In 1966, Francis Crick proposed the wobble concept to explain the observations that a single tRNA could bind to different codons. For many tRNAs, the first two nucleotides of the codon must pair perfectly to the anticodon, while the third nucleotide does not need to be an exact match.
Related to this concept, we can find some interesting patterns when examining the genetic code. All of the amino acids except methionine and tryptophan are able to be coded for by multiple codons. In each of these, the first two nucleotides are the same. There are exceptions, such as leucine and arginine having two subsets of these types of codons, but each subset follows this pattern.
More interestingly, if the first two nucleotides in a codon are guanine or cytosine (i.e., GG, CC, GC, or GC), then it will code for the same amino acid, no matter what the third nucleotide is.
What is the BEST explanation for this pattern involving guanine and cytosine?