DNA Manipulation & Profiling

Once DNA is extracted from cells, two workhorse techniques let us copy it and read it: PCR and gel electrophoresis.

PCR — copying DNA

The polymerase chain reaction makes millions of copies of a target region by cycling through three temperatures:

• Denature (~95 °C) — heat splits the double strand.
• Anneal (~55 °C) — primers bind either end of the target.
• Extend (~72 °C) — a heat-resistant polymerase (Taq) adds free nucleotides to build new strands.

Each cycle doubles the DNA, so the amount grows exponentially.

Gel electrophoresis — sorting DNA

Fragments are loaded into a gel and pulled through it by an electric field. DNA is negatively charged, so it moves toward the positive electrode; smaller fragments travel further, sorting the DNA by size into a pattern of bands.

Smaller fragments migrate further, producing a banding pattern read like a barcode.

DNA profiling

Everyone (except identical twins) has a unique pattern of short tandem repeats (STRs). Amplifying and separating these with fluorescent labelling produces a DNA profile — read from an electropherogram. Comparing profiles is used in forensic science, paternity and identifying remains. It also raises real ethical, economic and cultural questions about who collects and stores genetic data.