Question

When a small amount of DNA is found in human saliva on a glass at a crime scene, how do forensic scientists analyze the saliva to find a suspect?

Answer 1

@why.math I have answered this a while back. Go read about polymerase chain reaction (PCR) and short tandem repeats (STR) analysis. Google/bing either of those and add in forensics and you will get a wealth of explanations.

Answer 2

Thanks

Answer 3

@why.math Essentially though, PCR makes millions of copies of a specific region of DNA. We can view this as a copy machine. If we had our genome printed on paper, PCR would be like taking a region, say pages 34-451, and making millions of copies of just that region.

STR is what let's you ID people. STR are short segments of DNA that are repeated over and over again, and mutate freely because it does not hurt the organism. So if you sequence all these repeats you can ID a person. Think of it like this. We have three people. We are going to look at three different tandem repeats:

STR1: AAAAGCGCGCGAAAGCGT
STR2:TTTACGTATGCGTTGCGAC
STR3:GGGGGGGGGGGGAATCCC

So, everyone has three copies of STR1, so we cannot figure out who is who. So we look, at STR2. Person 1 and 2 have 3 copies of STR2 and person 3 has 9 copies of STR2. So, we can ID person 3, but cannot separate 1 and 2. So, we try STR3 and find that person 1 has only 1 copy and person two has 121. Now if we look at all three we can tell everyone apart. This is alway how we get statistical support like 1/1000000000000.

Answer 4

What about STR and CODIS?

Answer 5

Because if it Saliva then I think it should have enough of a DNA sample to not need to be replicated right?

Answer 6

@why.math CODIS is a data base of these STR patterns and that is how they match people up. No, there will not be enough DNA in saliva for STR analysis, it will have to go through PCR.

Answer 7

Aahh okay. I thought
CODIS was its own separate data base. I think I have a better understanding now.

Answer 8

@why.math PCR let's scientists can use a single cell for STR. In fact, we can even sequence an organism's entire genome from one cell, given the correct conditions.

Answer 9

@why.math I am not sure what you mean about CODIS and the database? It is a federal data base that the states put the DNA patterns of arrested and convicted felons into and allows for forensic scientists to check possible new, but certainly STR patterns from an unknown person against for identification.

Answer 10

Yes, I understand everything your saying. But for the CODIS, I take a FL Virtual class and I guess I wasn't reading careful enough. It was as if they were the same thing when I read it. I knew what CODIS was and what STR was but I thought they were the same in one. Thanks again for helping me understand

Answer 11

@why.math Ah. Well, in a strictly forensic sense I suppose they are nearly equivalent, basically CODIS is kind of like applied STR in the forensic sense.

Answer 12

I see. Thanks again.! You're amazing!

Answer 13

@why.math NP.