DNA fingerprinting is a laboratory technique used to identify individuals based on distinct patterns within their genetic material. It has become an important tool across various fields, including criminal investigations, paternity testing, and victim identification. This method relies on the principle that specific DNA regions vary uniquely among people, creating a genetic signature.
The Tiny Amounts of DNA
Forensic samples often contain only minute quantities of DNA, posing a significant challenge for analysis. Sources like a single hair follicle, a minuscule drop of blood, saliva residue, or skin cells can serve as sources. These trace amounts are too small to be directly analyzed by standard laboratory equipment. This limited material makes it impossible to generate a complete and interpretable DNA profile without further processing.
Making More Copies: The PCR Process
To overcome the challenge of limited DNA, scientists use Polymerase Chain Reaction (PCR), a process often described as molecular photocopying. PCR can amplify a tiny initial DNA sample into millions of copies. This process occurs in a series of temperature cycles within a thermal cycler.
Each cycle of PCR involves three main steps. First, denaturation heats double-stranded DNA to separate it into two single strands. Next, annealing lowers the temperature, allowing primers to bind to specific target regions. Finally, extension uses a heat-stable DNA polymerase enzyme to synthesize new complementary DNA strands. This three-step cycle is repeated 25-40 times, doubling the amount of target DNA with each cycle, leading to an exponential increase.
How Copies Enable DNA Fingerprinting
Millions of DNA copies are essential because analytical techniques for DNA fingerprinting are not sensitive enough to detect small numbers of DNA molecules. Modern DNA fingerprinting primarily relies on analyzing Short Tandem Repeats (STRs), short, repetitive DNA sequences varying in length among individuals. After PCR amplification, these millions of STR fragments are separated and detected, most commonly using capillary electrophoresis.
In capillary electrophoresis, amplified DNA fragments are drawn through a thin capillary filled with a gel-like polymer by an electric field. Smaller fragments move faster, separating based on size. As fragments pass a detector, they produce a signal recorded as peaks on an electropherogram. Sufficient amplified DNA is required to generate these distinct, measurable peaks, representing specific STR alleles. Without millions of copies, the signal would be too weak, rendering DNA patterns invisible or inconclusive.
What Happens Without Enough Copies
Insufficient DNA copies compromise the reliability and interpretability of DNA fingerprinting results. If amplification is inadequate, analytical techniques may yield weak or partial DNA profiles. A partial profile means not all targeted STR regions were detected, leading to incomplete genetic information.
Such ambiguous results can make it impossible to confidently include or exclude an individual in forensic investigations or establish definitive relationships in paternity cases. Factors like degraded DNA or very low initial quantities can prevent proper amplification, leading to inconclusive outcomes. Robust amplification is important to ensure the accuracy and utility of DNA fingerprinting in real-world applications.