How to collect DNA for criminal investigation

How to collect DNA for criminal investigation

The public security criminal investigation collects DNA to make a database mainly including the following steps:

Sample collection

Crime scene samples: Criminal investigation personnel will carefully investigate the crime scene to find various biological samples that may contain the suspect’s DNA, such as blood, saliva, hair, semen, skin tissue, etc. For the samples found, their location, morphology and other information must be recorded in detail, and they must be properly stored and marked to prevent the samples from being contaminated or damaged.
Suspect samples: Oral swabs are usually used to collect oral mucosal cells from suspects, which is a non-invasive and more convenient method. Blood samples can also be collected to obtain larger amounts of high-quality DNA. In addition, hair (with hair follicles) and semen samples can also be used as collection objects, but it is necessary to ensure that the collection process is legal and compliant. Generally, the suspect’s consent must be obtained and the collection must be carried out with his voluntary cooperation.

Samples of other relevant persons: In some cases, such as those involving family relationships, it may be necessary to collect DNA samples from the victim’s family or other relevant persons as references to assist in the case investigation.

Sample processing
Extraction: Use professional DNA extraction kits and equipment to separate the DNA in the sample according to standard operating procedures. Different types of samples have different extraction methods. For example, blood samples need to undergo red blood cell lysis and protein removal, while hair samples need to first process the hair follicles to release the DNA.
Purification: The extracted DNA may contain impurities, such as protein and salt, which may affect subsequent analysis and testing. Through the purification step, impurities can be removed to obtain DNA samples with higher purity. Commonly used purification methods include column chromatography and ethanol precipitation.
Amplification: Since the amount of DNA samples from crime scenes or other sources is usually small, it is difficult to directly detect and analyze them. Using polymerase chain reaction (PCR) technology, DNA fragments in specific areas can be amplified in large quantities to reach a detectable level to meet the needs of subsequent genotyping and other analyses.
Data entry
Genotyping: Genotyping the processed DNA samples, that is, determining the genotype of the DNA sample at a specific gene locus. Commonly used genotyping technologies include short tandem repeat (STR) typing technology. By analyzing the genotypes of multiple STR loci, a unique DNA map of an individual can be obtained.
Data entry and storage: The DNA data obtained by genotyping and related sample information, such as sample source, collection time, and basic information of the person being collected, are accurately entered into the public security criminal investigation DNA database. The entry process must follow strict data entry specifications and quality control standards to ensure the integrity and accuracy of the data. At the same time, in order to ensure the security and reliability of the data, the database uses advanced computer systems and storage devices, and establishes a complete data backup and recovery mechanism.

Database management and application
Data comparison: When a new criminal case occurs, the DNA samples collected from the crime scene are processed and genotyped, and then compared with the existing DNA data in the database. Through comparison and analysis, we can find individual information that matches the DNA at the crime scene, provide clues and evidence for case detection, and quickly identify the suspect.

Data update and maintenance: Regularly update and maintain the data in the database, including adding newly collected DNA sample data, correcting erroneous data, deleting invalid data, etc. At the same time, with the development of technology and the improvement of detection methods, re-analyze and compare the existing DNA data in the database to improve the quality and application value of the data.
Quality control and security assurance: Establish a strict quality control system to monitor and evaluate the quality of each link such as sample collection, processing, and data entry to ensure the accuracy and reliability of the data in the database. Strengthen data security management, take measures such as data encryption, access control, and audit tracking to prevent security incidents such as data leakage and tampering