Make of this controversial statement what you will. This correctly predicts the structure of A-DNA, a form the molecule takes when dehydrated. This is extremely incorrect: substituting ribose for deoxyribose results in RNA, the more functional of the nucleic acids and hypothesized have evolved first in the primordial world. At the time of this paper, DNA was known to be the hereditary molecule, so must possess a means of copying itself for future transmission. The base-pairing model presented provides a means by which this can occur, as each individual strand can serve as a template for the replication of the other. As such, replication of a single DNA double helix results in two new DNA molecules, and further replication follows 2$^n.$ In the 50's, Erwin Chargaff proposed a set of ratios whereby the percent of thymine in a genome equals the percent of adenine, the same relationship holding true for cytosine and guanine. Watson and Crick are here reiterating Chargaff's Rules in the context of their findings. The correct Pyrimidines are Cytosine and Thymine (and Uracil in RNA) and the correct Purines are Guanine and Adenine. To avoid mixing these up you can remember Pure As Gold - Purines are Adenine and Guanine and to CUT the Pie - Cytosine, Uracil, and Thymine are Pyrimidines Each residue of a DNA molecule is comprised of a deoxyribose, a nitrogenous base, and a phosphate. A complete unit of the three is called a nucleotide. Thus, nucleotides are spaced 3.4 Å apart and twist along in the right-handed or z-direction. It is worth noting that these latter two features have been found to not be absolute: DNA takes other forms in which the spacing between nucleotides and/or helical direction. The molecule Watson and Crick describe is B-DNA, the apparent most common form. Here they describe the basic chemical structure of DNA: phosphate molecules connected to deoxyribose sugar molecules. The bases to which they refer are the four nitrogenous bases: the pyrimidines, adenine and guanine, and the purines, cytosine and thymidine. They state that the backbones are identical but antiparallel (parallel in opposite directions), except for their bases, which connect to form a "dyad." Pauling and Corey suggested a triple-helix structure which Watson and Crick discount because: 1. The crystal structure resolved by X-ray diffraction is not the native ("free-acid") structure but instead, a salt of DNA that more readily crystallized. As a result, their structure appeared distorted. Specifically, the negatively-charged phosphate backbone would be neutralized in a DNA salt. Watson and Crick suggest that the resolved structure was an artifact. In a triple helix, the phosphate residues would be wound too closely together. Recalling that like charges repel, it is unclear how the molecule would be held together. 2. Some van der Waals distances, an approximate measure of the radius of an atom, are too small, implying that the atoms in the Pauling and Corey model are too close together to be real.