Which of the following statements about PCR is incorrect?

The correct answer is Option (4) → PCR detects very low concentrations of a virus by amplification of its proteins.

PCR (Polymerase Chain Reaction) makes many copies of a piece of DNA or RNA. Because of this amplification, a pathogen present in a very low amount can be found before any symptom appears. PCR is one of the techniques of early diagnosis.

Option (1) PCR serves the purpose of early diagnosis. — A correct statement. PCR, along with recombinant DNA technology and ELISA, serves the purpose of early diagnosis.

Option (2) PCR is used to detect HIV in suspected AIDS patients. — A correct statement. PCR is now routinely used to detect HIV in suspected AIDS patients.

Option (3) PCR is used to detect mutations in genes in suspected cancer patients. — A correct statement. PCR is also used to detect mutations in genes in suspected cancer patients.

Option (4) PCR detects very low concentrations of a virus by amplification of its proteins. — An incorrect statement. PCR amplifies the nucleic acid of the bacterium or virus, not its proteins. Proteins and other antigens of a pathogen are detected by ELISA.

NCERT Extract:
Molecular Diagnosis – “… Presence of a pathogen (bacteria, viruses, etc.) is normally suspected only when the pathogen has produced a disease symptom. By this time the concentration of pathogen is already very high in the body. However, very low concentration of a bacteria or virus (at a time when the symptoms of the disease are not yet visible) can be detected by amplification of their nucleic acid by PCR. … PCR is now routinely used to detect HIV in suspected AIDS patients. It is being used to detect mutations in genes in suspected cancer patients too.”