Translation Flashcards
Where does translation take place?
Prokaryotes (bacteria and archaea) –> in the cytoplasm (they don’t have a nucleus), directly.
Eukaryotes –> in the cytoplasm, following mRNA export from the nucleus, more specifically in the ER.
Where are eukaryotic ribosomes made?
In the nucleolus.
rRNA translocated from the nucleoplasm,
while ribosomal proteins translocate from
the cytoplasm.
Where are eukaryotic ribosomes made?
Ribosomal Features
- same in prokaryotes and eukaryotes
- The ‘A’ site = Acceptor Site
- Two subunits come together only when ribosomes is actively synthesising proteins.
BOOK- image
tRNA charging
Amino acids need to be attached to the
appropriate tRNA – with correct anticodon loop.
Mediated by aminoacyl tRNA synthetases –
one enzyme for each amino acid.
The enzyme binds to the amino acid, ATP and
the anticodon loop of the appropriate tRNA.
tRNA charging
Aminoacyl tRNA
- Each amino acid is attached to a particular tRNA.
- There are 20 amino acids.
- There are 64 different codon sequences available.
why are there 64 possible codon sequences?
There are four available bases, and each codon is made up of three bases.
1 base → 4 different possible codons (A, C, G, T) → 4
2 bases → 16 different possible codons (AA, AC, AG, AT, CA, CC, CG, etc) → 4x4
3 bases → 64 different possible codons (AAA, ACA, AGA, ATA, AAC, ACC, ACG, etc) → 4x4x4
Why is genetic code describes as degenerate.
The genetic code is described as degenerate because most amino acids are encoded by more than one codon. This means that there are multiple codons that can specify the same amino acid
- Methionine – AUG – start codon – only
one codon. - Stop codons – UAA UAG UGA.
using codon table, translate UGUGCCACA
CAT- Cysteine, Alanine, Threonine
How do ribosomes bind to mRNA in PROKARYOTES.
The small ribosomal subunit (30S in prokaryotes) associates with the mRNA at the ribosome binding site (RBS).
The shine-Dalgarno Sequence (found upstream of the start codon) helps align the ribosome with the start codon (AUG) for translation initiation. This sequence is complementary to a portion of the 16S rRNA in the 30S subunit, ensuring proper binding.
The initiation factor 3 (IF3) facilitates the binding of the 30S ribosomal subunit to the mRNA. It also stops the 30S subunit from prematurely associating with the large subunit (50S) before the mRNA is correctly positioned.
How do ribosomes bind to mRNA in EUKARYOTES?
The small ribosomal subunit (40s) binds to 5’
mRNA cap via initiation factors.
Sequence surrounding the start codon=called
Kozac sequence – indicates where
translation will start.
Translation Initiation- PROKARYOTES- E.coli
The first (initiator) aminoacyl-tRNA, which carries the amino acid methionine (the initiator tRNA), is recruited to the start codon (AUG) at the P site of the ribosome.
This recruitment is facilitated by IF2 (initiation factor 2) that is bound to GTP. IF2 helps ensure that the correct initiator tRNA is positioned at the start codon.
IF1 also binds to the A site of the small ribosomal subunit (30S), preventing any other tRNAs from entering prematurely.
With the initiator tRNA correctly positioned at the start codon, IF3 is released.
The binding of the initiator tRNA triggers the hydrolysis of GTP, which leads to the release of both IF2 and IF1.
With the initiation factors released, the large ribosomal subunit (50S) can now join the complex, completing the formation of the functional ribosome and allowing translation to begin.
Translation elongation Prokaryotes- E.coli
SAME BASIC MECHANISM IN BOTH EUKARYOTES AND PROKARYOTES !
The process begins with the recruitment of the second aminoacyl-tRNA to the A site of the ribosome.
The enzyme peptidyl transferase, which is part of the ribosomal RNA in the large subunit, catalyzes the formation of a new peptide bond between the incoming amino acid (attached to the tRNA in the A site) and the growing polypeptide chain (attached to the tRNA in the P site).
The growing polypeptide is transferred from the tRNA in the P site to the amino acid on the tRNA in the A site, facilitated by a reaction with tRNA deacylase. This process releases the growing chain from the tRNA in the P site.
After the peptide bond is formed, the peptidyl-tRNA in the A site translocates to the P site. This movement shifts the ribosome along the mRNA, allowing the next codon to enter the A site.
The spent tRNA, which is now without an amino acid, is released from the E site of the ribosome, freeing up the A site for the next aminoacyl-tRNA.
When the ribosome reaches a stop codon (UAA, UAG, or UGA) in the mRNA, release factors (RF1 or RF2) bind to the A site. These factors recognize the stop codons and trigger the termination process.
he binding of the release factors stimulates the peptidyl transferase activity of the ribosome, leading to the release of the newly synthesized polypeptide from the tRNA in the P site.
After the polypeptide is released, the spent tRNA leaves the ribosome from the E site, making space for the next steps in recycling.
RF3 assists in clearing the A site by promoting the release of the release factors and facilitating the disassembly of the ribosomal complex.
Following termination, ribosome recycling factors help to dissociate the ribosomal subunits (30S and 50S) from the mRNA, allowing them to be reused for new rounds of translation.
RECAP- Which initiation factor helps recruitment of the initiator aminoacyl tRNA?
IF2
