12. In bacteria many of the tRNA genes do not contain the CCA sequence found at the 3′ end of tRNA. In this context which one of the following statements represents the correct explanation?(1) In these tRNAs amino acylation occurs at the 3′ end of the tRNA irrespective of the presence of the CCA sequence.
    (2) CCA sequence is added to these tRNA transcripts in a DNA template independent manner.
    (3) These tRNAs exploit the process of trans- splicing to include a CCA sequence at their 3′ end.
    (4) The absence of CCA sequence occurred only in the last common ancestor (LCA) during the course of evolution and the current day tRNA genes always possess a sequence to encode the CCA end of the tRNA.

Why Do Some Bacterial tRNA Genes Lack the CCA Sequence at the 3′ End? (CSIR NET Life Science)

Correct Answer

(2) CCA sequence is added to these tRNA transcripts in a DNA template independent manner.

Detailed Explanation

In many bacteria, including Gram-positive species, not all tRNA genes encode the CCA sequence at their 3′ end. Despite this, all mature tRNAs must have this CCA sequence because it is essential for amino acid attachment and proper function during translation.

How Is the CCA Sequence Added?

• Post-Transcriptional Addition:
  For tRNAs whose genes do not encode the CCA, the sequence is added after transcription by a specialized enzyme called CCA-adding enzyme or tRNA nucleotidyltransferase.

• Template-Independent Process:
  This enzyme adds the CCA nucleotides directly to the 3′ end of the tRNA in a manner that does not require a DNA template—the enzyme recognizes the tRNA structure and catalyzes the stepwise addition of C, C, and A nucleotides.

• Universal Mechanism:
  This mechanism is conserved across bacteria, archaea, and eukaryotes, ensuring that all mature tRNAs are functionally complete regardless of their gene sequence.

Why Are the Other Options Incorrect?

• Option (1): Aminoacylation (amino acid attachment) requires the CCA sequence; it cannot occur without it.

• Option (3): Trans-splicing is not involved in CCA addition for tRNAs.

• Option (4): The absence of the CCA sequence is not just an ancient evolutionary relic; many present-day tRNA genes still lack it and rely on enzymatic addition.

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Summary:
In bacteria, many tRNA genes do not encode the CCA sequence at the 3′ end. This sequence is added post-transcriptionally by a template-independent enzyme, making option (2) the correct answer. For more free resources, video solutions, and expert guidance, trust Let’s Talk Academy for your CSIR NET Life Science journey.