tRNAs are transcribed as larger precursors, and these are finally cleaved into mature 4S tRNA molecules. tRNAs are generally rich in “unusual” bases, which are created following transcription. This illustrates the concept of posttranscriptional modification. The “cloverleaf” model of tRNA was proposed by Robert Holley.
All the tRNAs contain …CCA-3′ sequence at the 3′ end, and a …G-5′ at the 5′ terminus. The amino acid is covalently linked to the terminal adenosine residue of the …CCA-3′.
The “cloverleaf” structure of tRNA has following features:
1) The acceptor arm accepts the amino acid at the …CCA-3′ sequence.
2) The TψC arm is so named because of the presence of this triplet sequence. (ψ stands for pseudouridine, a modified base).
3) The anticodon arm lies at the opposite end of the acceptor arm. The anticodon arm carries the anticodon triplet in the centre of the loop.
4) The D arm is so named because it is rich in dihydrouridine, a modified base.
5) The so-called extra arm is the most variable and lies between the TψC and anticodon arm. The functional significance of this arm is unknown.