About twenty percent of the DNA produced by the PCR is copied inaccurately. Explain why it's not safe to use PCR to clone the CFTR gene for use in treating cystic fibrosis.

• A. Mutations could be copied into many cells, leading to unintended toxin production or malfunction
• B. PCR is too slow
• C. PCR cannot clone CFTR gene
• D. PCR yields only correct sequences

Answer: (A)

The key idea is that PCR is not error-free, so using it to produce a therapeutic gene can spread mistakes. PCR copies DNA many times, but the polymerase makes occasional errors. If about one in five copies carries a mutation, introducing those copies into patient cells could mean the CFTR protein is altered or nonfunctional, or even cause unintended harmful effects. In gene therapy, you need an exact, correct sequence to ensure proper function and avoid new problems, so the presence of frequent errors makes PCR-derived products unsafe for cloning CFTR.

The other statements don’t fit. PCR can amplify the CFTR region, so the issue isn’t an inability to get CFTR DNA. PCR isn’t inherently slow; it’s actually rapid. And PCR does not yield only correct sequences—errors do occur, which is exactly why it’s not suitable for this therapeutic purpose.