• Question: How do get the genes out of the bacteria, and then how do you change it into the letter sequence?

    Asked by to Bethany, Hannah, Keith, Peter, Ramya on 19 Jun 2014. This question was also asked by , .
    • Photo: Bethany Dearlove

      Bethany Dearlove answered on 19 Jun 2014:

      This is a really great question. In short, we use DNA sequencing. There are different types of sequencing nowadays, but it started in the late 1960s and 1970s with Fred Sanger, who won two Nobel prizes for his work (one of only four people to do so!). It wasn’t until 1995 that the first bacteria, Haemophilus influenzae was fully sequenced though. Sanger sequencing, as it’s now known, is still used today – though the technology has been updated over the years. I give a very simplified version of how it works below – I’ll admit that sequencing still manages to amaze me, as the chemistry behind it is beyond me!

      The first step, as you mention, is to get to the DNA (genes) so that we can read it. This is done by unzipping the two strands of DNA, so we’re left with just a single strand. If there’s not very much DNA (say, we only managed to get a small sample), we can use this as a template to make some more copies so we’ve got plenty to work with.

      The important thing to remember is that the nucleotides that make up DNA (what we actually mean by the letters A,C G and T) always join in pairs. A always pairs with T, and C always pairs with G. So if there’s an A in our single stranded DNA, it will always be a T that joins on to it. We put coloured light tags on to some spare nucleotides (one colour for A, another for C…and so on), and then put them with the single stranded DNA. The spare nucleotides attach to their pair mate, and then a special camera can then be used to read the colours at each step of the sequence. So if A had a red tag, C was blue, G was green and T was yellow, and we got:
      (red, yellow, green, red, blue, green)
      using our camera, then we’d know the sequence was AGTACG.

    • Photo: Peter Elliott

      Peter Elliott answered on 19 Jun 2014:

      Not much more to add to Bethany’s answer except to say how we physically get the gene out of the bacteria. Again there are several ways but the most modern one uses a kit of just three liquids.

      First you take your bacteria and put them into a liquid that contains detergents (something very similar to soap). This breaks open the cells and releases the bacterial DNA. If you add the enzyme RNase then all the RNA (riboneuceic acid) in the cell is destroyed – as we don’t want RNA if we are looking at the bacteria’s genes.

      The next solution is added to the first and this breaks down all the proteins in the solution as again we do not want them. You have to mix the solution very well to make sure all you remove all the proteins. We then use a centrifuge to get rid of the solid protein and transfer the liquid into a new clean tube.

      Finally to the solution you add something called isopropanol. This makes the DNA come out of solution and you can see the DNA as a fluffy white substance in the liquid. We then centrifuge the tube and remove the liquid from the DNA.

      The you are ready to carry on with Bethany’s explanation 😀

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