At present, when the public mentions nucleic acid testing, they automatically think that "nucleic acid testing = new crown testing". In fact, the nucleic acid test of the new coronavirus is only one of the nucleic acid tests. Nucleic acid detection methods are widely used in the detection of pathogenic microorganisms. As an important means of diagnosing diseases and monitoring viral load, how does nucleic acid detection "catch" viruses? Let me explain in detail today.

Step 1: Storage and Shipping

After sampling, the swab is placed in the virus transport media. The virus transport media can maintain the activity and integrity of the virus. At the same time, the preservation solution contains a variety of antibiotics, which can inhibit the growth of other bacteria and fungi in the sample, and defeat the "competitive partner". The virus can monopolize the "comfortable" environment and live "freely" in this paradise. .

Step 2: Shake.

After the virus sampling tube is sent to the testing laboratory, it is fully shaken, and the respiratory epithelial cells and virus adhering to the swab can not be "shaken" into the preservation solution.

Step 3: Extract Nucleic Acids

(1) Cracking

Denature the protein coat with guanidine salts. Only by pulling out this colorful protein "vest" can the nucleic acid of the virus be released, revealing the "true face" of the virus.

(2) Combine

Add magnetic beads to bind to nucleic acids. Say "goodbye" to all kinds of cell debris, proteins...!

"our target is?"

"Grab the nucleic acid!"

(3) Washing

Can you catch nucleic acid with one combination? How can it be so easy! It was caught, but there were still a small amount of debris, and the protein was still stuck together with the nucleic acid. How to improve the purity of nucleic acid? We have two magic weapons: add salt and add acid.

"Adsorbed Nucleic Acids - High Salt Low pH"

"Elution Nucleic Acids - Low Salt High pH"

Repeat the adsorption and elution two or three times to realize the extraction and purification of nucleic acid. The rest is "naked" nucleic acid.

Step 4: Detection of Nucleic Acids

The routine screening of respiratory virus nucleic acid detection adopts real-time fluorescent PCR method. PCR technology was invented in 1983 by the famous American chemist Kary Banks Mullis and won the Nobel Prize in Chemistry in 1993.

PCR reactions are like a large assembly shop. To amplify a certain viral fragment, first design a pair of primers according to its characteristics. Primers are the most powerful "grabs" for identifying and locating, with the precision of "where to hit". In a group of viral nucleic acids, if you want to catch the new crown or the flu, or whether it is rhinovirus, respiratory syncytial virus, etc., there are only things you can't think of, and nothing he can't catch.

When he identified the "characteristics" of the "target" virus - specific fragments, he immediately "captured" one after the other, then added raw materials such as dNTP, Mg2+, Taq enzyme, adjusted different temperatures, and went through "denaturation-annealing". -Extension" three steps to complete viral nucleic acid assembly. With each amplification, the DNA yield increases exponentially. The amplification of the new coronavirus generally requires 40 cycles. In theory, one copy can obtain 240=1,099,511,627,776 copies. Even if the virus content in the sample is very low, it can be detected by PCR amplification. Therefore, the PCR reaction has the characteristics of high specificity and high sensitivity.

As the number of amplifications increases, the amount of fluorescence accumulates, which is the graph we see, and we use it to determine whether there is a virus. The above is the complete process of nucleic acid detection to "catch" the virus. Overall, the first step is the process of preservation and transportation. It is extremely important. As the first stop for nucleic acid sample collection, the virus transport media is also indispensable. Due to the heavy task of nucleic acid detection, nucleic acid viruses are inactivated in the virus transport media, which means that they lose their original activity, which is convenient for transportation, and at the same time greatly reduces the infectivity and ensures the safety of detection.