Lesson 12: Translation Flashcards
How is the mRNA sequence read?
mRNA sequence is read in sets of three nucleotides (CODONS)
Do the nucleotides overlap in each codon?
No, the code is non-overlapping
How many reading frames can a mRNA have?
In theory, there are three potential reading frames. However, the selection of the correct reading frame is critical (allows for the same protein to be made each time).
How is mRNA converted to an amino acid/protein?
It uses tRNA!
- Transfer RNA is another example of a functional RNA
What is the structure of tRNA?
tRNA has a cloverleaf structure. Although tRNAs are single-stranded, they use complementary base pairing to form this structure.
What are some important parts of a tRNA?
1) On the bottom of the tRNA you have an anti-codon which complementary base pairs with mRNA in the ribosome.
- NOTE: anti-codon and codon are antiparallel
2) At the 3’ end an amino acid is covalently attached.
How is the correct amino acid attached to the tRNA?
Aminoacyl-tRNA synthetase
- Interacts with tRNA (anticodon), thus it recognizes one tRNA and adds the proper amino acid
Describe the structure of aminoacyl-tRNA synthetase.
There is a binding pocket for
- amino acid
- anti-codon
- amino-acid accepting arm (where the amino acid attaches to the tRNA)
Does aminoacyl-tRNA synthetase use energy?
Yes!
- ATP is used to covalently attach (conjugate) amino acid to tRNA.
NOTE: This leaves a high energy bond that is used to drive polypeptide chain formation in the ribosome (i.e: build our protein)
How does the genetic code cover all 20 amino acids with only 4 nucleotides?
The genetic code is DEGENERATE.
- Multiple codons encode the same amino acid
How is the genetic code degenerate?
1) Wobble Position: A single tRNA can often recognize more than one codon, allowing multiple codons to bind to one amino acid
2) Many times there is more than one tRNA for a single amino acid
Describe how the Wobble position works.
Some tRNAs do not need to have a perfect match in the 3rd codon position. This position has more flexibility and can allow for non-standard complementary base pairing.
Where does the mRNA get converted to protein?
Ribosome!
- It is a large complex, made of proteins and rRNA (a lot of proteins and a couple rRNAs)
- Has a large and small subunit
How does a ribosome look in a prokaryotic cell?
Since prokaryotes do not have compartmentalized organelles, transcription and translations happens simultaneously
How does a ribosome look in a eukaryotic cell?
Since transcription happens in the nucleus, the mRNA must leave the nucleus and go to the ribosome for translation
How are ribosomes SIMILAR in prokaryotic and eukaryotic cells?
1) Both the large and small subunits have a lot of proteins and a few rRNAs.
2) Ribosomes are free in the cytoplasm
- Note: In eukaryotes, some ribosomes are also attached to the rough ER
3) rRNA gives the ribosome its overall shape, while proteins stabilize the structure
More similarities between ribosomes in prokaryotes and eukaryotes!
4) RNA catalyzes the peptide bond reaction, so it is an example of a ribozyme.
5) Small Subunit Function: matches tRNAs to the codons of the mRNA by complementary base pairing
6) Large Subunit Function: RNAs in large subunit catalyze formation of the peptide bonds that covalently link the amino acids together into a polypeptide chain
What are the sites called in ribosomes?
EPA
E = exit site
- tRNA leaves this site
P = peptidyl site
- where our peptide bonds are formed
A = aminoacyl site
- where our new tRNAs are stored
What are the steps for translation?
Just like in transcription!
1) Initiation
2) Elongation
3) Termination
How does the initiation phase work in EUKARYOTIC translation?
While in theory, there are three reading frames, the initiation process allows for the right reading frame to be used
1) An initiator tRNA (hooked up with Met) binding in the P-site of the small ribosomal subunits which has translation initiation factors attached
2) Small subunit recognizes the 5’ cap on the mRNA and scans it until it finds the first AUG (codes for Met)
3) Large subunit then binds and translation begins
How does the initiation phase work in PROKARYOTIC translation?
1) An initiator formyl-Methionine tRNA recognizes a Shine-Dalgarno sequence in the small subunit of the ribosome.
2) The small subunit does not start scanning for AUG until after the Shine-Dalgarno sequence
3) Large subunit then binds and translation begins
Is elongation and termination the same in both eukaryotes and prokaryotes?
Yes!
How does the elongation phase work in translation?
1) A newly bound charged tRNA goes in the A site and complementary base pairs to the mRNA in the small subunit.
2) A new peptide bond is then formed with the amino acid that was attached to the tRNA that was just added
3) The large subunit moves 3 nucleotides. (so the “new” tRNA is now in the P site and the “old” tRNA is in the E site).
4) The small subunit moves 3 nucleotides to match up with the large subunit. This then kicks out the tRNA in the E site.
5) This then brings the ribosome back to the original position three nucleotides down to begin the cycle again.
How does the termination phase work in translation?
1) A release factor (which is a protein not a tRNA) catalyzes a hydrolysis reaction which releases polypeptide chain from the P-site tRNA
2) The ribosome then dissociates and falls off the mRNA
- Note: Since dissociated ribosomal subunits don’t get degraded it can bind a to a new mRNA and repeat the process.
Why does where the translation end have to be precise?
Mutations that alter a stop codon are called READ-THROUGH mutations.
- Does not stop and continue to add additional amino acids which can affect function of the protein
What does polycistronic mean?
One mRNA can make MORE THAN ONE protein
What does monocistronic mean?
One mRNA can make ONE protein
Why are bacterial mRNAs polycistronic?
In bacteria, ribosomes bind to the Shine-Delgarno sequences. Since there can be multiple Shine-Delgarno sequences in one mRNA, more than one protein can be made from one mRNA. It will start coding at the start codon and stop at the stop codon.
Why are eukaryotic mRNAs monocistronic?
In eukaryotes, ribosomes can only bind to the 5’ cap to start scanning for the start codon. Therefore, one protein is made from one mRNA.
What is a polyribosome?
Multiple ribosomes bind to one mRNA. This makes protein production more efficient (the more ribosomes on an mRNA, the more polypeptides being made)
- Note: polyribosomes create the same polypeptide chain because it binds to the same start codon.
How to decode an mRNA?
1) Look at your mRNA in 5’ to 3’
2) AUG (Met) is the start codon
3) Then in triplets, decode from 5’ to 3’ in the mRNA until you hit a “stop” codon
What is a point mutation?
When a single nucleotide is substituted for another one
ex: Silent, Nonsense, Missense
What is a silent mutation?
When a change in DNA doesn’t change the amino acid (this occurs due to the degeneracy of genetic code)
What is a nonsense mutation?
When a change in DNA creates a stop codon. This causes the rest of the protein to not be made (truncated protein)
What is a missense mutation?
When a change in DNA produces a different amino acid. There are two types – conservative and non-conservative
What is a conservative mutation?
The amino acid that is produced has similar properties to the original one. This might not affect protein function as much.
What is a non-conservative mutation?
The amino acid that is produced has different properties to the original one. This disrupts protein function.
What is a frameshift mutation?
When 1 or 2 nucleotides are inserted or deleted. This messes up the reading frame and every amino acid produced after will be different.
How can a frameshift mutation occur?
It can happen in alternative splicing or damage to the DNA (?)
What happens if you add or delete 3 (or a multiple of 3) amino acids?
If you add or delete 3 amino acids, then an amino acid will be added or deleted but then stay within the frame.
