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The gel-separated DNA fragments are then transferred to white nitrocellulose paper, so the paper now carries an exact replica of the DNA on the gel. The Southern blot equipment is quite Heath Robinson, involving trays, paper towels, and lots of solutions so can get quite messy.Heavy books are placed on top of the towels to squash everything down. Adding the radioactive probe This is the clever bit.
Extraction of DNA All cells (except red blood cells) in all living creatures contain DNA.
DNA can be thought of as a length of letters A, C, G and T (6 billion of them in a human cell).
The DNA is stored, dissolved in essentially water, in a small plastic tube and kept in a fridge until ready for the next stage.
Cutting up the DNA Freshly extracted DNA in water is quite sticky, because the DNA strands are very long.
We add a blue dye to the DNA fragments using a pipette, and use a pipette to move the blue DNA liquid from a colourless tube into the “well” – little hole – in the gel (see the top picture on the next page).
We use a blue dye to see where we have added the DNA on the gel – it’s just for our benefit so we don’t add two different DNA samples in the same hole!
Most of the method I have described is quite routine in the lab since the late 1970s and not developed by Alec but by Ed Southern at Oxford (hence “Southern blotting”).
Alec’s particular contribution was the choice of the “probe”.
This “restriction enzyme” doesn’t cut randomly in the DNA, but at specific letter sequences.
This stage involves adding the restriction enzyme (colourless liquid) to the DNA (another colourless liquid), using a pipette.