Ok a lot of people are wondering why I said PCR is prone to false positives and bad results so I'll try to explain it here.
For PCR to work it needs something to stick to the beginning and end of the DNA strand that is to be amplified. Now this thing are called primers and these primers are designed by being complements of the actual part it is supposed to stick to. Think of primers as having a shape this will fit a pattern and the pattern is supposed to be the DNA end. Sometimes if enough of the shape is close enough the primers can stick to another DNA which closely resembles the primer's original shape. So this would cause what is known as non-specific PCR amplification giving you amplification of some other part of the DNA than you originally intended. This problem can be solved be redesigning the primer to be more specific (making the shape more complex and such) or by altering the conditions to make it harder for wrong pairings to occur (increasing PCR temperature).
A PCR result does not confirm the identity of a gene amplified .. lets say you have a DNA and you design a primer to attach to certain parts of the DNA to create amplifications of a 2.2 kb (kilo base pairs , measurement unit for DNA) sized DNA and you do the PCR .. then you get a 2.2 kb sized band .. it doesnt mean it is the original band you targeted. Why ? Lets look at an analogy ....
Imagine the gene being amplified as a story book , Cinderella for example . Now the book Cinderella has a beginning and an ending. We design primers to stick to the beginning and end . We design it to recognise " Once apon a time " as the beginning and " they all lived happily ever after" as the ending. Now that we have our primers we do PCR on the book (DNA) we get results and the size that is amplified matches the original book (200 pages) ... this doesnt proof that the book is Cinderella since there are many fairy tale stories which begin and end the same way. The better way to know would be to read the whole book and compare it and for that other more reliable methods are used.
PCR of DNA for use in court cases has been modified so that it can be used to compare two DNA samples and say if they are the same or not so no problems there but remember to have extra proof of a DNA's identity (like sequencing the gene) before saying that the PCR DNA is your DNA of interest. Normally we call PCR result as putatative results since it has to be confirmed with other methods ... (yes sometimes we go on faith that if I was supposed to PCR Cinderella and I get an amplified DNA which matches the size that I predicted it should be Cinderella but that is for non-critical applications only)
Ok , hope this cleared up some questions. I can give the full 10 min leture when we meet up for DotA next if you guys want .. I can talk about DNA all day long if I have to but dont bother since normally people ask and get bored half way through :) |
At 9:27 AM,
Ok I get it, but this still doesn't explain why the murderer is the lab technician.
Please. Please!! PLEASE!! Do not speak of this on Saturday, I'll lose my mind and select Azwraith.
I produce DNA twice a day....
At 2:50 PM,
but i wont be at dota or seremban =(
At 3:48 PM,
How heartbreaking T_T
CLOWN!!!
CHINO!!!!
NEWBIE!!!!!
ANYONE?!?!?!?
Salty Peanuts III not coming and we need a number 6 as I'm not sure if my brother's friends are down.
At 7:37 PM,
DNA from the handler can accidentally enter the solution of the suspect DNA and get mixed up (a single skin cell , a drop of saliva, water from sneezing, eyelash, strand of hair etc etc) and when you amplify you will amplify the technician's DNA as well.
It's not really a problem with the procedure since an expert in PCR will know how to avoid stuff like this but it can still happen neverthesless .. but all you need to do is get frest DNA samples and do it again :)
The joke just alludes to the fact that PCR results has to be verified again just to be safe and mustn't be taken at face value.
At 2:18 PM,
Oh well, I'm sure that we have super incompetent people doing these tests for the police so there's bound to be a good number of slip ups.
I can give out DNA samples if you need 'em
At 3:16 PM,
yeee haaaa!!
i added the " DNA from the handler can accidentally enter the solution of the suspect DNA and get mixed up" in my report!!
YEAHOOOOOOOOOo