will be thymine (T). Also conversely, if thymine appears on one strand, adenine will be found opposite on the other strand. The same logic applies to analogous pairings with cytosine (C) and guanine (G). These base pairs present the horizontal connection, as it were, by their affinity for a weak chemical bond with their complementary partner on the opposite strand. But along the vertical axis (the rope's length), any of the four bases may appear next. Thus the rope—call it a single strand, either the sense strand or the antisense strand—of DNA can have virtually any sequence of A, C, G, and T. The other strand will necessarily have the complementary sequence. The code is simply the sequence of base pairs, usually approached by looking at one of the strands only.
In their quest to explain the complexity of life, scientists next turned to deciphering the code. Once it was realized that the four nucleotide bases were the basic letters of the genetic alphabet, the question became, How do they form the words? The answer was known within a decade: the 64 possible combinations of any given three of them—referred to as a triplet—taken as they are encountered strung along one strand of DNA, each delivered an instruction to "make an amino acid."
Only 20 amino acids have been found in plant and animal cells. Fitting the 64 "word commands" to the 20 outcomes showed that a number of the amino acids could be commanded by more than one three-letter "word sequence," or nucleotide triplet, known as a codon (Figure 5.2). The explanation remains an interesting question, and so far the best guess seems to be the redundancy-as-error-protection theory: that for certain amino acids, codons that can be mistaken in a "typographical mistranslation" will not so readily produce a readout error, because the same result is called for by several codons.
The codons serve, said Hanahan "to transmit the protein coding information from the site of DNA storage, the cell's nucleus, to the site of protein synthesis, the cytoplasm. The vehicle for the transmission of information is RNA. DNA, the master copy of the code, remains behind in a cell's nucleus. RNA, a molecule whose structure is chemically very similar to DNA's, serves as a template for the information and carries it outside the cell's nucleus into the cytoplasm, where it is used to manufacture a given sequence of proteins.
Once the messenger transcript is made, its translation eventually results in the production (polymerization) of a series of amino acids that are strung together with peptide bonds into long, linear chains that in turn fold into interesting, often globular molecular shapes due to weak chemical affinities between and among various amino acids.