Question

Arrange the steps of translation initiation in prokaryotes:
A. Binding of 50S ribosomal subunit
B. Binding of initiator tRNA
C. Binding of mRNA to 30S subunit
D. Formation of complete 70S initiation complex

• A. C $\rightarrow$ B $\rightarrow$ A $\rightarrow$ D
• B. B $\rightarrow$ C $\rightarrow$ A $\rightarrow$ D
• C. C $\rightarrow$ A $\rightarrow$ B $\rightarrow$ D
• D. B $\rightarrow$ A $\rightarrow$ C $\rightarrow$ D

Hint:

Always remember the assembly order: Small subunit first (with mRNA), then the starter tRNA, and finally the large subunit acts as the "lid" to close the complex.

Answer 1

The Correct Option is A

Translation initiation in prokaryotes involves the assembly of a functional ribosome at the start codon of the mRNA, ensuring that protein synthesis begins accurately. Let's break down the process step-by-step:

1. Binding of mRNA to 30S subunit (C): The initiation process starts with the binding of the mRNA to the small ribosomal subunit, which is the 30S subunit in prokaryotes. This step is facilitated by the ribosome binding site on the mRNA, also known as the Shine-Dalgarno sequence, which is complementary to a segment of the 16S rRNA in the 30S subunit.
2. Binding of initiator tRNA (B): Following the mRNA attachment, the initiator tRNA, which carries N-formylmethionine (fMet) in prokaryotes, binds to the start codon (AUG) on the mRNA. This tRNA is specifically guided to the P-site of the 30S subunit by initiation factors, particularly IF-2.
3. Binding of 50S ribosomal subunit (A): Once the initiator tRNA is in place, the 50S large ribosomal subunit binds to the initiation complex, forming a complete ribosome. This association is stabilized by the hydrolysis of GTP, associated with initiation factor IF-2, which subsequently leaves the complex.
4. Formation of complete 70S initiation complex (D): The final step is the assembly of the complete 70S ribosome, including both subunits along with the mRNA and initiator tRNA, ready for the elongation phase of protein synthesis. This marks the formation of the 70S initiation complex.

Thus, the correct sequence of events for translation initiation in prokaryotes is:

C → B → A → D

This sequence ensures that the translation machinery is correctly assembled before elongation begins, leading to accurate and efficient protein synthesis.