The four steps in DNA processing are extraction, quantitation, amplification, and separation. These steps form the standard workflow used in forensic laboratories to turn a biological sample (blood, saliva, skin cells) into a usable DNA profile. Each step builds on the last, and skipping or rushing any one of them can compromise the entire result.
Step 1: Extraction
Extraction is the process of pulling DNA out of cells and separating it from everything else in the sample: proteins, fats, carbohydrates, and other cellular debris. The goal is to end up with pure DNA, free of contaminants that could interfere with later steps.
To get there, lab technicians first break open the cells in a process called lysis. This can be done chemically, with enzymes, or with heat. Once the cells are cracked open and their contents released, the DNA needs to be isolated from the surrounding material. Several methods exist. One common approach uses a chemical mixture of phenol and chloroform: the sample is vigorously mixed with the solution, then spun in a centrifuge. DNA rises into the upper water-based layer, while denatured proteins sink into the lower organic layer. The DNA is then collected and concentrated by adding alcohol and salt.
Another widely used method is solid-phase extraction, where the liquid sample is poured through a small column. DNA binds to the column material while contaminants wash through. A final rinse with a special buffer releases the purified DNA. Magnetic bead methods work on a similar principle, using tiny charged beads that selectively bind proteins and debris, leaving clean DNA behind in solution. Whichever technique a lab uses, the end product is the same: a small tube of purified DNA ready for measurement.
Step 2: Quantitation
Before doing anything else with extracted DNA, the lab needs to know exactly how much is there. This step, quantitation, determines the concentration of human DNA in the sample. It matters more than most people realize. Too much DNA fed into the next step produces overloaded, unreadable results. Too little produces incomplete or unreliable profiles. Either outcome means the lab has to start over, wasting time and precious sample material.
The standard tool for this measurement is real-time PCR (also called qPCR). It works by running a small-scale copying reaction on the DNA while tracking fluorescence in real time. Short molecular probes, each tagged with a fluorescent dye, bind to human-specific DNA sequences. As copies are made, the probes break apart, releasing their dye and producing a glow. More DNA in the sample means more fluorescence, and the instrument translates that signal into a concentration number. This method is preferred over older techniques like UV light absorption because it can distinguish human DNA from bacterial or animal DNA that might be mixed in.
The International Society of Forensic Haemogenetics has recommended since 1992 that DNA quantification be performed before any amplification step. Once the concentration is known, the technician adjusts the amount of DNA going into amplification to hit an optimal range, ensuring the final profile will be clean and within the detection range of the instruments.
Step 3: Amplification
Amplification is where the lab makes millions of copies of specific regions in the DNA. The technique used is called PCR, or polymerase chain reaction. Forensic samples often contain only tiny amounts of DNA, sometimes just a few cells’ worth. PCR solves this by targeting the exact genetic markers needed for a profile and copying them over and over until there’s enough material to detect.
PCR works through repeated heating and cooling cycles, each with three phases:
- Denaturation: The sample is heated to about 95°C, which breaks apart the two strands of the DNA double helix.
- Annealing: The temperature drops to roughly 55°C to 72°C. Short synthetic DNA fragments called primers bind to complementary sequences on the now-single-stranded DNA, bracketing the target region.
- Extension: The temperature rises to around 75°C to 80°C, and a specialized enzyme builds a new complementary strand starting from each primer.
Each cycle doubles the amount of target DNA. After 25 to 30 cycles, a single starting copy can become billions. The specific markers being copied are short tandem repeats, or STRs. These are sections of DNA where a short sequence (typically 2 to 6 letters long) repeats a variable number of times. The number of repeats differs from person to person, which is what makes DNA profiles unique. A thermal cycler, basically a programmable heating block, automates the temperature changes so the process runs unattended.
Step 4: Separation and Detection
The final step takes the amplified DNA fragments and sorts them by size to reveal the actual profile. The standard method is capillary electrophoresis, where the DNA is injected into a hair-thin glass tube filled with a gel-like polymer. An electric current pulls the fragments through the gel. Smaller fragments move faster, larger ones move slower, so they spread out by size along the tube’s length. The relationship between migration time and fragment size is linear, making measurements precise.
As the fragments reach the end of the tube, a laser hits them and reads their fluorescent tags. During the amplification step, the primers used to copy each STR marker were pre-labeled with different colored dyes. Forensic kits typically use four or more fluorescent colors (common ones go by names like FAM, JOE, TAMRA, and ROX). This color-coding is essential because some STR markers produce fragments of similar sizes that would overlap if read by size alone. By assigning different colors to overlapping markers, the instrument can distinguish them spectrally even when they arrive at the detector at nearly the same time.
The output is an electropherogram, a graph showing peaks at specific positions. Each peak represents an STR marker, and its position indicates the number of repeats. Together, the peaks form the individual’s DNA profile. Laboratories are required to attempt typing at all core loci designated by the FBI’s CODIS database to ensure profiles are compatible with national searches.
How Labs Prevent Contamination
Because PCR is so powerful at copying tiny amounts of DNA, even a stray skin cell or a speck of previously amplified material can ruin a result. Forensic labs follow strict protocols laid out by the National Institute of Standards and Technology to prevent this. Pre-PCR and post-PCR work must take place in physically separated rooms with floor-to-ceiling walls and closed doors. Equipment, supplies, and protective gear are dedicated to one area and never moved to the other without decontamination. DNA extracts are stored only in pre-PCR areas, while amplified products stay in post-PCR areas. Even waste from post-PCR rooms cannot pass through pre-PCR spaces without being double-bagged.
How Long the Process Takes
In a well-staffed lab processing a straightforward sample, the four steps can be completed in a single day. In practice, forensic labs handle large caseloads with limited resources, and turnaround times are much longer. The Colorado Bureau of Investigation, for example, recently cut its average processing time for sexual assault cases from 450 days to 190 days, a 58 percent reduction, and is targeting a 90-day turnaround for all DNA cases by the end of 2027. Rapid DNA instruments, designed for use at booking stations, can compress the entire four-step workflow into under two hours, though they are currently approved only for specific sample types.
Newer sequencing technologies are also changing the landscape. Next-generation sequencing platforms can read thousands to millions of DNA fragments simultaneously, and some long-read methods skip the amplification step entirely, eliminating a potential source of error. These systems are gradually being validated for forensic use, though capillary electrophoresis remains the standard in most accredited labs today.