Nucleic Acids 7- Translation and post-translational modification Flashcards
(37 cards)
Why is Spatial and temporal coordination of transcription and translation important
Ensures that energy is now wasted on transcribing mRNA or translating proteins only for them to be degraded as they are not needed
How is the DNA read
In a triplet code 300nt of exon= 100 amino acids
What is a group of 3 nucleotides called
A codon
Describe the typical structure of mRNA
5’ “cap” (7-Methyl Guanosine) – entry site for ribosome
UTR – untranslated regions – may be involved in regulation of translation or mRNA stability
polyA – protects mRNA from degradation
UTR present after 5’ cap util start codon
UTR also present after stop codon up until poly-A-tail
What are the triplet codes that encode stop codons
UAA
UAG
UGA
What does it mean if an amino acid has lots of codons that code for it
It is commonly used
What is the start codon
Methionine (AUG)
Describe simply how mRNA is read
Ribosome scans from 5’ end of mRNA (cap)
Translation starts at first AUG, continues in frame, i.e. with immediately succeeding triplet codon (CAU) and so on…..
Translation stops at first in frame termination codon
Describe the structure of tRNA
RNA molecules can fold into 3-D structures by forming base pairs between different regions of the molecule, if these regions are extensive they will fold into double-helical molecules, this happens in tRNA, producing a molecules that looks like a cloverleaf. The cloverleaf undergoes further folding to form a L-shaped structure, further stabilised by Hydrogen bonds between the different regions.
There are two regions of unpaired nucleotides:
5’- anticodon
3’- amino acid.
Describe the binding between the mRNA codon and the tRNA anticodon
Antiparallel binding, 5’-3’ 3’-5’
Complementary base pairing
How does each tRNA molecule recognise the correct amino acid
Aminoacyl-tRNA synthetases covalently couple each amino acid to the appropriate set of tRNA molecules, different synthetase enzyme for each amino acid.
Aminoacyl-tRNA synthetases attach the amino acid to the 3’ end of the amino acid, coupled by the hydrolysis of ATP– AMP +PPi. This process is known as charging and it produced a high energy bond between the charged tRNA and the amino acid, the energy of this bond can be used later to form peptide bonds between adjacent amino acids.
What are the steps of translation
Initiation, elongation, termination
What is the function of ribosomes
Hold the tRNAs in position
Capture complementary tRNAs
Move along the mRNA
What is the structure of a ribosome
In eukaryotes- 80s. Made up of one 40S unit and one 60S unit
Describe simply the steps of initiation
Step 1: dissociation of ribosome subunits (eukaryotic 40S + 60S)
Step 2: assembly of preinitiation complex
containing Met-tRNA + eIFs + 40S subunit
Step 3: binding of mRNA to preinitiation complex
Step 4: binding of 60S subunit
Describe the assembly of the pre-initiation complex
eIF4E and eIF4G bind to cap and are recognised by 40S/Met-tRNA/eIF2
Only initiator Met-tRNA can bind to 40S subunit alone
40S subunit is primarily involved in tRNA and mRNA recognition- searches for AUG start codon of mRNA.
Describe the binding of the mRNA to the pre-initiation complex
initiator Met binding sets the frame of the translation
Describe the binding of the 60 S subunit
Translation initiation factors dissociate
GTP— GDP + Pi (ensures correct pairing)
Where is the Met-tRNA found
On the P site of the 40S subunit.
What happens to methionine when the final polypeptide is synthesised
It is usually removed by a specific protease.
Describe, simply, the process of elongation
Step 1: binding of new tRNA carrying second amino acid to “amino acyl” (A) site
Step 2: catalysis of peptide bond formation between the two amino acids by peptidyl transferase (PT) on the large ribosomal subunit
Step 3: translocation of tRNA to P site and dissociation of first tRNA
How is the charged tRNA carrying the next amino acid added.
It binds to the vacant A site of the ribosome, where it base pairs with the mRNA codon that is based there
The next mRNA codon is in frame as the A and P sites are sufficiently close that they form base pairs with mRNA codons that are contiguous, meaning that there are no stray bases between them.
Describe the formation of the peptide bond between adjacent amino acids
The carboxyl group of the amino acid is uncoupled from the tRNA at the P site and is joined by a polypeptide bond to the free amino group of the amino acid bound to the tRNA at the A site.
This reaction is catalysed by peptidyl transferase (PT) on 60S subunit
Describe the translocation of the large subunit
The shift moves the two tRNAs to the E and P sites respectively. Elongation Factors (EFs) are proteins that promote movement of ribosome along mRNA using GTP