- PCR (Polymerase Chain Reaction) Center
- HIV AIDS Myths and Facts Slideshow Pictures
- Take the HIV/AIDS Quiz
- AIDS Retrospective Slideshow Pictures
What is PCR (polymerase chain reaction)?
PCR (polymerase chain reaction) is a method to analyze a short sequence of DNA (or RNA) even in samples containing only minute quantities of DNA or RNA. PCR is used to reproduce (amplify) selected sections of DNA or RNA. Previously, amplification of DNA involved cloning the segments of interest into vectors for expression in bacteria, and took weeks. But now, with PCR done in test tubes, it takes only a few hours. PCR is highly efficient in that untold numbers of copies can be made of the DNA. Moreover, PCR uses the same molecules that nature uses for copying DNA:
- Two "primers", short single-stranded DNA sequences that are synthesized to correspond to the beginning and ending of the DNA stretch to be copied;
- An enzyme called polymerase that moves along the segment of DNA, reading its code and assembling a copy; and
- A pile of DNA building blocks that the polymerase needs to make that copy.
How is PCR (polymerase chain reaction) done?
As illustrated in the animated picture of PCR, three major steps are involved in a PCR. These three steps are repeated for 30 or 40 cycles. The cycles are done on an automated cycler, a device which rapidly heats and cools the test tubes containing the reaction mixture. Each step -- denatauration (alteration of structure), annealing (joining), and extension -- takes place at a different temperature:
- Denaturation: At 94 C (201.2 F), the double-stranded DNA melts and opens into two pieces of single-stranded DNA.
- Annealing: At medium temperatures, around 54 C (129.2 F), the primers pair up (anneal) with the single-stranded "template" (The template is the sequence of DNA to be copied.) On the small length of double-stranded DNA (the joined primer and template), the polymerase attaches and starts copying the template.
- Extension: At 72 C (161.6 F), the polymerase works best, and DNA building blocks complementary to the template are coupled to the primer, making a double stranded DNA molecule.
With one cycle, a single segment of double-stranded DNA template is amplified into two separate pieces of double-stranded DNA. These two pieces are then available for amplification in the next cycle. As the cycles are repeated, more and more copies are generated and the number of copies of the template is increased exponentially.
What is the purpose of doing a PCR (polymerase chain reaction)?
To do PCR, the original DNA that one wishes to copy need not be pure or abundant. It can be pure but it also can be a minute part of a mixture of materials. So, PCR has found widespread and innumerable uses -- to diagnose genetic diseases, do DNA fingerprinting, find bacteria and viruses, study human evolution, clone the DNA of an Egyptian mummy, establish paternity or biological relationships, etc.. Accordingly, PCR has become an essential tool for biologists, DNA forensics labs, and many other laboratories that study genetic material.
How was PCR (polymerase chain reaction) discovered?
PCR was invented by Kary Mullis. At the time he thought up PCR in 1983, Mullis was working in Emeryville, California for Cetus, one of the first biotechnology companies. There, he was charged with making short chains of DNA for other scientists. Mullis has written that he conceived of PCR while cruising along the Pacific Coast Highway 128 one night on his motorcycle. He was playing in his mind with a new way of analyzing changes (mutations) in DNA when he realized that he had instead invented a method of amplifying any DNA region. Mullis has said that before his motorcycle trip was over, he was already savoring the prospects of a Nobel Prize. He shared the Nobel Prize in chemistry with Michael Smith in 1993.
As Mullis has written in the Scientific American: "Beginning with a single molecule of the genetic material DNA, the PCR can generate 100 billion similar molecules in an afternoon. The reaction is easy to execute. It requires no more than a test tube, a few simple reagents, and a source of heat."
What is RT PCR?
RT-PCR (Reverse transcriptase-polymerase chain reaction) is a highly sensitive technique for the detection and quantitation of mRNA (messenger RNA). The technique consists of two parts:
- The synthesis of cDNA (complementary DNA) from RNA by reverse transcription (RT) and
- The amplification of a specific cDNA by the polymerase chain reaction (PCR).
Previous contributing author and editor:
Medical Author: Frederick Hecht, MD, FAAP, FACMG
Medical Editor: Leslie J. Schoenfield, M.D., Ph.D.
Health Solutions From Our Sponsors
"Tools for genetics and genomics: Polymerase chain reaction"
Has your doctor done a PCR test to diagnose you or your child's genetic disease? What is the condition? Describe your experience.Post
Top PCR (Polymerase Chain Reaction) Related Articles
Breast Cancer (Facts, Stages)Breast cancer is an invasive tumor that develops in the mammary gland. Breast cancer is detected via mammograms, breast self-examination (BSE), biopsy, and specialized testing on breast cancer tissue. Treatment of breast cancer may involve surgery, radiation, hormone therapy, chemotherapy, and targeted therapy. Breast cancer risk may be lowered by managing controllable risk factors.
Genital Herpes in Women OverviewGenital herpes is a sexually transmitted disease (STD) caused by the herpes simplex virus (HSV). Symptoms of genital herpes include painful blisters and often fever, body aches, and swollen lymph nodes for first time infection. Genital herpes is diagnosed with lab tests to test for the presence of the virus. Treatment for genital herpes includes antiviral medications to shorten the duration of the outbreak or reduce the risk of future outbreaks. There is no cure for genital herpes. Condoms may help prevent the spread of genital herpes.
Hepatitis CHepatitis C is an inflammation of the liver due to the hepatitis C virus (HCV), which is usually spread by blood transfusion, hemodialysis, and needle sticks, especially with intravenous drug abuse. Symptoms of chronic hepatitis include fatigue, fever, muscle aches, loss of appetite, and fever. Chronic hepatitis C may be cured in most individuals with drugs that target specific genomes of hepatitis C.
HIV TestingHIV antibody tests detect antibodies the body produces to neutralize the virus. HIV RNA testing uses polymerase chain reaction to detect HIV RNA in a person's blood. It usually takes one to three days to get results.
Malaria FactsMalaria is a disease that is spread by the bite of an infected Anopheles mosquito. Malaria symptoms include fever, chills, nausea, vomiting, and body aches. Treatment involves supportive care and antibiotics.
MeningococcemiaMeningococcemia is a bloodstream infection caused by Neisseria meningitides. Meningococcemia symptoms include fever, headache, fatigue, and body aches. Meningococcemia is treated with intravenous antibiotics. There is an effective and safe vaccine to protect against most serogroups of meningococcus that cause meningococcemia.
Mycobacterium MarinumMycobacterium marinum (M. marinum) is bacteria found in fresh and saltwater that can infect the skin through cuts or scrapes, causing granulomas to appear on the skin near the site of infection. This infection may be treated with a long course of oral antibiotics.
PertussisWhooping cough (pertussis) is highly contagious respiratory infection that is caused by the bacteria Bordetella pertussis. There are an estimated 300,000 plus deaths annually from whooping cough (pertussis). Whooping cough commonly affects infants and young children but can be prevented with immunization with the vaccine. First stage whooping cough symptoms are a runny nose, sneezing, low-grade fever, a mild cough with the cough gradually becoming more severe. After one to two weeks, the second stage of whooping cough begins.
Ramsay Hunt SyndromeRamsay Hunt syndrome is an infection of a facial nerve that causes a red painful rash with blisters and facial paralysis. Other symptoms of Ramsay Hunt syndrome may include:
- ear pain,
- hearing loss,
- dizziness (or vertigo),
- dry eye,
- and changes in taste sensation.
- antiviral agents,
- steroids, and
- pain medications.
Tuberculosis (TB) FactsMycobacterium tuberculosis is the bacterium that causes tuberculosis (TB). Symptoms and signs of TB include bloody sputum, fever, cough, weight loss, and chest pain. Treatment depends upon the type of TB infection.
Worried About Zika Virus?A quick guide to the Zika virus, its causes, symptoms, and treatment options. Find out how Zika is transmitted by mosquitoes, Zika's effects on pregnancy, as well as how to protect yourself from the Zika virus.