Living cells typically have biosynthetic pathways to synthesize at least some of the amino acids used in making proteins. Some strains of E. coli, a prokaryote, can synthesize the amino acid tryptophan, while other E. coli strains cannot. Similarly, some strains of the yeast S. cerevisiae, a eukaryote, can synthesize tryptophan, while other S. cerevisiae strains cannot.

Which of the following describes the most likely source of genetic variation found in the tryptophan synthesis pathways of both species?

Crossing over and random assortment occur during meiosis for eukaryotic organisms.

E.coli and S. cerevisiae are not viruses.

Errors in DNA replication may cause mutations which change the genome of the organism, thus changing the amino acids produced.