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DNA (deoxyribonucleic acid) carries design information between generations, and thus accounts for inherited biological traits (phenotypes). At conception, a father's sperm injects a set of DNA molecules into a mother's egg, which already contains a nearly matching set. Those molecules contain the designs for all the material components their child needs for growth, development, and daily living.

Structure of DNA

The designs are called genes. Some genes play a role in regulating other genes, and some design ribonucleic acid, a close relative of DNA. But mostly, the designs in DNA are for the class of chemicals called proteins. The human body contains tens of thousands of kinds of proteins, which do all the body's work. Interactions among those proteins, and interactions between them and environmental factors account for the processes and structures of the body. Those processes and structures are manifested as inherited traits. DNA is comprised of chains of chemical subunits called nucleotides, each of which contains one nitrogenous base: adenine (A), thymine (T), cytosine (C), or guanine (G). The design instructions in DNA are spelled out as particular sequences of these four bases. This is analogous to conveying instructions in printed books by particular arrangements of the twenty-six letters of the alphabet. In the case of genes, however, there are only four letters in the alphabet. Hundreds of nucleotides are linked in a DNA chain in a sequence that spells out instructions for a single gene.

There are two complementary chains in the structure of DNA. Each nucleotide in DNA has a sugar component joined to a phosphate group at one point on the sugar, and to a nitrogen-containing base attached at another point. The chains in DNA have the phosphate of one nucleotide linked to the sugar of the next nucleotide to form a strand of alternating sugars and phosphates with dangling nitrogenous bases. DNA contains two such chains, twisted around each other to form a double-stranded helix with the bases on the inside. Every A on one chain forms weak bonds with a T on the other strand, and every C on a strand bonds weakly to a G on the opposite chain. The two strands, held together weakly by the pairing of A with T, and G with C, are thus complementary, and the sequence in one can be deduced from the other's sequence.

Design information is transmitted as new DNA to new cells during development and growth. The complementarity of the two DNA strands allows their information to be copied. Each old strand is used as a template in synthesizing a new complementary one. Intricate cellular machinery makes new copies of the DNA when a fertilized egg divides into two progeny cells. When each of the progeny divides again, the new progeny all receive complete copies of the parental DNA. As the fertilized egg grows to become successively an embryo, a fetus, a child, and finally an adult, cells go through many rounds of division with replication of the DNA in each round. Finally, adult humans have trillions of cells, each one (except sperm and ovum) containing complete copies of the DNA initially contributed by the parents.

On rare occasions mutations (changes) are made in nucleotides by chemicals, radiation, or errors in copying DNA. In a nucleotide chain, one nucleotide may be substituted for another, or one or more nucleotides might be inserted or deleted. Sometimes the change in DNA structure has little or no effect on the function of the gene's product, but it frequently harms the function to some degree, or very rarely enhances it. Harmful mutations cause gene-based diseases, but enhancing mutations allow organisms to evolve new or more effective functions. Like normal phenotypes, disease phenotypes usually require the products of multiple genes, so most defective genes predispose an organism to disease rather than directly causing it. The accumulation of mutations within the human species accounts for such phenotypic differences as eye color, stature, or skin pigmentation. The number of mutations among human genes is so large that no two persons, except for identical twins, have exactly the same nucleotide sequence in the three billion bases of their DNA.

Control of gene expression

DNA information is expressed as proteins and their feedback networks. The information resident in nucleotide sequences is used not only for replicating DNA, but also for synthesizing proteins. Proteins are chains of a few hundred subunits called amino acids, of which there are twenty kinds. The amino acids in a protein are arranged in a specific sequence by cellular machinery that translates the genetic information coded in DNA. The sequence of nucleotides, read three at a time, corresponds to the sequence of amino acids in a protein. The amino acids differ among themselves in chemical character so that every kind of protein differs in chemical character from others. For the work of the human body many thousands of proteins are needed, each having a highly specific function like catalyzing a chemical reaction or transporting oxygen. Observable phenotypes are the result of protein action, usually the coordinated action of many proteins. The functions of many proteins are integrated into large networks, and these webs of chemical processes act as feedback control systems allowing organisms to shift the balance of their activities to adapt to changes in the demand for the system's output. Often the networks possess alternate pathways for achieving a desired output.

Differentiation into specialized cells requires the control of gene expression. The development of a human being starts with a single-celled, fertilized egg. As the egg divides into two cells, and as successive rounds of cell division occur, every progeny cell receives a complete copy of parental DNA. In the first few divisions, the cells produced are identical in all observable characteristics, but as cell division continues, cells are produced that differ in phenotype even though all the cells continue to have identical DNA. In this differentiation, particular genes are controlled by blocking their expression, not by changing nucleotide sequence. Regulatory molecules block particular sites in DNA preventing translation of the corresponding genes into their products. Specific blocking thus generates different patterns of gene expression. Changing patterns of gene expression produce distinct populations of cells, diverging in phenotype as differentiation progresses. Eventually, differentiation in humans produces more than two hundred cell types, organized into different tissues and organs. In any one cell type the majority of its approximately 35,000 genes is repressed, leaving a small subset of expressed genes that differs from the subsets expressed in other cell types. Phenotypic differences between progeny in a given cell generation depend on the location of the cells in different microenvironments. During differentiation cells adapt to a succession of environmental changes produced by changes in their neighboring cells and extracellular fluids. Each successive adaptation is superimposed on its predecessor so that each terminally differentiated cell manifests the entire history of its lineage and not merely its immediate state. Since differentiation is irreversible in animals, (except in special cases), history as well as DNA designs a person, even in the material sense.

Feedback networks and regulation of genes allow individual organisms to adapt to changing conditions throughout life. When environment increases the need for the product of a network of chemical reactions, the overall process will be accelerated, and when need decreases the process will be inhibited. Obviously, adaptation to environment is induced by contact with physical and chemical forces, but adaptation can be evoked even without physical contact, as in the adaptation of the brain through learning, and emotional reaction. Many of these adaptive responses affect patterns of gene expression, and therefore environment, as well as history, joins with DNA in designing persons.

At the level of populations, long-term adaptation to environment occurs more by changes in gene structure than by changes in the expression of genes. The mechanism for this adaptation is the natural selection that underlies evolution. For example, skin pigmentation may be an adaptation that protects against exposure to the sun, and the genes that design the pigment systems would be naturally selected in successive generations that are exposed to much sunlight. Similarly, sickle-cell hemoglobin seems to have evolved in Africa because it offers resistance to malaria that is prevalent there.

Long-term adaptation through natural selection is most obvious in the case of physical and chemical aspects of human beings. Less obvious is the adaptation of behaviors through natural selection of genes, a possibility actively studied under the title "sociobiology." Although the mechanisms producing material phenotypes may seem more obvious than those producing social behaviors, a mechanism giving rise to a certain behavior may be thoroughly materialistic, although far more complex. Behavior modification by psychoactive drugs reveals a material mechanism for behavior. A mechanism can be pictured, for example, in the courting and mating behaviors that are correlated with the release of hormones from the brain, when an animal or human senses that a potential mate is near. Those released hormones induce particular chemical reactions at many sites throughout the body, giving rise to an appropriate pattern of bodily actions. Moreover, feedback responses between the mates guide further behavioral interactions between them. The hormonal system that links brain functions to bodily functions is, of course, designed by genes, and the mechanism just sketched is clearly materialistic. The frequent association of natural selection with notions of "survival of the fittest," makes altruism an especially challenging kind of behavior to study in testing the validity of sociobiology theory, and much of the research of sociobiologists is focused on the evolution of a gene for altruism.

Genes affect behavior, but as is the case with most human phenotypes, genes act in combinations and their expression is modulated by the histories and environments of individuals, as already described. Through the invariability of individual histories and environments, natural selection must be able to recognize the difference between organisms that possess a particular behavioral gene, and those that do not possess it. In order for a behavioral gene to evolve through natural selection it must be powerful enough in determining the behavior, to avoid substantial compromise by variable non-genetic factors. Sociobiology, then, tends to favor a strongly deterministic and materialistic view of behavior.

Human nature and genetic determinism

Choosing is part of human nature, but its degree of autonomy is debated. All agree that choice is constrained by genes, history, and environment, but does any degree of freedom remain? Science describes material brain mechanisms as chains of causes and effects, but every cause is an effect having a prior cause. Since the initial cause is not recognized by science, some say thought initiation is due to chance. Others look for initiation outside the material realm of science by distinguishing between mind and brain, or even spirit and brain.

Some degree of genetic determinism is necessary in describing human nature. All the possible scenarios of a person's life must conform to the designs in DNA, and thus genes set rigid, though spacious boundaries on what a person can be and do. But genes are insufficient for explaining what actually happens. What actually happens within the boundaries set by genes, depends on factors that control genes, including environment, history, and mental state. The question arises whether spiritual forces can be added to the list of controlling factors. Material determinism argues that a complete physicochemical description of the history and state of a person would explain everything without including a spiritual component. Some, however, argue that human spirituality is a capacity that emerged as gene-based human biology evolved, and that its activity cannot be fully comprehended at the molecular level. Still others add spirit as a control factor in human nature in accepting a dualism where body and spirit are distinct, though coexistent, in a person. The disparity in these views of human nature has theological consequences.

A view of human nature according to material determinism fits atheism and deism. It provides no locus for personal interaction with God, although deists might suppose that God influences humans through environment. Belief in human spirituality, either as an emerged capacity or as a distinct part of human nature does provide such a locus. Scientific understanding of gene-based human biology does not perceive a spiritual component in human nature, but it might not be expected that a physicochemico-molecular description of humans would be capable of such discernment in the first place.

See also GENE PATENTING; GENETIC DEFECT; GENETIC DETERMINISM; GENETICS; HUMAN GENOME PROJECT; MUTATION; NATURE VERSUS NURTURE

R. DAVID COLE