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If the DNA from a crime scene is damaged, why would it be helpful to replicate it by PCR (polymerase chain reaction)? Even if we get billions of those copies, it is still incomplete, isn't it? How can the PCR make the forensic process easier?

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    $\begingroup$ As far as I know (and I know little in this domain), there are statistical to deal with damaged sequences. Typically, different nucleotides have different probabilities of being altered with time and this can be taken into account. However, on a crime scene, there is no reason for DNA to be damaged in any. The main issue is probably a potential problem of contamination. $\endgroup$
    – Remi.b
    Jul 29 '16 at 1:21
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This process helps with the signal to noise ratio. In theory, you could use the fingerprinting techniques to dice up the few billion copies of the DNA and carefully measure them. However, it is far easier to replicate the DNA to increase the number of molecules of DNA available to the process.

Remember, fingerprinting for crime investigations doesn't involve reading the DNA, but merely uniquely identifying it.

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  • $\begingroup$ does that mean we need about billion of that same copies to show on the result of gel electrophoresis. so we need to replicate it by PCR $\endgroup$
    – Snake
    Jul 29 '16 at 3:00
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    $\begingroup$ Yeah. If you think about it, the final step of the process is to stain the DNA fragments after they have been separated by weight. You need enough material at each position along the gel to make a noticeable stain. You also need enough of them to be able to use statistics to account for any random variation in your gel. $\endgroup$
    – Cort Ammon
    Jul 29 '16 at 3:17
  • $\begingroup$ sorry,could you explain what you mean by "any random variation"? and why the process need to be helped with "signal to noise ratio" $\endgroup$
    – Snake
    Jul 29 '16 at 4:14
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    $\begingroup$ Electrophoresis is dependent on diffusion, which is a rather inexact process. If a single molecule of your source material finds an easier path than average, it will move further than it should have. However, if you have billions of said molecules, it is far less likely that all of them will find the easy paths. $\endgroup$
    – Cort Ammon
    Jul 29 '16 at 5:56

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