Gene Sequencing: How It Works

By Shlomo Maital  

  Gene sequencing has proved a powerful tool in understanding the coronavirus’s mutations.

   Ever wonder how it is done?  How scientists can precisely identify the genetic structure of, say, a virus? 

    The Economist weekly has a fine section, “Simply Science”.  In general, the reporting in The Economist, both political and economic, is way above the level in any other publication, in my opinion.  While pro-free market, it is generally unbiased and reliable. 

     Here is how they sequence genes:  (in 300 words): 

(First, crucial information:      DNA is made up of four building blocks called nucleotides: adenine (A), thymine (T), guanine (G), and cytosine (C). The nucleotides attach to each other (A with T, and G with C) to form chemical bonds called base pairs, which connect the two DNA strands.)

      “Reading a genome requires some chemistry. * You start by extracting it from the cell and then heating it up in a test tube to break the DNA chain into pieces. Other molecules, known as primers, then bind with these pieces of DNA and begin to produce copies. These copies, like the DNA itself, are built sequentially, one nucleotide at a time. This is all done in the presence of four special molecules, each of which will bind to only one of the four nucleotides in the sequence. When they do, they stop the primer from working, and the sequence of nucleotides is frozen.   *  With the right balance of primers and molecules that cut off the reaction, you end up with a test tube filled with countless strands of DNA that have been cut off at every conceivable point along the length of the genome. The trick is that each of the cut-off molecules is also fluorescent, emitting its own unique wavelength of light.      *    By shining light onto these new strands of DNA and recording how they fluoresce as a result, you can figure out which strand endings correspond to which nucleotide, and begin to fit the sequence of nucleotides together.   This tells you which nucleotides are in the genome, but not the order in which they appear.*  To obtain that information you need to flush your many strands of DNA through a gel, under an electric field. This spreads them out in order of mass—the smallest strands are pulled further through the gel by the electric field than the larger strands. The order in which the four different colours of light appear through the gel tells you the order of nucleotides in your sample, and thus an organism’s genome sequence.

      This, broadly, is the sequencing technique discovered by Fred Sanger, a British scientist, in 1977.