The first position of the anti-codon, the "Wobble" position, forms hydrogen bonds less well than do the second two. This means that the last position of the codon has less coding potential than the first two. The reason is that the anticodon is at the bottom of the anticodon loop of the tRNA, and so there backbone of the tRNA is bending back to pair with itself. The nucleotides do not hold their bases flat and regular in relation to each other.

Here is a picture of the anticodon loop. In this case 5'-CAU-3' is the anticodon for 5'-AUG-3', so it would be the C, right at the sharpest part of the bend in the anticodon loop, that would pair poorest.

Here is an interactive model where you can spin the tRNA/mRNA around and see that not all the hydrogen bonds are equal length, nor are all the bases coplanar.

The first position of the anti-codon, the "Wobble" position, forms hydrogen bonds less well than do the second two. This means that the last position of the codon has less coding potential than the first two. The reason is that the anticodon is at the bottom of the anticodon loop of the tRNA, and so there backbone of the tRNA is bending back to pair with itself. The nucleotides do not hold their bases flat and regular in relation to each other.

Here is a picture of the anticodon loop. In this case 5'-CAU-3' is the anticodon for 5'-AUG-3', so it would be the C, right at the sharpest part of the bend in the anticodon loop, that would pair poorest.

Here is an interactive model where you can spin the tRNA/mRNA around and see that not all the hydrogen bonds are equal length, nor are all the bases coplanar.