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?
(A) Exchange of genetic information occurs through crossing over.
(B) Viral transmission of genetic information required to synthesize tryptophan occurs.
(C) Random assortment of chromosomes leads to genetic variation.
(D) Errors in DNA replication lead to genetic variation.
Answer: (D) Errors in DNA replication lead to genetic variation.
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