All organisms contain more than 20 types of tRNAs: this is because there is at least one tRNA for each of the amino acids.
The length of all tRNAs can vary from 60 to 95 nucleotides (i.e., 18 - 28 kDa); most tRNAs have lengths of about 76 nucleotides.
For a typical 76 nucleotide tRNA, there are 15 invariant positions and 8 semi-invariant positions that mostly occur in loop regions.
Figure 8.10: Modified Bases in tRNAs
Most tRNAs have modified bases; nonethelss, the three most common modified bases (see figure 8.10) in tRNA are:
Figure 8.11: Secondary Structure of tRNA
All tRNAs fold into a clover leaf-like structure and have a 5’ phosphate group (see figure 8.11).
Furthermore, there are four components to a typical tRNA molecule:
Acceptor / amino acid arm
This is a 7 base pair stem that has a protruding 3’-single-stranded element (i.e., “CCA”) to which an amino acid can be attached to.
D arm
This is a 3 - 4 base pair stem that ends in a loop and contains multiple dihydrouridine bases.
Anticodon arm
This is a 5 base pair stem that ends in a 7 nucleotide loop that contains the anticodon: a triplet of bases that is complementary to the mRNA codon that specifies the tRNA.
T (i.e., T\(\psi\)C) arm
This is a 5 base pair stem that contains the sequence T\(\psi\)C
Also note that all tRNA molecules end with the sequence “CCA” (which has a free 3’ OH group).
The tertiary structures of tRNAs is maintained via base stacking interactions.
Figure 8.12: L-Shaped tRNA
The L-shaped conformation of tRNA (see figure 8.12) is formed by the acceptor and the T arms that have been folded into a continuous A form-like double helix. The acceptor arm is comprised of D and anticodon arms.