Crick-Bernner had done an experiment to prove that a codon is actually three letters long and not more than three. For example, if every three bases on the resultant mRNA (messenger ribonucleic acid) make a word, after that the reading frame might be recognized by acquiring the first three bases from the end as the first word, the next three as second word and so on. Then addition or deletion of a single pair on the DNA would transfer the reading frame on the mRNA and from that point causing all subsequent words to be misread.
The order of three additions or three deletions have been revealed together to bring back a wild-type phenotype. By their experiment it is confirmed that a word in the genetic code possess of three successive nucleotides or a triplet. The three insertions or three deletions inside a gene automatically return the reading frame in the mRNA only if the words are triplets.
No, a codon cannot be composed of six bases. It was revealed by the experiments of Crick-Bernner.