Question

Why is it sometimes necessary to use polymerase chain reactions to create copies of DNA fragments?

Answer 1

Because many downstream analyses or uses require far more molecules of a particular DNA segment than are present in the original sample. PCR (polymerase chain reaction) lets you make millions–billions of copies of a chosen fragment quickly and specifically. Reasons you would need to do that include:

- Extremely small starting amount: clinical, forensic, archaeological or single-cell samples often contain only picograms–nanograms of target DNA; PCR amplifies it to detectable/usable levels.
- Low-abundance target in a background of other DNA: PCR’s primer specificity lets you enrich a particular sequence from a complex mixture (pathogen detection, rare mutation detection).
- Downstream requirements: sequencing, cloning, restriction analysis, many biochemical assays and some preparation methods require micrograms or at least nanograms of target DNA that you usually do not have without amplification.
- Speed and convenience: PCR amplifies DNA in vitro in hours without need to culture organisms or clone into cells.
- Preserving the original sample: amplification produces copies so you can run destructive tests without exhausting the original material.

(Practical caveat: PCR is extremely sensitive to contamination and requires appropriate controls.)