Learning Objective: By the end of this section, the learner should be able to describe the structure, function, and synthesis of tRNA, explain how RNA editing modifies RNA molecules post-transcriptionally, and relate these concepts to clinically relevant examples.

TRANSFER RNA (tRNA)

tRNA plays a central role in translation by delivering the correct amino acid to the ribosome according to the mRNA codon.

1. Structure & Function of tRNA

tRNAs share a conserved secondary structure (the classic cloverleaf), although each variant carries a specific amino acid.

Key features

• Amino acid attachment site → 3′ end (“CCA” tail).
• Anticodon loop → recognizes complementary mRNA codon.
• D-loop and TψC-loop → assist in proper folding and ribosome interaction.

Function

• Each tRNA is charged with only one specific amino acid by its aminoacyl-tRNA synthetase.

• The charged tRNA (aminoacyl-tRNA) brings the amino acid to the ribosome for incorporation into a growing polypeptide.

2. Synthesis of tRNA

• Enzyme: RNA polymerase III.
• Location: Nucleoplasm.
• After transcription, tRNAs undergo:
  • Trimming of ends
  • Modification of bases
  • Addition of “CCA” at the 3′ end
  • Folding into their mature functional form

RNA EDITING

RNA editing refers to post-transcriptional modifications that alter the nucleotide sequence of RNA.

Activity

1. Types of RNA Editing

• Insertion or deletion of nucleotides
• Base modifications (e.g., deamination)
  • Cytosine → Uracil
  • Adenine → Inosine

Occurs in mRNA, rRNA, and tRNA depending on cell type.

2. Clinical Example: Apoprotein B Editing

• Apoprotein B100: produced in the liver.
• Apoprotein B48: produced in the intestines.
• Mechanism:
  • Cytosine is deaminated to uracil in the intestinal mRNA
  • CAA → UAA (stop codon)
  • Produces shorter ApoB48

This demonstrates how editing can change protein length and function.

Activity