CSI to Clinics: PCR Applications Explained

If a crime scene sample contains only a few cells, then the total amount of DNA is too small for standard testing. If PCR can create millions of copies of specific target regions, then it provides a sufficient quantity for identification; therefore, it is used to amplify the sample.

If a virus is present in a patient's blood or respiratory tract, then its unique genetic material is also present. If PCR primers are designed to match only that viral code, then a positive result confirms an infection much faster than traditional cell cultures.

If individuals have different numbers of repeating units at specific locations in their genome, then those regions are highly variable. If PCR is used to measure the length of these Short Tandem Repeats (STRs), then a unique DNA profile can be created.

If a genetic disorder is caused by a known DNA mutation, then PCR primers can be designed to target and amplify that specific mutated sequence. While it detects DNA changes, it does not measure transient levels like blood sugar or proteins like blood type antigens.

If standard Taq polymerase can only use DNA as a template, then an RNA virus cannot be copied directly. If the enzyme Reverse Transcriptase is added first to create a cDNA copy of the viral RNA, then the PCR process can proceed to amplify the signal.

If a child inherits half of their chromosomes from their mother and half from their father, then every STR allele in the child's PCR-amplified profile must match a marker present in one of the two biological parents.

If the PCR reaction is to only amplify a specific pathogen's DNA and ignore the host's DNA, then the starting points must be unique. If the primers only bind to the pathogen's code, then only that code will be copied.

If a doctor needs to know how much virus is in a patient's system rather than just "if" it is there, then they must count the copies. If qPCR uses fluorescent dyes to track growth during every cycle, then the speed of growth reveals the starting amount.

If PCR can recover genetic information from small, old, or damaged samples, then it can be used for identification and matching in criminal cases. However, DNA cannot determine physical weight or predict future choices, which are not encoded in the sequence.

If animals shed skin cells or waste into their environment, then their DNA is left in the water or soil. If PCR is used to amplify these trace amounts of "environmental DNA," then scientists can confirm a species is nearby without ever seeing the animal.

If PCR creates millions of fragments of varying lengths, then they must be sorted to be read. If an electric current pulls the DNA through a porous gel, then the fragments will separate by size, creating a visible "barcode" or profile.

If different people have small DNA variations (SNPs) that affect how their liver enzymes process medicine, then a drug might be toxic to one person but helpful to another. If PCR identifies these variations, then a doctor can prescribe the correct dose.

If a forensic sample is tiny or old, the Southern Blot usually fails because it needs a large amount of intact DNA. If PCR can target and copy even small, broken bits of DNA very quickly, then it is the superior tool for high-speed investigations.

If DNA has a chemical half-life of roughly 521 years, then it completely breaks down into unusable fragments after about 6 million years. If dinosaurs lived 66+ million years ago, then their DNA is too degraded for PCR to work.

If the Latin root "amplus" means large, then the biological process of making a genetic signal "larger" or more numerous is called amplification.

If law enforcement agencies need to compare DNA from different crimes, then they need a central digital library. If CODIS stores the specific marker sets generated by PCR, then it allows for the rapid cross-referencing of suspects.

If a surface has come into contact with human tissue or fluids, then it likely contains cells with DNA. Stamps, saliva, and skin flakes all provide the necessary template; a dry footprint or clean metal lacks the biological material.

If a scientist adds multiple sets of primers with different colored fluorescent tags to one reaction, then the machine can track several genes at once. If this saves time and sample material, then multiplexing is a highly efficient technique.

If the PCR process requires heating the sample to 95°C in every cycle, then a normal enzyme would be destroyed. If the enzyme from the hot-spring bacterium Thermus aquaticus is used, then it remains functional throughout the entire run.

If PCR can find "one in a million" sequences and copy them in a few hours, then it provides a level of detail previously impossible. If this speed and sensitivity changed forensics and diagnostics, then it is the defining advancement of the field.