Scientists have successfully engineered the bacterium Escherichia coli (E. coli) to produce human insulin, an important endocrine peptide in mammals. When the human gene that encodes insulin, flanked by certain regulatory sequences, is integrated into the genetic material of E. coli cells, the resulting E. coli cells can produce insulin that is active in humans. Such E. coli strains are called recombinant E. coli. The insulin-producing trait of recombinant E. coli can be passed down to new generations when cells are grown in a proper selective medium. Why can the recombinant E. coli strain produce biologically active human insulin?

Gene transfer has changed the genetic code of E. coli.
Gene transfer has changed the way E. coli regulates gene expression.
E. coli cells and human cells have similar compositions of nucleotides and amino acids.
E. coli cells and human cells have similar enzymes and processes for synthesizing mRNA and protein.