Translation Flashcards
What is the primary enzyme used in translation?
Ribosome
What is the template used for translation?
mRNA
What are the subunits assembled during translation?
Amino acids
What are the two subunits that make up a ribosome?
Large and small subunits. Both are made of at least one RNA and lots of proteins
What does tRNA do during translation?
It reads the mRNA sequence and brings the right amino acid to be added next. They are physically bound to an amino acid that corresponds to the codon on the mRNA
How are tRNAs recycled?
After leaving the ribosome, they go to the tRNA synthetase and it binds to the anticodon and attaches another amino acid to the tRNA
How many tRNA-synthetases are in a cell?
20, one for each amino acid
What defines the beginning and end of translation?
The start and stop codons
Are the start codons always at the very beginning of an mRNA transcript?
No, most of the time it isn’t. It will be further downstream so there will be a 5’ untranslated region on the mRNA
Are the stop codons always at the very end of an mRNA transcript?
No. There will be an untranslated region 3’ of the stop codon
How does translation initiation occur in prokaryotes?
- The small ribosomal subunit recognizes and binds to the ribosome binding site
- The small subunit slides along the mRNA until it finds an AUG
- The small subunit recruits tRNA and the large ribosomal subunit and translation begins
How does the ribosome recognize the ribosome binding site?
Complementary base pairing with rRNA
Why is an mRNA able to encode more than one polypeptide in prokaryotes?
There are multiple ribosome binding sites along the transcript. The polypeptide that gets produced will depend on which ribosome binding site the ribosome binds to
In eukaryotes, what needs to happen before the small subunit can bind to the mRNA transcript?
Eukaryotic initiation factors need to bind to the 5’ cap, the polyA tail, and to each other to circularize the mRNA. Then the small subunit gets recruited
Where does the small ribosomal subunit bind in eukaryotes?
At the 5’ end
How does the small ribosomal subunit determine which AUG is a start codon and which AUG is just a methionine in eukaryotes?
Eukaryotes use the scanning mechanism, so they start at the 5’ end and scan until it finds an AUG. The first AUG the small subunit finds will be the start codon
Why is the mRNA circularized?
It makes it easier to reload the ribosomes and translate multiple times, reusing the mRNA to make more proteins
Can a eukaryotic mRNA encode multiple polypeptides like in prokaryotes?
No, because of the scanning mechanism there will always be only one start codon. The ribosome won’t bind anywhere but the 5’ end
What is the A site in the ribosome?
The aminoacyl tRNA site. It is where the tRNA with the next amino acid will bind with complementary base pairing between the codon and anticodon
What is the P site in the ribosome?
The peptidyl tRNA site. The growing peptide chain is physically bound to the amino acid on the tRNA coming in from the A site
What is the E site in the ribosome?
The exit site. The peptide chain is transferred to the next tRNA and the tRNA in this site leaves the ribosome and go to a tRNA synthetase
How do we know that it’s the rRNA that is catalyzing the formation of peptide bonds and not the proteins?
If the large ribosomal subunit is treated with a protease, it can still catalyze the formation of peptide bonds
What are the steps in translation elongation?
- The tRNA comes into the A site and complementary base pairs to the codon
- The tRNA moves into the P site
- The growing peptide chain is transferred to the amino acid carried by the tRNA
- The tRNA moves into the E site
- The tRNA is detached from its amino acid
- the tRNA leaves the ribosome to be reloaded by a tRNA synthetase
How is translation terminated?
A release factor binds to a stop codon in the A site and hydrolyzes the bond between the tRNA in the P site and the amino acid it is attached to. The protein, ribosome, mRNA, and tRNA all dissociate
What is a polyribosome?
An mRNA with a lot of ribosomes attached and translating
How are transcription and translation coupled in prokaryotes?
Translation can begin as soon as a ribosome binding site is available, even while the RNA is still being transcribed
Why do transcription and translation need to stay separate in eukaryotes?
Organelles, there’s membranes in the way. Transcription occurs in the nucleus and translation occurs in the cytoplasm. This ensures that the mRNA gets all modifications before it is translated
