6) Translation Flashcards
(22 cards)
What is the overall process of translation?
1) Ribosome goes to mRNA to do the job of translation
2) tRNAs bring amino acids to the ribosome in an order specified by the mRNA sequence
RNA—–> Protein
What are some of the components of translation?
Ones which will give special attention to:
Ribosomes
tRNA
Aminoacyl Synthetases
mRNA Initiation Factors Elongation Factors Releasing Factors Amino acids
What is the structure of ribosomes?
Large and Small subunit
Both are made up of RNA and protein, with many different proteins per subunit
Overall secondary structure: RNA predominates
Large subunit: Made up of more than one RNA molecule= Bacteria: 23S and 5S, 3 in euk
Small subunit: Contain single RNA (bacteria= 16S and euk= 18S)
What is the active site of a ribosome called?
Peptidyl Transferase
No ribosomal protein was near the active site
rRNA (ribosome RNAs)= between the 2 subunits, and is on the active site
Protein part of the ribosome is on the outward facing sides of the structure
What do ribosomal RNAs do?
Fold up into complex secondary structures
rRNAS= Actually catalyse peptide bonds
rRNA is a single continuous molecule= has 4 separate domains
Important that a secondary structure is stable so that accessory proteins can attach to a scaffold
Can also have non-standard base pairs forming (e.g. G-U) in double stranded regions of an RNA molecule
What do tRNAs do?
Bring amino acids to the active site of the ribosome
Amino acids are selected to be added to the growing polypeptide chain in an order specified by the sequence of the mRNA
Each amino acid is brought to the chain by transfer RNA specific for particular amino acid
What are the 2 crucial regions of tRNA?
1) Anticodon= Complementary to the codon on mRNA
3’ end always ends in -CCA
2) Acceptor arm= Binds to amino acid
Formed by binding together of 7 bp of the 5’ and 3’ ends of the molecule, amino acid attaches to the adenosine at the 3’ end
Amino acid attaches at the adenosine at the 3’ end
Where the specificity of tRNA come from?
Shape and modifications to tRNA
tRNA can then be recognised by its specific aminoacyl tRNA synthesise
Many different modifications= One of the most common is methylation of the base or sugar component of the nucleotide
What does aminoacyl tRNA synthesase do?
Attaches an amino acid to tRNA
Key enzymes that power protein synthesis
Each one recognises the correct amino acid to link to the correct tRNA
One aminoacyl tRNA synthesise for each of the 20 amino acids
Aminoacyl= General term for amino acid
VERY specific + Activate amino acid with ATP
Energy maintained when linked to tRNA
Energy for protein synthesis comes from this chemical bond
Amino acid now passenger to the tRNA
How does aminoacyl tRNA synthetase add amino acid to tRNA?
1) Activated amino acid intermediate has link between carboxyl group and AMP
ATP is cleaved= Produces energy to join amino acid to the tRNA
2) Amino acid-AMP remains bound to the enzyme and tRNA takes place of AMP= Forms aminoacyl-tRNA
What are the 3 steps of translation? (Names only)
1) Initiation
2) Elongation
3) Termination
Just like transcription!
What is the first step of translation?
1) Ribosomes: Large and small subunit remain separate in cytoplasm until need to assemble
2) Translation= Ribosome assembles on mRNA at ribosome binding site= Sequence upstream of first codon (AUG) that is complementary to short sequence of 16S rRNA
3) Initiator tRNA (Met-tRNAfmet ) is brought to the right position on the complex by IF-2 which accompanies the small subunit along with molecule of GTP= Energy while IF-3= Prevents large subunit from attaching until right time. This brings the Methionine amino acid
4) Large subunit attaches by using energy from GTP= Release of initiation factors= Makes process irreversible
What are the different sites for binding tRNA molecules on ribosomes?
A= Aminoacyl site= tRNA must be bound to an amino acid that corresponds to the amino acid dictated by mRNA codon
P site= Peptidyl site= Holds tRNA bound to the growing polypeptide chain
E site= Exit= Releases now empty tRNA from the ribosome
Met-tRNAfMet is the only aminoacyl-tRNA that can load directly into the P-site. It does not need to go to the A site first.
GOES to A –> P —> E
What is phase 2 of translation? (Overall)
ELONGATION
1) Individual amino acids are attached to the carboxyl (-COOH) terminus of the growing polypeptide= First peptide bond
2) Assembling the ribosome out of the 2 subunits has created 2 sites at which aminoacyl tRNAs can bind
Elongation involves the next amino acid binding to the A-site and then being moved long to the P-site where is can be joined to the growing peptide chain (so far the “chain” is only one aa – the methionine- but it’s going to get bigger).
What are the 3 elongation factors?
EF1A (EF-Tu) Loads aa-tRNA into the A-site. Commonest protein in cell. EF-1B (EF-Ts) Recycles EF-Tu EF-2 (EF-G) Checks that translocation is successful
What happens during elongation in translation?
1) 2 parts of the ribosome are now together= P-site is already occupied by initiator tRNA
2) Next aminoacyl-tRNA will bind to A site
3) EF-1A protein in bacterial cell and complexes with GTP and aminoacyl-tRNAs= pool of these in each cell and ONLY the correct amino acid can load their amino acid into the empty A-site
4) Matching aminoacyl-tRNA is loaded, EF1-A will hydrolyse GTP= Release energy to allow formation of a peptide bond by the ribozyme peptidyl transferase
5) EF-1B recycles EF-1A by removing GDP and allow free GTP to bind, which can now pick up new aa-tRNA
6) Ribosome must now move 5’ to 3’ 3 bases to move the next codon into A-site, aided by EF-2 which hydrolyses GTP upon successful translocation
What happens during translocation (elongation)?
First 2 amino acids have been joined= Attachment between first amino acid and its aminoacyl tRNA are broken
Translocation= Ribosome moves 3 nucleotides along mRNA= ejecting deacylated initiator tRNA from P site= Now occupied by 2nd tRNA attached to first 2 amino acids of the polypeptide
New codon can now enter A site
Translocation= hydrolysis of a molecule of GTP and is mediated by EF-2
Ejected aminoacyl tRNA leaves via 3rd site= Exit site
Ribosome can slide along the mRNA so it repositions itself each time a new amino acid needs to be added
Where does the synthesis of the peptide bond occur?
P-site
Peptide transferase enzyme
The peptide bond is synthesised by the transfer of the growing polypeptide chain from the P-site tRNA to the free amino group on the amino acid on the tRNA in the A-site.
What are releasing factors?
Proteins that mimic aminoacyl-tRNA and can load into the A-site
RF-1: Recognises UAG and UAA
RF-2: Recognises UGA and UAA
RF-3: Stimulates dissociation of RF-1 and RF-2
What happens during phase 3? (Termination)
1) Termination codon (UAA, UGA or UAG) moves into A-site= signal for ribosome to release the now complete protein
2) Releasing factors loads onto A site (Either Rf-1 + RF-3 OR RF-2 + RF-3) and bond between the last amino acid of the protein and the adenine of the tRNA in the P-site is hydrolysed
3) Releases the protein as it tries to make another peptide bond but there is no amino acid in the A-site= peptide is transferred not to the amino group of an amino acid but to water
4) Ribosome is separated into its 2 subunits by the ribosome recycling factor (RRF)
What happens when you alter each base in the Genetic code?
Altering 3rd base= Usually no effect
Altering 1st base= Will change one amino acid to a biochemically similar one
All in the first column= Hydrophobic amino acids
All in second column=
Polar amino acids
Many in 3rd column are basic or acidic
What does the ‘wobble’ term describe?
The fact that one tRNA can recognise more than one codon for the same amino acid if the codon differs only in its 3rd position (3rd base)
Example: Can recognise GCC or GCU
