Week 6: Translation Flashcards
Where does translation take place?
In the cytosol
True or False: the genetic code is the sequence of DNA that makes up the organism
False
What makes up the genetic code?
3 nucleotides and 1 amino acid (UGU codes for Cysteine)
How are codons read?
mRNA triplets
How does the genetic code account for the 64 codons but only 20 amino acids?
Redundancy - multiple codons code for the same amino acid (e.g. GUU, GUC, GUA, and GUG all code for Valine)
How are reading frames determined?
They are determined by translation initiation bat the 5’ AUG
How do reading frames with the same nucleotide sequence differ from each other?
Different reading frames may begin translation at different nucleotides and because nucleotides are read 3 at a time, different amino acids can be coded for
Is the start site for translation the same at the start site for transcription?
No, there is UTR before the first AUG
How are mutations classified?
They are classified by the effect on the nucleotide, the amino acid, and protein
What are the three different types of nucleotide-pair substitutions?
Silent, missense, nonsense
What is a silent mutation?
The nucleotide change does not affect the amino acid coded for
What is a missense mutation?
The nucleotide change changes the amino acided coded
What is a nonsense mutation?
The nucleotide change causes an early stop codon (no amino acid)
What are the effects of nucleotide pair deletions/insertions
They can cause an immediate change to the frameshift (i.e. a missense or nonsense mutation)
What makes a 3 nucleotide-pair deletion different from the rest?
Because 3 nucleotides are deleted, the frame shift does not go through any immediate changes, there is just one amino acid missing
At which end is the Amino Acid attached to in tRNA?
the 3’ end
In what direction is the genetic code read in?
5’-3’
How do tRNAs translate mRNAs?
tRNA recognizes the codon on mRNA via its anticodon and brings the correct amino acid
About how long are tRNAs?
80 nucleotides long
How is codon redundancy in genetic code managed during translation?
- More than 1 tRNA for many amino acids
- Some tRNAs can recognize and base pair with more than 1 codon
What is the wobble position?
The wobble position is where there’s a bit of flexible base pairing between the anticodon of the tRNA and the codon of the mRNA. It’s the 3’ position of the codon and the 5’ position of the anticodon.
What is aminoacyl-tRNA synthease?
Aminoacyl-tRNA synthase is a key player in fidelity and error correction via hydrolytic editing. It gets the amino acid puts it on the correct tRNA.
How is recognition achieved beteween a specific tRNA and its synthase?
- Identifying the tRNA anticodon nucleotides
- Recognizing the nucleotide sequence of the acceptorstem/arm
- Reading nucleotide sequences at additional positions on the tRNA
What makes up a eukaryotic ribosome and what are they made of?
Large subunit: ~49 ribosomal proteins + 3 rRNA molecules
Small subunit: ~33 ribosomal proteins + 1 rRNA molecule
Where are ribosomes located (in eukaryotes)?
In the endoplasmic reticulum and in the cytosol
What do each of the sites on a ribosome stand for?
A= aminoacyl site
P= peptidyl site
E = exit site
What makes peptide synthesis energetically favourable?
The energy stored in covalent bonds between the amino acid and tRNA in P site
What catalyzes peptide bond formation in translation?
The peptidyl transferase activity of the rRNA in the large subunit
Why is a ribosome a ribozyme?
Because of RNA molecules that posses catalytic activity
In the elongation of the amino acid chain, what does EF-Tu (EF1 in euk.) check for in aminoactyl tRNA?
- The right amino acid
- the right anticodon-codon base pair
What happens if the base-paring is not correct between an anticodon and codon?
EF-Tu/EF1 is not released, denying the formation of a peptide bond
What happens if the base-pairing is correct between anticodon and codon?
GTP is hydrolyzed & EF-Tu/EF1 released
Why is there a slight delay before formation of a peptide bond between amino acids?
It allos for one last check for accurate base-pairing
What is EF-G/EF2’s role in elongation?
It helps the ribosome to move the mRNA forward one codon and helps speed up elongation of the polypeptide chain
Can protein synthesis happen without the use of elongation factors?
Yes but it would be much slower, inefficient, and would most likely have more mistakes
What does it mean when bacteria has polycistronic mRNA?
There is more than one protein for one mRNA
Describe the initiaiton of translation in prokaryotes
- Shine-Dalgarno sequences on mRNA base pair with rRNA in small ribosomal subunits
- Positioning of small subunits to initiating AUG codons on mRNA also requires Initiation Factors (IFs)
- fMethionine aminoacyl tRNA binds to initiator coon
- Large ribosomal subunit binds
Describe the initiation of translation in Eukaryotes
- tRNA conects to small ribsomal subunit (with initiation factors bound) without mRNA
- both then connect to mRNA strand at 5’ cap
- once recognizing 5’ most AUG, anticodon binds to AUG
- translation initiation factors dissociate
- large ribosomal subunit binds
- tRNA binds to the second codon
- first peptide bond is formed
What terminates translation?
A translation release factor, which in humans is a protein, not tRNA. It recognizes and binds to the stop codon, releasing the newly synthesized polypeptide chain.
What is a polyribosome/polysome?
It is when there are multiple ribosomes making the same protein simultaneously, which is relatively slow and about 80 nulcelotides apart
What helps proteins fold?
Chaperone proteins (e.g. Hsp60, Hsp 70)
What are examples of post-translational modifications?
- phosphorylation
- glycosylation
Why might covalent modifications be required in post-translational modifications?
It may be required to:
- make protein active
- recruit protein to correct membrane or organelle
Proteins that are targeted for degredation have a small protein call ubiquitin covalently attached to them, which directs them to the ____________ where they are degraded by ____________, and the amino acids recycled into new proteins by the cell
proteasome; proteases
What do many antibiotics target?
bacterial translation
If chemicals are active on bacteria and eukaryotes or only eukaryotes, can they be considered antibiotics?
NO! They’re poison and most likely used in the lab for experiments.
