The DNA bases themselves, those rungs that join between the twin backbones, have their own purpose, and now we're getting closer to DNA sequencing. But not quite yet... we still need to know a little bit more about what these rung-like bases are, and what they do, before we can move on to their sequencing.
Remember how I said that one whole 'rung' is actually cut in half, that it is made from two half-rungs, or bases? I know it can be confusing: in books you'll keep reading DNA parts referred to as singular when they are really double; like the DNA strand is actually two strands, it is not actually one long polymer but two; and you'll also read that the DNA base is the whole rung of the ladder. But that's a generalization for ease of use. Remember that we've now cut that rung in half; technically, each half of the rung is a single base. If we take our capital T turned on its side, which is called a nucleotide, now we've pulled apart the horizontal bar - the base - from the vertical bar - the backbone. Visually, we no longer have a sideways T, we now have the single horizontal bar and the single vertical bar. We already learned a little about the vertical bar - the DNA backbone, now here's a little about the DNA base.
There are only four substances a DNA base can be made out of; all those millions of bases, joining in twos to form millions of ladder rungs, are made up of only these four substances, in differing sequences. Yes, we're getting closer to sequencing! But not quite yet. First, we need to know about the little units that are being sequenced, the DNA bases.
The four substances DNA bases are made of, and their abbreviations, are: Adenine (A), Cytosine (C), Guanine (G), and Thymine (T). These bases pair together (called the obvious name of DNA base pairing, or base pairs) to form a whole base or rung of the ladder. Each of these substances has its separate purpose and each needs its pairing with a similar yet opposite substance, just as the two DNA backbones run in opposite directions to complement and complete each other. In a DNA base pairing, only two bases are meant for each other, to complement each other, hence another term used - complementary base pairings: Adenine always links up with Thymine, and Quanine always links with Cytosine, these DNA base pairings never mix with each other in any other configuration. So each rung or base is either an adenine-thymine pairing or a quanine-cytosine pairing. Below: a variety of illustration examples, each a little different but showing some view of DNA base pairings.
These DNA base pairings are joined together and to their backbones by hydrogen bonds. And now that you have a simplified picture of DNA as a whole, and can see visually how it is separated into constituent parts of the whole, we can take a look at those little ladder rungs, those DNA base pairings, and see how and why they are put into specific sequences!