In eukaryotic cells, tRNA (transfer RNA) is synthesized through a process called transcription, primarily by RNA polymerase III.
Here's a breakdown of the tRNA synthesis process:
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Transcription Initiation: RNA polymerase III, a specialized enzyme, binds to specific DNA sequences (promoters) upstream of the tRNA gene. These promoters are typically located within the tRNA gene itself, making them internal promoters.
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RNA Polymerase III Action: Once bound, RNA polymerase III unwinds the DNA double helix and begins synthesizing a pre-tRNA molecule. This pre-tRNA is a longer precursor to the mature tRNA.
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Pre-tRNA Processing: The pre-tRNA undergoes several processing steps to become a functional tRNA molecule:
- 5' Leader Removal: The extra nucleotides at the 5' end of the pre-tRNA are cleaved off by an enzyme called RNase P.
- 3' Trailer Removal and CCA Addition: The 3' end of the pre-tRNA is trimmed, and a CCA sequence (cytosine-cytosine-adenine) is added. This CCA sequence is crucial because it's the site where the amino acid will be attached during translation.
- Intron Splicing (if present): Some pre-tRNAs contain introns (non-coding sequences) that must be removed through splicing.
- Base Modification: Many of the bases within the tRNA molecule are chemically modified. These modifications are essential for tRNA folding, stability, and recognition by aminoacyl-tRNA synthetases (the enzymes that attach the correct amino acid to the tRNA). Common modifications include methylation, deamination, and reduction.
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Folding and Export: After processing, the mature tRNA folds into its characteristic cloverleaf structure, which is critical for its function. The properly folded tRNA is then exported from the nucleus to the cytoplasm, where it participates in protein synthesis.
In summary, tRNA synthesis involves transcription by RNA polymerase III to create a pre-tRNA, followed by extensive processing steps including trimming, CCA addition, splicing (if necessary), and base modifications, ultimately leading to a functional tRNA molecule.
