Question

Alanine is encoded by four different codons: GCA GCG GCU GCC How many tRNAsare needed to be able to read all four alanine codons?

Arginine is encoded by six different codons: CGA CGG CGU CGC AGA AGG How many tRNAsare needed to be able to read all six alanine codons?

Answer 1

There are 61 sense codons that encode 20 different amino acids. Theoretically it would require 61 distinct tRNAs for 61 sense codons. But due to the 'wobble' rules one tRNA can bind with more than one sense codon thus we don't require 61 distinct tRNAs. According to the wobble rules, pairing at the 5' end of the first & second bases of mRNA is according to the Watson-Crick pairing rules (A with U & G with C). However paring between the third base of the mRNA codon & first position (5' end) of the anticodon of tRNA can be paired with flexibility. The following are the wobble rule for third base pairing-

First position of the anticodon	Third position of the codon
C	G
G	U or C
A	U
U	A or G
I (Inosine)	A, U or C

Now coming to the question, alanine can be encoded by 4 different codons- GCA, GCG, GCU & GCC. Also these codons are different only in the third base. From the above table we find that third position of the codon A or G can be paired with a single tRNA having first position of the anticodon as U & third position of the codon U or C can be paired with a single tRNA having first position of the anticodon as G. So, alanine can be read by only 2 tRNAs.

Arginine is encoded by six different codons- CGA, CGG, CGU, CGC, AGA & AGG. From these codons four are identical in the first two bases (CGA, CGG, CGU & CGC) & two more are identical in the first two bases (AGA & AGG). From wobble rules, CGA & CGG can be read by single tRNA; CGU & CGC can be read by single tRNA; AGA & AGG can be read by single tRNA. So, arginine can be read by only 3 tRNAs.