Forensic Fingerprinting: The Essential Role of DNA Polymerase in DNA Analysis

What enzyme is used in fingerprinting?

While no single enzyme is directly employed in the primary chemical or physical methods for visualizing latent fingerprints, enzymes play a crucial role in the subsequent forensic analysis of biological material, particularly DNA, that may be present within fingerprint residues.

The Nature of Fingerprints: What They Are Made Of

Fingerprints are unique patterns formed by the ridges on the skin of our fingers, palms, and soles. When we touch a surface, we leave behind a latent print, which is an invisible impression composed of the natural secretions from our skin. These secretions are a complex mixture, primarily consisting of:

• Water (98-99%): The largest component, which evaporates over time.
• Organic Materials:
  • Amino Acids: Such as serine, glycine, and alanine, which are stable and react with certain chemicals.
  • Lipids: Fatty acids, triglycerides, and waxes from sebaceous glands.
  • Proteins and Urea: Other nitrogenous compounds.
• Inorganic Materials:
  • Salts: Primarily sodium chloride (NaCl) from eccrine glands.

The goal of fingerprint detection is to make these invisible components visible without destroying the delicate ridge patterns.

Traditional Fingerprint Detection Methods

The methods used to visualize latent fingerprints are designed to react with or adhere to these specific chemical components, none of which directly involve enzymatic reactions for visualization:

• Physical Methods:
  • Powders: Fine powders (e.g., black powder, fluorescent powder) adhere to the moisture and fatty components of the print.
  • Cyanoacrylate Fuming (Super Glue Fuming): Cyanoacrylate esters react with moisture, amino acids, and fatty acids in the print, polymerizing to form a white, stable print.
• Chemical Methods:
  • Ninhydrin: Reacts with the amino acids present in the sweat residues to produce a purple-blue color (Ruhemann's Purple). This is a highly effective method for porous surfaces like paper.
  • 1,8-Diazafluoren-9-one (DFO): Also reacts with amino acids, producing a fluorescent product when viewed under specific light wavelengths.
  • Silver Nitrate: Reacts with chloride ions (salts) in the print to form silver chloride, which turns dark when exposed to light.
  • Physical Developer: A silver-based solution used for water-soluble components, particularly on porous surfaces that have been wet.

It is crucial to understand that these methods rely on chemical reactions or physical adhesion, not on the catalytic activity of enzymes.

The Role of Enzymes in Forensic Science (Beyond Direct Fingerprint Detection)

While enzymes are not used for the visualization of fingerprints, they are absolutely critical in a subsequent, highly advanced stage of forensic analysis: the analysis of DNA recovered from the cellular material within a fingerprint. Even a seemingly invisible latent print can contain enough skin cells (keratinocytes) to yield a DNA profile.

The primary enzyme of interest in this context is DNA Polymerase.

• DNA Polymerase and PCR: When only trace amounts of DNA are available from a fingerprint (which is common), forensic scientists use a technique called Polymerase Chain Reaction (PCR). PCR is an enzymatic process that amplifies (makes many copies of) specific regions of DNA.
  • Mechanism: DNA polymerase, typically a heat-stable variant like Taq polymerase (originally isolated from the bacterium Thermus aquaticus), is central to PCR. It synthesizes new DNA strands by adding nucleotides to a primer, using the existing DNA as a template. This process is repeated through cycles of heating and cooling, exponentially increasing the amount of target DNA.
  • Importance: Without DNA polymerase, the minute quantities of DNA often found in fingerprint residues would be insufficient for profiling using techniques like Short Tandem Repeat (STR) analysis.

Other enzymes, such as restriction enzymes, are also fundamental to molecular biology and DNA analysis, though less directly tied to the initial processing of a fingerprint for DNA than DNA polymerase. Restriction enzymes cut DNA at specific recognition sequences and are used in various genetic engineering and DNA analysis techniques, including some older DNA fingerprinting methods (RFLP) or specific research applications. However, for routine forensic DNA profiling from trace evidence like fingerprints, DNA polymerase for PCR is the key enzymatic player.

Why No Direct Enzyme for Fingerprint Visualization?

The reason enzymes are not used for direct fingerprint visualization lies in their nature and the composition of latent prints:

• Specificity: Enzymes are highly specific to their substrates. The primary components of latent prints (water, salts, lipids, amino acids) are not typically substrates for readily available enzymes in a way that would produce a visible, stable, and distinct ridge pattern suitable for forensic analysis.
• Stability: Many enzymes are sensitive to environmental conditions (temperature, pH) and can degrade, making them impractical for field use or long-term storage required for forensic reagents.
• Complexity: Enzymatic reactions are often complex and could lead to less reliable or less distinct visualization compared to simpler chemical or physical interactions.

Conclusion

In summary, the direct visualization of latent fingerprints relies on physical and chemical interactions with the non-enzymatic components of skin secretions. No specific enzyme is employed in these primary detection methods. However, in the broader scope of forensic science, enzymes, most notably DNA polymerase, are indispensable for the amplification and analysis of trace DNA evidence that may be recovered from a fingerprint, allowing for individual identification where traditional methods might only provide pattern recognition. This highlights the multifaceted nature of forensic investigation, integrating various scientific disciplines.