Gene expression is the combined process of the transcription of a gene into mRNA and its translation into protein. Regulation of gene expression occurs primarily at the level of transcription in both prokaryotes and eukaryotes. However, in eukaryotes regulation also occurs through modifications of DNA, post-transcriptional and post-translational events. In prokaryotes the genes involved in a metabolic pathway are often present in a linear array called an operon, e.g. the lac operon. An operon can be regulated by lac repressor, induction of expression and catabolite repression. Mutations are produced when changes occur in the nucleotide sequence in DNA either due to mistakes in base pairing, by the covalent modification of bases and/or by deletion and insertion of bases. Point mutation results from single base substitution. Frame shift mutation result from deletion or insertion of nucleotide in DNA.