Translation - Ribosomes and overview of process Flashcards
What are the two steps of translation?
Capturing complementary tRNA molecules and holding them in position (to allow the tRNA anticodons to base-pair with mRNA codons)
Covalently linking the amino acids that tRNAs carry so as to form a protein chain
What is the protein manufacturing machine?
The ribosome
Where are ribosomal subunits made in eukaryotes?
Made in nucleus
What does the small ribosomal subunit do?
Matches tRNAs to codons of mRNA
What does the large ribosomal subunit do?
It catalyses formation of peptide bonds that link amino acids together into a polypeptide chain
Where and why do the 2 ribosomal subunits come together?
They come together on an mRNA molecule - near 5’ end
This allows them to begin synthesis of protein
How does the ribosome work?
2
It moves along mRNA
As it does so it translates the nucleotide sequence into an amino acid sequence, one codon at a time
How does the ribosome work with tRNA
The ribosome uses tRNAs as adaptors to add each amino acid in the correct sequence
What happens to the ribosome when protein synthesis is completed?
The 2 subunits of the ribosome separate
How many amino acids are added per second to a polypeptide chain by ribosomes in eukaryotic cells?
Ribosomes add 2 amino acids to a polypeptide chain per second in eukaryotes
How much faster are prokaryotic ribosomes than eukaryotic ribosomes?
Prokaryotic ribosomes are ten times faster than eukaryotic
What are the three binding sites for tRNA?
P site
A site
E site
What is the P site of a ribosome?
2
The peptidyl-tRNA site
It holds the tRNA that is carrying the growing polypeptide chain
What is the A site of a ribosome?
2
The aminoacyl-tRNA site
It holds the tRNA that is carrying the next amino acid to be added to the chain
What is the E site of a ribosome?
2
The exit site
This is where discharged tRNAs leave the ribosome
Why do the three ribosome binding sites need to be close together?
So that tRNA anticodons in the binding sites can form base-pairs with the adjacent codons on mRNA molecule
How does mRNA bind to a ribosome?
mRNA has a single binding site in the small subunit
In total how many binding sites do ribosomes have?
4
1 for mRNA and 3 for tRNA
List the order of the tRNA binding sites on a ribosome
A site, P-site, E-site
What are the three stages of translation?
Chain initiation
Chain elongation
Chain termination
What do all three stages of translation need?
They all need protein factors
What provides the energy needed for translation?
Energy is provided by the hydrolysis of GTP
What is GTP?
Guanosine triphosphate
How does translation begin?
Translation begins with start codon (AUG) and a special initiator tRNA
What does initiator tRNA always carry?
The amino acid methionine
What is always found at the N-terminal end of a protein?
A methionine
How do the amino acids join together?
2
The carboxyl end of the polypeptide chain uncouples from tRNA at the P-site
It then joined by a peptide bond to the amino acid linked to tRNA in the A site
What happens after a new amino acid has been added to the polypeptide chain?
Each tRNA moves along the mRNA a space until the A site is free again
What are the three stop codons?
UAA
UAG
UGA
What binds to the stop codon?
Release factors
What happens when the release factors bind to a stop codon?
2
This frees the carboxyl end of the growing polypeptide chain from its attachment to tRNA
The completed polypeptide is then released into the cytoplasm
What must proteins do before they can become a fully functioning protein?
They must undergo post translation modification
