The human genome has about 3 billion base pairs, making up around 20000 genes, which then make op our 24 chromosomes. The base pairs are made up of four nucleotides, represented by the letters A, C, G, & T. So how do we accurately obtain the sequence of 3 billion of these letters? The method used today is called the termination method, and will be explained below.
Since the ratio of regular to terminator nucleotides is very high, duplicated strands will be anywhere from 2 base pairs long to hundreds of base pairs long before a terminator happens to stop the process.
Now we have millions of DNA strands of varying length, with special terminator nucleotides on the end of each one. These terminators are extra special because each one of the four has the ability to reflect light when a laser is shown on it! The colors reflected can be seen in the illustration just above.
|Illustrations courtesy of pbs.org: NOVA online|
This process of sequencing is repeated for all of the cut chromosome fragments and different vectors until the entire sequence is obtained!