• Question: how do you identify DNA

    Asked by schmick to Anil, Blanka, Cees, Emma, Mike on 29 Jun 2012.
    • Photo: Emma Trantham

      Emma Trantham answered on 29 Jun 2012:


      Great question 🙂

      I don’t know how much you’ve learnt about DNA at school so I will start with some of the basics.

      DNA is found in all of us, every animal, all bacteria and even some viruses. It basically provides the code for how we develop and live (like a computer programme code provides all the instructions needed for the programme to run.)

      The code, rather than being written in numbers like binary code is, is made up of 4 different bases: adenine (A), guanine (G), cytsoine (C), thymine (T)

      So to get to your question – we look for these A G C and Ts to identify whether any DNA is present.
      We can tell what the DNA is by the order of the A G C and Ts.

      The way we look for them was developed by some really ingenious scientists and relies on the fact that DNA likes to be double stranded with bonds between the 2 strands.

      So first of all we design the sequence of AGCT that we are looking for and make a single strand of this (for example AATGGCTTTGAC) and we can label this with something that fluoresces.

      Then we heat up the DNA that is in the sample we are investigating so that the two strands come apart.

      We add our test sequence along with some enzymes to our sample. The test sequence will bind to a sequence on the sample if it matches it and then it will fluoresce. We look for this fluoresence – if it is there we know that DNA sequence is present in the sample.

      Now we can also sequence whole genomes (which means that say for example we have some DNA from a bacterium, or even a dog we can find out exactly what order the As Cs Ts and Gs come across the whole length of its DNA). That usually uses a different process but I can talk about that too if you would like?

    • Photo: Blanka Sengerova

      Blanka Sengerova answered on 29 Jun 2012:


      That’s a great answer, Emma.

      I will just add that when you have small pieces of DNA, such as the ones I come across in my experiments (unlike DNA in our bodies which is millions of bases – A,T,C,G – long, my bits of DNA are only 20-60 of these letters). some of these I chew up with my protein (called digestion) and when I stop the reaction by heating up the enzyme to denature it (stop it from working) I need to separate all the different bits of DNA to see what happened to my initial DNA that I added in. To do this I run my reaction mixture on a gel. This is very similar to jelly that you make as a pudding. I have my jelly between two glass plates and add the reaction mixture at one end and then apply a current. Since DNA is negatively charged, it will travel towards the positive end of the battery and the smaller pieces of DNA will travel further through the gel and the larger ones will travel less far. When I visualise them (using either fluorescence – visible light – or radiactivity) I can see what happened to my reaction.

      Does this make sense?

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