A final proof of our ideas can only be obtained by detailed studies on the alterations produced in the amino acid sequence of a protein by mutations of the type discussed here. Francis Crick Read Quote
Attempts have been made from a study of the changes produced by mutation to obtain the relative order of the bases within various triplets, but my own view is that these are premature until there is more extensive and more reliable data on the composition of the triplets. Francis Crick Read Quote
Do codons overlap? In other words, as we read along the genetic message do we find a base which is a member of two or more codons? It now seems fairly certain that codons do not overlap. Francis Crick Read Quote
For simplicity one can think of the + class as having one extra base at some point or other in the genetic message and the – class as having one too few. Francis Crick Read Quote
The balance of evidence both from the cell-free system and from the study of mutation, suggests that this does not occur at random, and that triplets coding the same amino acid may well be rather similar. Francis Crick Read Quote
The meaning of this observation is unclear, but it raises the unfortunate possibility of ambiguous triplets; that is, triplets which may code more than one amino acid. However one would certainly expect such triplets to be in a minority. Francis Crick Read Quote
If poly A is added to poly U, to form a double or triple helix, the combination is inactive. Francis Crick Read Quote
If, for example, all the codons are triplets, then in addition to the correct reading of the message, there are two incorrect readings which we shall obtain if we do not start the grouping into sets of three at the right place. Francis Crick Read Quote
It would appear that the number of nonsense triplets is rather low, since we only occasionally come across them. However this conclusion is less secure than our other deductions about the general nature of the genetic code. Francis Crick Read Quote
It is one of the more striking generalizations of biochemistry – which surprisingly is hardly ever mentioned in the biochemical textbooks – that the twenty amino acids and the four bases, are, with minor reservations, the same throughout Nature. Francis Crick Read Quote
