As astounding as it may be, all of the information that we need in order to live is contained in only four different gene letters:

(1) A = Adenine
(2) T = Thymine
( 3 ) G = Guanine
( 4 ) C = Cytosine

The ring-formed chemical substances of which the gene letters are composed are called bases.

These bases are arranged in sequence on the DNA strand. The DNA resembles a nearly indefinite, helically twisted rope-ladder. The bases are the rungs of this helical ladder.

Two bases at a time link together to form a rung of the rope-ladder. This is what scientists call a base pair. It is always A and T as well as C and G, respectively, that form a pair, because Adenosine fits with Thymine or Cytosine fits with Guanine like a key in the lock or like a lid on a pot. The trick is that although the individual bases are of different sizes, their preferred pairings create units of identical shape and size—A pairing with T and C with G. Otherwise too long, too short or crooked “rungs” would be generated in the DNA rope ladder.

One can imagine the four bases as red, green, purple and yellow rods which are consecutively tied to the ropes of the rope-ladder: where a red rod protrudes from one rope, a green one would protrude from the same spot on the opposite rope. Together, they would form a red-and-green rung. A yellow rod together with a purple rod would form a yellow-and-purple rung.