The detection of many unsettled cases is related to DNA evidence. For example, the case of dismembering a corpse in Hangzhou not long ago, and the murder case of Nanjing Medical University, which has not been solved for decades, was detected last year. < / P > < p > in the crime scene investigation, all the evidence from the suspect is the key material evidence. Like fingerprints, blood, sweat. But we all know that fingerprints are easy to identify and criminals know. As criminals evolved, they learned how to counter detect without leaving fingerprints. DNA evidence, by contrast, is more defensible for criminals. < p > < p > fingerprints only exist on your fingertips. There is DNA in every cell of your body. Whether you are male or female, how old you are this year, and whether you have congenital diseases are clearly written on them. As long as the criminal accidentally leaves something on the scene, such as a hair with hair follicle cells, a drop of blood, a little semen or dandruff. As long as there are cells, it is possible to extract DNA. There is an important profession in forensic medicine, crime scene investigator (CSI). Maybe your forensic enlightenment comes from that American TV show of the same name, but CSI in reality is more grounded – they will appear on the scene of a crime and be tested in the forensic laboratory. However, they will not have direct contact with prisoners, or participate in reasoning, interrogation and arrest. This is an absolutely rational profession, whose mission is to record the real scene and restore the iron evidence. < / P > < p > I guess you haven’t tried to irradiate your saliva and urine with ultraviolet light. In fact, these body fluids are naturally fluorescent substances, such as urine, semen, saliva. CSI will use black light (which can emit ultraviolet light), blue LED flashlight, green LED flashlight and so on with filters of specific colors. This is because under different wavelengths of excitation light, the body fluid will release a specific color of fluorescence. The fluorescence of < / P > < p > became more and more obvious. This means that even if a criminal cleans up a crime scene with his own ingenuity, what looks like a clean surface in the blue light is a different picture. < / P > < p > note that the blood is not fluorescent. But if you’re a criminal detective fan, you’ll already know that spraying luminol solution can make the blood stain blue fluorescent. < / P > < p > it is not difficult to extract DNA from biological tissue samples. Even trace samples can be amplified by PCR. Xiaobian was once a biological dog. There is a saying in the circle: “young people don’t work hard, but grow up PCR.” PCR technology is common enough and standardized in the laboratory. < / P > < p > if the police already have a suspect, things can still move forward. We need to compare the DNA sequence of the scene with that of the suspect. You may subconsciously think that the DNA sequence of the scene will be tested, and then the sequence of the suspect will be measured out, and the computer will be used to compare and work together. The biodog will tell you again that the cost of whole genome sequencing is not small. Human has 23 pairs of chromosomes, more than 3 billion base pairs, and has been known to encode 20000 genes. The key is that 99.9% of the gene sequences of each person are the same. Obviously, the method to detect all the DNA sequences is too stupid, so we need to choose a few with personality. It’s a coincidence that these personalized sequences are not coding sequences, that is to say, they are excluded from 20000 genes. They are short tandem repeat (STR), short tandem repeat (STR). “Short clip” is composed of 2-6 bases. The specific number of repeats varies from person to person. In DNA comparison, we only need to amplify these STR loci, and then compare whether the str repeat numbers of suspects and field samples are the same. At present, there are 20 STR loci for comparative analysis. In this way, accurate benchmarking can be achieved and the workload is greatly reduced. The FBI has a fingerprint database, the integrated automated fingerprint identification system (IAFIS). If fingerprints are collected at the scene, the database, after a day or so of analysis, can give 20 candidates with similar matching patterns – anyone in the population that may match. This is equivalent to directly helping the police determine the subjects of investigation. < / P > < p > can we access the DNA database in a similar way? This is where DNA analysis is difficult to use. The FBI also has a DNA database, or CODIS, where everyone has 20 STR loci. The problem is that most of the people’s information does not exist in CODIS. Only those who have been convicted by the public security organs and have criminal records will be included. China’s police also have such a database, information collection is a similar process. The world’s largest national DNA database is in the UK, with more than 6 million files covering only 10% of the UK population. < / P > < p > why not build a national DNA database like fingerprints? Genetic information is the ultimate personal privacy, and its collection and use will involve many ethical and legal issues. In fact, such information is easy to get. Since the 1950s, British born babies have left their own blood samples for screening for genetic diseases. The information is stored on gsrika, where genetic information helped identify a Briton killed in the 2004 tsunami in Indonesia. However, the use of geslika is not regulated for forensic identification. < / P > < p > if through the database search, the DNA samples on the spot do not find a match in the database, then the DNA comparison road will not be so smooth. Fortunately, this DNA is genetic material in the final analysis, which is still well documented. With the continuous expansion of the database, if a criminal’s lineage or collateral blood relatives left information in the police’s DNA database, wouldn’t it be possible to follow suit to investigate? It is in this way that the murder case of Nanyi University mentioned above was solved. Moreover, it is a highly conserved “Y chromosome”, which is transmitted only to male and not to female, which is the same in males of the whole family. < / P > < p > the current second-generation sequencing technology is more powerful, and DNA criminal investigation will have a greater use. At school, the teacher said: the 21st century is the century of biology. Well, now I believe it.