Now we've actually managed to extract our DNA (like this) we get to cut it all up into tiny pieces. It might seem a bit pointless, but this is how to determine that we actually have the right bits of DNA in our extracts. Cutting DNA with little enzymes called restriction enzymes produces different restriction patters depending on what DNA you have.
Restriction enzymes are naturally produced by most bacteria, and what they do is cut pieces of DNA at very specific points. EcoRI, for example, cuts DNA after the G in the DNA sequence GAATTC. As each viral genome has a different DNA sequence, each one will produce a different restriction map, producing a characteristic number of bands on a gel:
(this picture is not from my research, it is from here)
Each band is a blob of a certain size of DNA lit up with ethidium bromide (which is a dye, nothing very exotic). Different restriction enzymes, and different genomes, will produce different band patterns on the gel.
So, what do the bacteria need to produce DNA cutting enzymes for? The answer (naturally) is bacteriophages! One way the bacteria can protect themselves against viral invasion is to have lots of these enzymes around. As soon as the viruses inject their DNA into the bacteria cell, the restriction enzymes chop it all up.
But bacteria also contain DNA, and unlike people (and other eukaryotes), they don't keep it all tucked up in a nuclear membrane. So how do they stop the restriction enzymes from cutting up their DNA? One of the most common ways is to methylate the DNA, essentially sticking a methyl group (a carbon atom attached to three hydrogen atoms) onto some of the bases. in the example shown above, therefore, the restriction enzyme is looking for the sequence GAATTC. It sees this in invading DNA and slices it up, but in the bacterias own DNA it sees GA(methylated)A(methylated)TTC, which it doesn't recognise. And therefore, does not cut.
Restriction enzymes were first discovered my Daniel Nathans, Werner Arber, and Hamilton Smith. They won the Nobel Prize for it in 1978. (see here)
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