The atomic basis of biological symmetry and periodicity.

Geometric derivations and mathematical formulas have shown how the symmetrical patterns of plants, animals and crystals, follow similar mathematical solutions. These transformations have had the great value of indicating relationships between different types of patterns. However, they could neither shed light on the material processes that led to the emergence of symmetries in nature, nor explain their transfer to higher levels of organization, as evolution proceeded from simple molecular systems to complex living organisms. Several phenomena contribute to elucidate the material processes that have led to the emergence and transfer of symmetries. Their characteristics can be summarized: (1) symmetries are inherent to the structure of matter, since they already occur in the elementary particles, such as the neutrino; (2) simple atoms, such as oxygen and hydrogen produce a six-ray symmetry in water crystals a pattern from which these crystals cannot depart; (3) the mechanism of twinning, common in minerals, obliges several components to associate in a regular fashion guided by the properties of their atoms; (4) the symmetries of crystals are decided by the electronic properties of their constituent atoms. Since living organisms consist of the same atoms that are found in the minerals, it is not surprising that the symmetries of the minerals were transferred intact to the cell and organism levels; (5) another atomic property that has been preserved is periodicity. The existence of the Periodic Table of the elements shows how this phenomenon is so well established at the atomic level. This periodicity was also transferred to the living organism level. Both biological structures and functions display periodicity; (6) the recurrence of the same type of symmetry in minerals, plant organs and animal structures is an expression of the preservation of this periodicity; (7) another manifestation of the periodicity is found in the recurrence of the same symmetry in flowers of a large number of plant families which are not closely related in evolutionary terms; (8) the re-emergence of the same symmetry at different levels of organization is also elucidated by the fact that different atom combinations can display the same form and even the same function. This is what has been called molecular mimicry. Examples are the minerals with quite different chemical compositions which display the same symmetry and the proteins, that although they consist of different amino acid sequences, result in the same structural pattern and the same function. Due to the occurrence of molecular mimicry, in the cell's main macromolecules, an organism does not even need to have the same genes to exhibit a symmetry that appeared long ago in evolution; and (9) support for the concept that the biological periodicity is anchored on the chemical periodicity is found, among other features, on the fact that the six atoms that build the main macromolecules of the cell: the nucleic acids and proteins are all simple atoms that are located in a 'niche' on the right side of the Periodic Table of the chemical elements. The basis of biological symmetry and periodicity is now starting to be elucidated in atomic terms.