Regarding galactose sugar metabolism, normally, a repressor protein binds to the galact ase gene initiation site , that is the operator site. Genetic transcription of the enzymes needed to metabolize the galactose sugar is then allowed to occur. When the sugar again becomes absent, repressor protein, now free of the sugar, binds once again to the operator region of the gene and inhibits transcription by RNA polymerase. Some mutations are beneficial , some are harmful . Harmful mutations are the mechanism of some methods of killing bacteria or virus or inhibiting bacterial growth.
Radiation is a common means of killing bacteria by inducing mutations.
It codes for a peptide 4 amino acids long, because the gene consists of only 4 codons. Suppose that CGC was the start of another gene. May or may not cause a codon reading error, because of the fact that the genetic code is redundant, that is multiple codons often code for the same amino acid. At most, only one amino acid of the peptide coded for would be effected. Cause a frameshift mutation, much more serious than a substitution.