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Compiling information in chromosomes.

Nucleic acids, composed of chains of nucleotides, are particularly noble materials. They are linear sequences, chains of molecules as in proteins, and potential message carriers. In terms of information they are comparable to conversation, music, and, in general, to our written or recorded messages.

Cell organization is three-dimensional, and we could even add a fourth dimension: time; but the manipulation of information in two or three dimensions is difficult. We have examples of it in games like crossword puzzles (two dimensions) and the Rubik's cube (three dimensions), which test our patience and ingenuity. The same difficulty is reflected in the management of information at the cellular level and the solution adopted has ended up by defining the genetic and evolutionary mechanism. Our brain provides representations of a great complexity, as well as generating "lines of thought."

Biological information is expressed in different "languages" and it can be translated from one into another. From the nucleic acids (deoxyribonucleic, DNA) of the nuclei and genes, it goes first to the ribonucleic acids (RNA), which act as intermediate carriers of information and are subject to a higher probability of spontaneous alteration; and then to enzymes that operate as catalysts on the substrates. Ribosomes are scattered in the cell and are essential intermediaries in the flow of information from RNA to peptide chains. There exists a complete hierarchy in the organization of the cell and of life which we will not study here but which is important to keep in mind for the following reasons. Firstly, because it is the best-known structure which is close to the basis of life. Secondly, because it is the model or paradigm of a style of construction and functioning that is found time and again at other levels in the biosphere. Thirdly, because genetic manipulation adds new possibilities to the interaction between man and nature. The most persistent information carriers are the nucleic acids, polynucleotides made of chains of modular units called nucleotides. Each nucleotide consists of a sugar: deoxyribose (in DNA) or ribose (in RNA), a phosphate and a base, generally adenine (A), guanine (G), cytosine (C) or thymine (T). Each molecule of DNA is made up of two parallel chains in the form of a helix; when they separate each of them may automatically induce the replication of another complementary chain. This requires a complex enzymatic system for the replication. Normally the guanine of a simple chain pairs with the cytosine of a complementary chain and the adenine with the thymine.

The chains of DNA that form the "hard" memory of organisms are very long and have a characteristic organization, which facilitates their operation. The RNA of a well-studied bacterium (Escherichia) is a filament consisting of three million pairs of nucleotides; human DNA is one thousand times longer. Along the axis of the double helix, the distance between two successive nucleotide is approximately one third of a nanometre (nm), so the total filament of human DNA is about 3 ft (1 m) long and divided among the 23 chromosomes of each haploid set. In the chromosomes the DNA filaments are repeated, organized, and folded in various ways in order to fit into microscopic structures of a few micromillimeters long since the cell nuclei which house them are ordinarily no larger than 5 [micro]m in diameter.

If we translate from a code with four symbols (nucleic acids) into a code with some 20 symbols (amino acids), it is necessary to use three of the former to specify each of the latter. Since the codification of one amino acid requires a group of three successive bases, the nucleotide "triplets" are called codons. Three thousand million nucleotide pairs, approximately the number found in mammals and in man suffice to specify at least 750 million amino acids and at least a million types of proteins, although it seems that the real number of proteins that are required and actually synthesised is less than one tenth of that. DNA also contains "punctuation marks," various instructions, many inactive sections and even viruses. It could be compared to a computer memory that has been used over a long period of time. Chromosomes are very complex structures, a good part of whose information is latent.

Nucleic acids are very resistant and have a limited turnover rate. DNA chains inform RNA chains, which are shorter, easier to renew, and more subject to accidental changes. RNA chains are models for the enzymes, and the enzymes manipulate the substrates, which are molecules with a high turnover rate that usually need relatively more energy. Such a situation offers the typical image of a kind of relationship found in different natural situations: the same opposed flows of energy and information may be recognized along the trophic chains of ecosystems, from the primary producers (plants) to carnivores, as well as in the technology of our civilization.

Genetic mechanisms and the expression of genetic information in development explain how the biosphere works and evolves. Interest in the conservation of species and biotic diversity has to refer to their respective genotypes. It is also important to anticipate the results, which may be expected from what is now possible in the field of genetic engineering.

Humanity's most important role will not be one of producing more mutation-type changes, because nature has always had plenty of time to try them out and, in fact, evolution could have gone faster if selection conditions had not slowed it down. The importance of genetic engineering lies rather in being able to realize in vitro a sequence or combination of changes that would not have a chance in the wild, especially in the case of transitional forms of doubtful or low viability. Just as culture has been assimilated because it increases the possible speed of evolution, genetic engineering, in as far as it can accelerate the assimilation of complex (or otherwise impossible) changes, may eventually become an acceptable mechanism in the global evolution of the planet.
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Publication:Encyclopedia of the Biosphere
Date:Jan 1, 2000
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