Analysis of the Core Technology of DNA in Solving Crimes

The core process of DNA crime solving

The entire process is divided into four key stages: on-scene collection -> laboratory analysis -> database comparison -> identification and solving the case.

Stage 1: Collection of biological samples from the crime scene

This is the starting point and the most critical step. If the sample at the scene is not found or is contaminated, all subsequent techniques will be useless.

Forensic technicians will systematically search the crime scene for any items that may contain human biological material, such as:

Blood, saliva, semen stains, hair (with hair follicles), sweat, and skin flakes: These are common sources of DNA.

Potential carriers:

Cigarette butts: Saliva.

Used water bottles, cups, and straws: Saliva.

Leftover food and chewing gum: Saliva.

Masks and toothbrushes: Saliva and oral cells.

Murder weapons and tools (such as crowbars and daggers): The handles may contain the perpetrator’s sweat or skin flakes.

Clothing, hats, and gloves: Sweat, dander, or hair on the inside.

Residue between fingernails: If the victim struggled with the perpetrator, skin tissue may be obtained.

Collectors will wear a hood, gloves, a mask, and shoe covers. They will carefully extract samples using sterile tweezers, cotton swabs, and other tools. Each sample will be placed in a dedicated evidence bag, with detailed information such as the location and time of discovery recorded to prevent cross-contamination.

Phase Two: Laboratory Analysis and DNA Profiling
The collected samples will be sent to a forensic DNA laboratory for scientific analysis.

DNA Extraction: First, scientists use chemical methods to purify DNA molecules from cells, separating them from other cellular components such as proteins.

Quantification: The amount of extracted DNA is measured to determine whether it is sufficient for further analysis.

PCR Amplification (Polymerase Chain Reaction): This is a crucial step. The amount of DNA in evidence at the scene is often extremely small (even just a few cells). PCR technology acts like a “biological photocopier,” exponentially amplifying specific DNA fragments millions of times to produce sufficient quantities for testing.

STR analysis (short tandem repeat analysis): This is currently the most popular technique in forensic DNA identification. It detects highly variable, repetitive sequences within DNA strands. The number of repeats of these sequences is unique to each person (except for identical twins). Typically, laboratories test for around 20 core STR loci.

Generate a DNA profile: After analysis, a computer generates a unique DNA typing profile, typically represented by a series of numbers (each number represents the number of repeats of a specific STR locus). This profile serves as the “genetic ID” for the biological sample.

Phase 3: Database Comparison
After obtaining the suspect’s “genetic ID,” investigators compare it to a database.

Comparison with physical evidence at the scene: If DNA from an unknown suspect has been found in previous cases, cross-checking can be performed to determine whether the crime was committed by the same individual.

Comparison with DNA databases of criminals:

This is crucial for solving cases. China has established a massive national DNA database for public security agencies.

According to legal regulations, public security agencies compulsorily collect DNA samples from arrested criminal suspects (such as those being investigated for theft, robbery, or violent crimes) and enter them into the database.

Unknown DNA profiles extracted at the scene are automatically compared with the vast database of known DNA profiles. Once a “hit” is found, the suspect’s identity is immediately confirmed.

Family Lineage Investigation and Y-Chromosome Identification (Y-STR):

Scenario: When an exact match for a suspect cannot be found in the database, police employ another powerful strategy.

Principle: The Y chromosome is male-specific and is generally inherited paternally (from father to son). Male members of the same paternal family have highly similar Y chromosome STR profiles.

Application: Police can use the Y-STR profile of male suspects obtained at the scene to compare their paternal lines against the database. Even if the criminal himself has no criminal record, his father, brothers, uncles, cousins, and other male relatives may have had their DNA collected for other reasons and entered into a database. By matching these family lines, the scope of investigation can be greatly narrowed, zeroing in on a specific family. Focused investigations and targeted DNA collection within that family can then be conducted to ultimately identify the true perpetrator. This technology has been instrumental in solving many long-standing cases in recent years, such as the Baiyin Serial Murder Case.

Phase 4: Identification of the Suspect and Closure of the Case (Identification)

Preliminary Identification: The target suspect is identified through database matching or family lineage investigation.

Sample Verification: Police will, in accordance with the law, conduct a direct DNA sample from the suspect (usually a buccal swab or blood sample) and compare the DNA typing with that of the physical evidence at the scene.

Confirmation of Identity: If the two typings are identical, the laboratory will issue a “Forensic Evidence Report,” indicating a high probability (usually a statistical value greater than 99.99%) that the physical evidence at the scene originated from the suspect.

Building a Chain of Evidence: DNA evidence is powerful scientific evidence, but it often needs to be corroborated with other evidence (such as surveillance footage, witness testimony, motive, and chronological evidence) to form a complete chain of evidence that ultimately brings the perpetrator to justice.

Summary and Features
Steps
Key Actions
Description

1. Collection: On-site investigation and search for biological samples. Prevent contamination and maintain detailed records. This is the foundation for success.
2. Analysis: Laboratory extraction, PCR amplification, and STR analysis generate a unique “DNA genetic ID” (a string of numbers).
3. Comparison: Comparison with DNA databases and family tree screening (Y-STR) are key steps. Directly match the suspect or narrow down the scope through their family.
4. Identification: Direct sampling and verification to form a chain of evidence. DNA evidence must be combined with other evidence to establish a consistent identification.
   Advantages of DNA Technology:

Extreme Accuracy: Its ability to identify individuals is extremely strong, earning it the nickname “the king of evidence.”

Traceability: Even extremely small amounts of sample can be tested.

Stability: DNA is stable, allowing samples stored for years to be analyzed.

Precautions:

Ethics and Privacy: The establishment of a large-scale DNA database raises issues of personal privacy and requires strict legal and regulatory oversight.

Contamination Risk: Field collection and laboratory operations must be conducted with extreme caution; contamination could lead to erroneous conclusions.

Auxiliary Use: DNA evidence cannot be used in isolation but must be integrated into a complete chain of evidence.