"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"

This quote by Francis Crick is describing the principle of hereditary messages, which are the instructions that figure out the attributes of an organism. The + class is a type of genetic message that has one additional base, which is a system of hereditary information, compared to the - class. The - class has one fewer base than the + class. This quote is emphasizing the simplicity of the principle of hereditary messages, as it is easy to comprehend the distinction between the two classes. By having one additional or one less base, the genetic messages of the + and - classes stand out from each other. This quote is also highlighting the value of the base in hereditary messages, as it is the unit of information that figures out the characteristics of an organism.