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Extraction: Why do I know that word? Inside the Crime Lab: What happens next?

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Presentation on theme: "Extraction: Why do I know that word? Inside the Crime Lab: What happens next?"— Presentation transcript:


2 Extraction: Why do I know that word?

3 Inside the Crime Lab: What happens next?

4 How do they get the DNA off the shoes? Real answer: -They extract it.


6 Lets talk about how this is SUPPOSED to be done: Photo courtesy of Dr. Mitch Holland

7 Picture courtesy of Dr. Mitch Holland

8 The scientist is then to return the sample, remove the paper, clean the worksite, change their gloves before moving on to the next sample What happens if you dont??

9 Questioneds before Knowns

10 Exceptions

11 Rule

12 Can you look into the future and tell me why this is a good idea?

13 Practical Application

14 Practical application further: If the questioned sample is contaminated with the defendants known sample at the lab, every time you test the questioned sample, it will show the defendants profile – even if he was in Topeka at the time of the crime.

15 Finally, when they want your clients sample – and they will... The American Bar Association recently released the ABA Criminal Justice Standards on DNA Evidence.

16 2.2 (b)(i)(C) (i) If the person from whom the sample is to be collected is suspected of committing a crime, an order should issue only upon an application demonstrating: (A) probable cause that a serious crime has been committed, and (B) if the sample is to be collected from a person is: (1) a sample collected by a physically noninvasive means, reasonable suspicion that the person committed the crime charged; and (2) a sample collected by physically invasive means, probable cause that the person committed the crime charged; and (C)that the sample will assist in determining whether the person committed the crime.

17 Controls! The idea is that if there is a contamination, it will show up in the control. Using the previous example, if the defendants profile contaminates the questioned, it ideally will also contaminate the control.

18 Controls: Positive Controls Negative Controls –reagent blanks –Negative amplification control Blind Controls/QC sample.

19 Processing controls The controls are treated just as if they were case samples.

20 Positive Control The Positive Control has a known, correct answer. It comes with the kit and is the same every time.

21 Negative control


23 Blind Control The Blind Control also has a right answer. However, this right answer is not immediately known. It is the DNA profile of one of the scientists in the lab. When they do DNA testing, they check out a DNA profile from the lab supervisor, who keeps track of whos sample was given. The scientist must compare the answer known to the supervisor with the answer they got during testing to confirm the test was done correctly. Failure to match SHOULD result in a retest.

24 Controls generally The idea behind controls, of course, is to have a way to see if you contaminated your sample. Controls arent the exclusive or only way to tell, however.

25 Other ways to detect contamination Known samples, such as from a victim or a suspect, should generally contain no more than two alleles (remember that word?) at each locus.

26 A few words about alleles in single source samples: Heterozygote In the first blue locus, the individual has a 16 and a 17 (one from mom and one from dad, remember?). Homozygote At the next locus, we only see a 15. Freak of nature?? No!! This person got a 15 from mom AND a 15 from dad.

27 How do you extract it?

28 Dawn takes grease out of your way... HOW?

29 Extraction in pictures – a photo essay Put the cells in the test tube...


31 Extraction photo essay continued Break open the walls of the cell, using detergent and salt.

32 For cell walls

33 Final steps... More chemicals are added to break open the yolk – or nucleus.

34 For the nucleus walls

35 Recall the yolks represent the nucleus of the cell.




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