A molecular biologist used a retroviral vector to introduce a gene coding for a certain human enzyme into mouse cells. One cell line was isolated that was able to make the human enzyme, but it had lost the ability to express an endogenous, normally expressed gene in the process. What is the best explanation for these results?

• A. The virus caused the mouse cells to become diseased.
• B. The virus had transferred a gene from one mouse cell to another.
• C. The virus was too small to carry the entire gene.
• D. The enzyme acted as a nuclease enzyme, cutting up mouse DNA.
• E. The virus inserted the gene encoding the human enzyme within the sequence of a normally expressed endogenous gene.

Answer: (E)

Verified step by step guidance

1. Understand the role of a retroviral vector: Retroviral vectors are used to introduce new genetic material into cells. They integrate their genetic material into the host cell's genome, which can disrupt existing genes.
2. Consider the integration process: When the retroviral vector inserts the human gene into the mouse cell's genome, it can integrate randomly. This random integration can disrupt the function of endogenous genes if the insertion occurs within or near them.
3. Analyze the loss of gene expression: The loss of expression of a normally expressed endogenous gene suggests that the insertion of the human gene interfered with the normal function of this gene, likely by disrupting its sequence or regulatory elements.
4. Evaluate the options: The other options do not adequately explain the observed results. The virus causing disease, transferring genes between cells, or being too small to carry the gene do not account for the specific loss of endogenous gene expression.
5. Conclude with the most plausible explanation: The most logical explanation is that the retroviral vector inserted the human gene into the sequence of a normally expressed endogenous gene, disrupting its expression and leading to the observed results.