodon associates with the mRNA codon (via complementary base pairing)
- The T arm associates with the ribosome (via the E, P and A binding sites)
- The D arm associates with the tRNA activating enzyme (responsible for adding the amino acid to the acceptor stem)
3
Q
tRNA Activation
A
- Each amino acid is recognised by a specific enzyme (the enzyme may recognise multiple tRNA molecules due to degeneracy)
The binding of an amino acid to the tRNA acceptor stem occurs as a result of a two-step process:
- 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
*stored energy will provide the majority of the energy required for peptide bond formation during translation
4
Q
Translation (HL) Initiation
A
- The small ribosomal subunit binds to the 5’-end of the mRNA and moves along it until it reaches the start codon (AUG)
- Next, the appropriate tRNA molecule bind to the codon via its anticodon (according to complementary base pairing)
- Finally, the large ribosomal subunit aligns itself to the tRNA molecule at the P site and forms a complex with the small subunit
5
Q
Translation (HL) Elongation
A
- A second tRNA molecule pairs with the next codon in the ribosomal A site
- The amino acid in the P site is covalently attached via a peptide bond (condensation reaction) to the amino acid in the A site
- The tRNA in the P site is now deacylated (no amino acid), while the tRNA in the A site carries the peptide chain
6
Q
Translation (HL) Translocation
A
- The ribosome moves along the mRNA strand by one codon position (in a 5’ → 3’ direction)
- The deacylated tRNA moves into the E site and is released, while the tRNA carrying the peptide chain moves to the P site
- Another tRNA molecules attaches to the next codon in the now unoccupied A site and the process is repeated
7
Q
Translation (HL) Termination
A
- Elongation and translocation continue in a repeating cycle until the ribosome reaches a stop codon
- These codons do not recruit a tRNA molecule, but instead recruit a release factor that signals for translation to stop
- The polypeptide is released and the ribosome disassembles back into its two independent subunits
8
Q
Eukaryotic ribosomes
A
ribosomes are separated from the genetic material by the nucleus
- After transcription, the mRNA must be transported from the nucleus (via nuclear pores) prior to translation by the ribosome
- This transport requires modification to the RNA construct (e.g. 5’-methyl capping and 3’-polyadenylation)
9
Q
Prokaryotic ribosomes
A
lack compartmentalized structures (like the nucleus) and so transcription and translation need not be separated
- Ribosomes may begin translating the mRNA molecule while it is still being transcribed from the DNA template
- This is possible because both transcription and translation occur in a 5’ → 3’ direction
10
Q
Polysome
A
group of two or more ribosomes translating an mRNA sequence simultaneously
11
Q
Features of the polysome
A
- The polysomes will appear as beads on a string (each ‘bead’ represents a ribosome ; the ‘string’ is the mRNA strand)
- In prokaryotes, the polysomes may form while the mRNA is still being transcribed from the DNA template
- Ribosomes located at the 3’-end of the polysome cluster will have longer polypeptide chains that those at the 5’-end
12
Q
Protein destination
A
is determined by the presence or absence of an initial signal sequence on a nascent polypeptide chain
- The presence of this signal sequence results in the recruitment of a signal recognition particle (SRP), which halts translation
- The SRP-ribosome complex then docks at a receptor located on the ER membrane (forming rough ER)
- Translation is re-initiated and the polypeptide chain continues to grow via a transport channel into the lumen of the ER
- The synthesized protein will then be transported via a vesicle to the Golgi complex (for secretion) or the lysosome
- Proteins targeted for membrane fixation (e.g. integral proteins) get embedded into the ER membrane
- The signal sequence is cleaved and the SRP recycled once the polypeptide is completely synthesized within the ER
13
Q
Protein structure - PRIMARY
A
- The first level of structural organization in a protein is the order / sequence of amino acids which comprise the polypeptide chain
- The primary structure is formed by covalent peptide bonds between the amine and carboxyl groups of adjacent amino acids
- Primary structure controls all subsequent levels of protein organization because it determines the nature of the interactions between R groups of different amino acids
14
Q
Protein structure - SECONDARY
A
- The secondary structure is the way a polypeptide folds in a repeating arrangement to form α-helices and β-pleated sheets
- This folding is a result of hydrogen bonding between the amine and carboxyl groups of non-adjacent amino acids
- Sequences that do not form either an alpha helix or beta-pleated sheet will exist as a random coil
- Secondary structure provides the polypeptide chain with a level of mechanical stability (due to the presence of hydrogen bonds)
15
Q
Protein structure - TERTIARY
A
- The tertiary structure is the way the polypeptide chain coils and turns to form a complex molecular shape (i.e. the 3D shape)
- It is caused by interactions between R groups; including H-bonds, disulfide bridges, ionic bonds and hydrophobic interactions
- Relative amino acid positions are important (e.g. non-polar amino acids usually avoid exposure to aqueous solutions)
- Tertiary structure may be important for the function of the protein (e.g. specificity of active site in enzymes)
