7.3 Translation Flashcards
What are 4 features included in ribosome structure?
- Proteins and ribosomal RNA molecules (rRNA)
- two sub-unit, one large one small
- three binding sites for tRNA on the surface of the ribosome. two rRNA molecules can bind at the same time to the ribosome (E - exit, P, and A)
- there is a binding site for mRNA on the surface of the ribosome
What are 3 features of the tRNA structure?
- a triplet of bases called anticodon which is part of a loop of seven unpaired bases
- two other loops
- the base sequence CCA at the 3’ end which forms a site for attaching an amino acid
What is the T and D arm on tRNA associated with?
T - ribosome
D - tRNA activating enzyme (responsible for adding the amino acid to the acceptor stem)
How is a tRNA molecule activated? What happens next?
- by the attachment of an amino acid to the 3’ terminal of the tRNA by an enzyme called tRNA-activating enzyme
- There are 20 different tRNA- activating enzymes that are each specific to one of the 20 amino acids and the correct tRNA molecule
- The active site of the activating enzyme is specific to both the correct amino acid and the correct tRNA
- Enegy from ATP is used for the attachment of amino acids
- Amino acid is activated by the formation of a bond between the enymes and adenosine monophosphate (AMP)
- then the activated amino acid is covalently attahced to the tRNA
- Energy from this bond is later used to link the amino acid to the growing polypeptide chain during translation
What are the 3 steps of translation?
- initiation
- elongation
- termination
What happens in initiation?
Involves the assembly of the componenets that carry out the process (mRNA, tRNA and ribosome)
* the same ribosomal subunit binds to the 5’ end of the mRNA and moves along it until it reaches the start codon (AUG)
* Appropriate tRNA molecule (carrying the amino acid methionine) bind to the codon via its anticodon (according to complementary base pairing)
* The large ribosomal subunit aligns itself to the tRNA molecule at the P site and forms a complex with the small subunit
What happens in elongation?
- second tRNA molecule binds 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
- The ribosome moves (translocates) aong the mRNA strand by one codon position (in a 5’ to 3’ direction)
- The deacylated tRNA from the P site moves in to the E site and is released, while the tRNA carrying the peptide chain moves to the P site
- Another tRNA molecule attaches to the next codon in the now unoccupied A site and the process is repeated
What happens in termination?
Final stage invovles the disassembly of the components and the release of a polypeptide chain
* Elongation continues until the ribosome reaches a stop codon
* Stop codons reruit a release factor that signals for translation to stop
* The polypeptide is released and the ribosome disassembles back into its two independent subunits
What are SRP?
signal recognition particle
What does the presence of a SRP do?
- SRP halts the translation in the cytoplasm
- SRP docks with SRP receptor on the RER membrane
- Translation is re-initiated
- the translation continues and the polypeptide chain grows via a channel protein into RER
- Goes into the ER lumen
Why are prokaryotes able to begin transcription and translation at the same time?
they lack compartmentalised structures and so transcription and translation need not be separated
