Which of the following statements is incorrect for the polymerase chain reaction (PCR)?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → PCR is useful only when pathogen concentration is very high in the body.
PCR (polymerase chain reaction) makes many copies of a chosen piece of DNA. In diagnosis, a pathogen is usually suspected only after it causes symptoms, when its numbers in the body are already very high. PCR amplifies the pathogen’s nucleic acid, so it can find the pathogen even when it is present in a very low concentration. The question asks for the incorrect statement.
Option (1) PCR involves the amplification of nucleic acids. — A correct statement. PCR works by amplifying nucleic acid, that is, making many copies of a DNA piece.
Option (2) PCR can detect pathogens before they produce symptoms. — A correct statement. PCR can find a bacterium or virus before the symptoms appear. It is used, for example, to detect HIV in suspected AIDS patients.
Option (3) It is used to detect genetic defects. — A correct statement. PCR is used to detect mutations in genes, as in suspected cancer patients, and to identify many other genetic disorders.
Option (4) PCR is useful only when pathogen concentration is very high in the body. — The incorrect statement. The value of PCR is the reverse: it works even when the pathogen concentration is very low. A high concentration is the stage that ordinary methods need.
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. It is a powerful techqnique to identify many other genetic disorders.”