a. Acridine orange produces frame shift mutations. It is an effective mutagen. Frame shift mutations make alteration of reading frame and create null alleles. The codons are groups of three nucleotides. If one or more nucleotide is inserted or deleted the number of nucleotides will not be in the multiples of three and this can alter the reading frame.
Acridine orange is an intercalating agent. These agents are planar molecules. They act like base pairs and slip themselves in between the stacked nitrogen bases at the core of the DNA double helix. Acridine orange in the intercalated position can cause an insertion or deletion of a single nucleotide pair and produce null alleles.
b. A certain acridine-like compound induces a mutation with single insertions. Some revertants are produced when this mutation is treated with the same compound. This can be explained as follows. A single insertion means it causes + 1 frame shift mutation which alters the reading frame by adding one nucleotide. It can be reverted by two further single insertions so that the codon of three nucleotides will be formed and the reading frame is re - established.