| 1. | Label 4 test tubes labelled A, T, C and G. Into each test tube add: a sample of the DNA to be sequenced (containing millions of individual molecules), the 4 DNA nucleotides and the enzyme DNA polymerase. |
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| 2. |
In each test tube add a small amount of a special modified nucleotide * that cannot form a bond and so stops further synthesis of DNA. Tube A = A*, tube T = T*, tube C = C* and tube G = G*. The * nucleotides are present at about 1% of the concentration of the normal nucleotides. |
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| 3. |
Let the DNA polymerase synthesise many copies of the DNA sample. From time to time at random a * nucleotide will be added to the growing chain and synthesis of that chain will then stop. A range of DNA molecules will be synthesised ranging from full length to very short. The important point is that in tube A, all the fragments will stop at an A nucleotide. In tube T, all the fragments will stop at a T nucleotide , and so on. |
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| 4. |
The contents of the four tubes are now run side by side on an electrophoresis gel. |
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