[Music] [Caitlin:] You might be asking, "How do scientists read the sequence of base pairs in DNA?" [Felicia:] Good question! [Caitlin:] We use a process called Sanger sequencing, in honour of the scientist who pioneered the technique, Frederick Sanger. Sanger was an amazing scientist who received two Nobel prizes. His first Nobel, in 1958, was for his work on understanding the structure of proteins. But more to the point here, in 1980, he shared a Nobel prize with two other researchers for figuring out a way to sequence DNA. The process is both brilliant and simple, as elegant solutions in science often are. You may remember that in Week 2, Felicia cooked up some PCR samples in her thermocycler so we could identify the DNA of Dr Earl N. Myer's killer. Well, DNA sequencing is a very similar process. First, we use PCR to make many, many copies of our DNA sample with all of the same base pairs of sequence. [Felicia:] The heat and unzips the strands of DNA. The primer marks the spot where we'll start to copy the DNA strand. Taq DNA polymerase starts at the location of the DNA primer and begins adding bases one at a time, according to Watson-Crick rules: A pairs with T, C pairs with G. Eventually, the DNA polymerase will add a base that puts the brakes on the duplication process: a ddNTP. [Caitlin:] ddNTPs come in four different flavours: A, T, C, and G. Just like the regular bases. Here's the genius part: The ddNTPs have been specially treated, so they light up in different colours. The colours let you know where the end of the strand is. Even better, the colour can tell you which base is last on this fragment of DNA. You put many strands of DNA in the test tube. So, there are many reactions going on in there. Different strands will stop duplicating at different places. So, the newly-replicated DNA will have many different lengths. If you could take a quick look into the tube, you might see something like this... Now, we just need to sort these DNA fragments, based on their size, to see each base in sequence. [Felicia:] We know how to do this already. We can use gel electrophoresis -- with a minor twist. Instead of a giant slab of gel, we have a very thin capillary tube with gel. As the sample runs through this thin tube, the molecular sieve sorts the DNA fragments out by their length. The colours of each ddNTP associated with a different termination length of DNA is detected using the colour properties of each ddNTP. [Caitlin:] The computer output is in the form of these colourful peaks called a chromatogram. Each peak corresponds to a different colour assigned to each of the four ddNTPs. Also, each peak is detected at a specific length of the DNA. So, all you have to do now is read the peaks from left to right in order to figure out the sequence of DNA. In this case, we have AGTCAGTC. [Felicia:] So, next time you see one of these images on TV, you'll know exactly what it means. That Sanger guy and his pals were brilliant!