​The following statements were made about the initiation of translation in eukaryotes:

(A) eIF2 facilitates correct recognition and binding of ribosomal subunits.
(B) eIF2B activates eIF2 by replacing its GDP with GTP.
(C) eIF3 binds to the 60S ribosomal subunit and inhibits its reassociation with the 40S subunit.
(D) eIF5 promotes association between the 60S ribosomal subunit and the 48S complex.
(E) eIF6 binds to the 60S ribosomal subunit and blocks reassociation with the 40S subunit.

Which one of the following options shows the correct combination of all correct statements?

Correct option is B

​Explanation :

Statement A: eIF2 facilitates correct recognition and binding of ribosomal subunits.

• Incorrect. eIF2 is not responsible for ribosomal subunit binding.
• Instead, eIF2 binds GTP and Met-tRNAi (initiator tRNA), forming the ternary complex that helps in translation initiation.
• Ribosomal subunit binding is regulated by eIF3, eIF6, and eIF5B.

Statement B: eIF2B activates eIF2 by replacing its GDP with GTP.

• Correct. eIF2 is active in its GTP-bound form, allowing it to deliver Met-tRNAi to the 40S ribosomal subunit.
• eIF2B is a guanine nucleotide exchange factor (GEF) that replaces GDP with GTP, reactivating eIF2 for another round of translation initiation.

Statement C: eIF3 binds to the 60S ribosomal subunit and inhibits its reassociation with the 40S subunit.

• Incorrect. eIF3 binds to the 40S ribosomal subunit, NOT the 60S subunit.
• It prevents premature 60S binding, ensuring correct pre-initiation complex assembly.
• eIF6, NOT eIF3, binds to the 60S subunit to prevent reassociation.

Statement D: eIF5 promotes association between the 60S ribosomal subunit and the 48S complex.

• Correct. eIF5 functions as a GTPase-activating protein (GAP) that facilitates GTP hydrolysis on eIF2.
• This step triggers the joining of the 60S subunit to the 48S initiation complex, leading to the formation of the 80S ribosome.

Statement E: eIF6 binds to the 60S ribosomal subunit and blocks reassociation with the 40S subunit.

• Correct. eIF6 is a major inhibitor of ribosomal subunit joining.
• It binds to the 60S subunit and prevents premature reassociation with the 40S subunit.
• eIF6 is crucial for ribosome biogenesis and recycling of ribosomal subunits.

Information Booster:

1. eIF2 is crucial for initiating translation as it binds Met-tRNAi and delivers it to the ribosome.
2. eIF2B functions as a nucleotide exchange factor, converting inactive eIF2-GDP to active eIF2-GTP.
3. eIF3 ensures that the 40S subunit remains free until proper initiation factors and mRNA are recruited.
4. eIF5 hydrolyzes GTP on eIF2, allowing the 60S subunit to join the 48S complex, forming the 80S ribosome.
5. eIF6 prevents premature association of the 40S and 60S subunits, ensuring proper ribosome recycling.

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