How does DNA testing work for crime scenes?

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How is DNA physically collected? What’s revealed? How is it matched to a database of people? And how are samples collected without contamination?

In: Biology

5 Answers

Anonymous 0 Comments

DNA is basically a chemical fingerprint that appears in almost all cells.
Hair, skin and blood are the common ones as well as semen for things like rape.
At a crime scene these are usually present in one form or another.

It is matched against a database of known offenders and if there is no result it can end up as a cold case until that person is caught either for that crime or another crime.

Anonymous 0 Comments

It’s usually collected on a sterile swab (pretty much a one sided q-tip) and placed in boxes or paper bags to transport to the lab for testing. Crime scene techs can also collect items to send to the lab.

DNA is unique to individuals except for identical twins so it can link a person to a crime scene/weapon/etc. There’s a database of people arrested or convicted of certain crimes that can be searched after a DNA profile is obtained.
Techs and scientist wear masks, lab coats, gloves, and clean with bleach to help prevent contamination.

Anonymous 0 Comments

A common misconception about DNA tests is that your DNA is not actually being sequenced. Your DNA contains long repetitive regions which can be “stained” using chemicals, and the length and positions of these regions are unique to every person. That’s how DNA probes can be matched to individuals. But it’s not the case that your entire genome gets analyzed, that would be way too expensive

Anonymous 0 Comments

Once DNA is collected, a DNA profile must be created. One method is using STRs or ‘Short Tandem Repeats’. Basically, in our DNA we have repeating regions that do not code for a gene. For example, GCAGCAGCAGCA is an STR, with ‘GCA’ being the repeating unit. The length of these is unique to the person and his or her lineage. So lab techs take the DNA out of the cells they collected at the scene, and they amplify (increase the number of copies) of about 10 different STR regions. Then, they run it through a machine that sequences the regions and can tell how long that region is. Say, for example, you found a sample of DNA and took it to the lab for analysis. One of the STR regions you sequenced came out to be 300 units long. You have a suspect in custody and get a sample of his DNA. That same STR region in his DNA is only 120 units long. Well, the conclusion would be that this guy is NOT your killer.
Stuff like that. Sorry if that was a bit too complex!

Anonymous 0 Comments

We leave our DNA absolutely everywhere, whether we intend to or not. A skin cell or a microscopic drop of blood or saliva contains your entire genome. Crime scene technicians swab and sample absolutely everything at crimes scene, especially obvious things like blood stains, in the hopes of picking up some DNA. They go through great trouble to avoid contamination. That means using sterile equipment, collection tools, and wearing things like gloves, surgical masks, and even shoe coverings to prevent contamination.

Once collected, it’s put through some chemical processes in a lab that will identify if there’s any DNA present, and if so, sequence it into a readable form. People arrested and/or convicted of certain types of crimes (depending on the country) will have their DNA stored in a database. If there’s a match, you know it’s that person. If not, police will have to try and obtain the DNA of suspects to look for a match. Recently, police have been able to match DNA collected at crime scenes to the suspects by finding their family members on DNA testing services like 23andMe.