Lecture 15 - The ribosome and translation Flashcards
What is the structure of a prokaryotic ribosome?
70S
Made up of a large and small subunit
Large subunit = 50S
Small subunit = 30S
Why doesn’t the size of the two subunits in a prokaryotic ribosome add up to 70S?
Due to svedberg, which is a measure of the sedimentation rate of suspended particles centrifuges under standard conditions, shape and size.
What are the main stages in translation?
Activation
Initiation
Elongation
Termination
How is initiation set up?
The start codon is embedded in either the Shine-Dalgarno sequence (prokaryotes) or the Kozak sequence (eukaryotes)
The first methionine to start the polypeptide chain is in the Shine Dalgarno sequence, and it is carried in a special tRNA
This positions it in the right place in the ribosomes
The tRNA involves is tRNAi
What is the process of initiation?
Initiation factors and GTP bind to the 30S subunit
The subunit positions the tRNA at the right place on the mRNA
The large subunit is recruited
It has binding pockets, the P pocket and A pocket
Methionine is tRNA is in the P pocket
As it is ejected, GDP and ATP are used up
What are the initiation factors involved in initiation?
IF1 which blocks the A site to tRNAi-methionine, and inhibits interaction between 30S and 50S before its ready
IF2 which tags tRNAi and regulates entry into ribosome
IF3 which stops premature 30S-50S interaction, stabilises 30S and does accuracy check for tRNAi methionine binding
What is the process of elongation?
Peptidyl-tRNA is in the P pocket and theres an empty pocket above the second codon
Aminoacyl-tRNA comes into the acceptor pocket and binds to the A site
This is driven by elongation factors
A peptide bond joins between the amino acids
The ribosome translocates. Peptidyl-tRNA is in the P site, and A site is free for next aminoacyl-tRNA
What are the two elongation factors?
EF-Tu which mediates aminoacyl tRNA entry to the ribosome
EF-G which mediates translocation
What is the process of termination?
There is a pocket over a termination codon
tRNA isn’t recruited and a protein called a release factor is recruited instead
There are three types of release factors
Hydrolysis of the polypeptide chain from the tRNA
Dissociation of tRNA and release factors
What are the 3 types of release factors?
RF1 = UAA
RF2 = UAA/UGA
RF3 = Helps RF1 or RF2 to bind to ribosomeW
What is GTP and what is its function?
Guanosine triphosphate
It provides energy by hydrolysisW
When is GTP involved in protein synthesis?
Initiation, during the addition of the large subunit
Elongation, during the addition of aminoacyl-tRNA by EF-Tu, peptide synthesis and translocation
Termination, at the release of the peptide chain and dissociation of the ribosome
