Translation Process
Translation is the process where
amino acids are polymerized to form a polypeptide. Here's an overview of the
steps involved:
·
Peptide Bond Formation: During
translation, amino acids are joined together by peptide bonds to form
polypeptides. This process requires energy.
·
Energy Requirement: The
formation of peptide bonds requires energy, and in the initial phase, amino
acids are activated in the presence of ATP.
Activation of Amino Acids
·
Activation Process: In this
phase, specific amino acids are activated and linked to the 3’ end of their
corresponding tRNA. This step is known as the charging of tRNA or
aminoacylation of tRNA.
·
Enzymatic Catalysis: The enzyme
aminoacyl-tRNA synthetase catalyzes this reaction, ensuring that the correct
amino acid is attached to its respective tRNA.
(B) Initiation of polypeptide synthesis
Ribosome Structure
In its inactive state, the
ribosome exists as two separate subunits: a large subunit and a small subunit.
The ribosome itself is composed of structural RNAs and approximately 80
different proteins.
Translation Initiation
·
Encounter with mRNA:
Translation begins when the small ribosomal subunit encounters the mRNA.
·
mRNA Binding: The mRNA binds to
the small subunit of the ribosome, following the base pairing rules between the
mRNA bases and the rRNA bases. This process is facilitated by initiation
factors.
Ribosomal Binding Sites
·
P-site and A-site: The larger
ribosomal subunit contains two key sites: the P-site (peptidyl site) and the
A-site (aminoacyl site).
·
Assembly of Subunits: The small
subunit, with the mRNA attached, joins the large subunit in such a way that the
initiation codon (AUG) aligns with the P-site.
·
Initiator tRNA Pairing: The
anticodon UAC of the initiator tRNA (methionyl tRNA), which carries the amino
acid methionine, pairs with the initiation codon AUG of the mRNA through
hydrogen bonds at the P-site.
(C) Elongation of polypeptide chain
¾ A second tRNA charged with an appropriate amino acid binds to the A-site of the ribosome.
Translation
¾ A peptide (CO-NH) bond is formed between the carboxyl group of methionine and the amino group of the second amino acid (The ribosome also acts as a catalyst (23S rRNA in bacteria is the enzyme- ribozyme) for the formation of peptide bond.
¾ The ribosome moves from codon to codon along the RNA in the 5’-3’ direction.
¾ Amino acids are added one by one in the sequence of the codons and become joined together to form a polypeptide.
(D) Termination of polypeptide synthesis
¾ When one of the termination codons comes at the A-site, it does not code for any amino acid and there is no tRNA molecule for it.
¾ As a result, the polypeptide synthesis (or elongation of polypeptide) stops.
¾ At the end, a release factor binds to the stop codon, terminating translation and releasing the complete polypeptide from the ribosome.
¾ A translational unit in mRNA consists of the RNA sequence between the start codon (AUG) and the stop codon, coding for a polypeptide. Additionally, mRNA contains untranslated regions (UTRs) at both the 5' end (before the start codon) and the 3' end (after the stop codon). These UTRs are essential for efficient translation..
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