How complex is dna?

Question

The cops took my daughters dna because I am 15 and he is 20. He was denying her so that is why I gave it up(enough said) Now my grandmother keeps saying she wonders why it is taking so long. I tell her the state crime lab is probably backed up and that dna is really complicated. How can I explain to her that dna is very unique and can be hard to work with? Perhaps tell me the basics of dna and why its hard to work with?

Thanks in advance

Answer 1

Genetics, as a science, is pretty complicated, but the physical DNA molecule itself is fairly simple.  

In everyone's DNA, there are repeating sequences called RFLPs.  They don't necessarily do anything on their own, but given the sheer amount of DNA that a person has, RFLPs are going to show up.  Since each person's DNA varies just a little bit, different people will have a different number of RFLPs, and they will be in different locations in the genome.

Imagine a person's genome as a book.  What are the chances of finding the letters "eny" in that order anywhere in the book?  On this very web page, there's one instance in the word "denying" in your question.  It doesn't mean anything by itself, it's just part of another word.

Now, why are RFLPs important?  In the lab, technicians can take a sample of DNA and add something called Restriction Enzymes.  They're kind of like molecular scissors, and they cut the DNA strand, but only where there is a RFLP.  

So, time for more analogies:  imagine DNA as a book, only instead of on pages, it's written in one line along one giant strip of paper.  Say our RFLP sequence is the "eny" that I mentioned earlier.  You take a pair of scissors (the restriction enzymes) and cut the paper wherever you find a word that contains the letters "eny."  In the end, you'll have a bunch of strips of paper of different lengths.

It stands to reason that if you were to do this with two different stories, then each story would produce strips of paper of different lengths, since "eny" will show up in different places in each story.  It's the same with people.  RFLPs are located in different locations in different people.  *But* since a child gets half of their DNA from each parent, they will also inherit some of the RFLPs.

So, to test the DNA, a sample is taken from the child and each parent.  Each set of DNA is amplified using a process called PCR (which basically makes many copies of the DNA).  The DNA is then treated with restriction enzymes, which chop it up at the RFLP sites.  Finally, the resulting fragments are placed on a gel under an electric current.  The smaller pieces of DNA move through the gel faster than the larger pieces (the same way it's easier to push a needle into syrofoam than it is to push through a thick sharpie marker).  The gel is taken out of the electric current and stained, resulting in the DNA "fingerprint" that you may have seen before, with the DNA fragments arranged from largest (at the top) to smallest (at the bottom).  By comparing the child's DNA fingerprint to the parents', it's possible to determine whether the parents are actually the child's parents.

It all sounds very complicated, and the science behind it took a long time to work out.  The process itself is fairly simple.  Take a sample of DNA, put it into the PCR machine with the right chemicals, then add the restriction enzyme, put the samples on a gel for 30 minutes, stain for 10 minutes, then take a picture.  

The reason it takes so long isn't because the DNA is complicated, but because there are many samples to run through, the technicians are overworked and underpaid, and because they must be extra careful because of the importance of the samples.  I'm a research scientist, and if I accidentally contaminate my sample of DNA, the results will look funny and I'll have to repeat the experiment.  In the worst case, it will lead to a inaccurate conclusion that I won't discover until later, and I'll have to backtrack and sort out a whole confusing mess.  It's a pain, but no serious harm done.  

If the crime lab technicians mess up, though, it could potentially send someone to jail or give someone the wrong paternity results.  Since so much is riding on the results, they need to be extra careful handling the samples and filling out paperwork.  When you combine this with many, many samples (and not enough technicians to run them all), it can take a long time to run what would otherwise be a simple procedure.

Answer 2

DNA tests done by the Police often take months as there is a necessary high standard of accuracy expected in the results, and a long backlog of samples awaiting tests. Consider:

- reliable DNA testing has only been available for about 10 years, however they have samples from old crimes dating back decades.

-more and more investigations are using DNA evidence and more and more samples are being taken from each crime.

-While DNA sequencing is fairly easy, the level of accuracy required for criminal cases, and the need to avoid contamination of the samples, delays the process.

Also, shows like CSI do not accurately portray the time-lines involved in forensic investigation. The public has however come to expect resolution of crimes in under 60 minutes (including commercials).