Chapter 6 - The Human Phenotype
We have seen in previous chapters how the algorithm of natural selection created, step by step, the different biological species, in a cumulative process of design similar to the R&D of a large corporation. We have explained how this has been possible, with no need to postulate the existence and intervention of a conscious designer, internal or external to nature. The force of circumstances, enormous time lapses, and the regular operation of physical laws were sufficient to perform such a huge feat. It is important to underline how, throughout this long evolution, the possibility of a new invention always counted on an irreplaceable ally, systematically overlooked by creationists: the design accumulated thus far by the process of evolution. To ignore this recursive accumulation, which limits and conditions each subsequent step of evolution, is tantamount to claiming that, in the history of human technology, each new invention would have had to be generated from scratch by its creators. In fact, every invention results from a relatively small improvement on the sum total of standing achievements. As someone said concerning the creative power of history, each generation is a dwarf riding on the shoulders of a giant, namely the sequence of dwarfs having labored before. In what follows, we will be tackling a complementary problem as arduous as the evolution of living species, and equally important for a humanistic conception founded on realities, not on illusions: the construction of each of the individual beings constituting the species.
Many thinkers, led by the suggestive power of compared embryology, used to belief, in the dawn of contemporary biology, that the solution to this problem was that, somehow, the individual organism rerun, in the maternal womb and in a compressed fashion, the different stages traversed by the species' evolution. There are great difficulties with this conception. In the first place, many differences exist between the two processes, ontogenesis (the development of the individual organism) and phylogenesis (the species' evolution). Secondly, the pressures of natural selection which impels phylogenesis is clearly absent in ontogenesis. Thirdly, the reason why the development of a specific individual stops exactly when it reaches the level of its own species is left unexplained, as well as why individual development chooses at each fork the appropriate course following the evolutionary march of the species throughout the geological stages. Above all, the theory, suggestive as it might be, lacks the features of rigor and causal connection we are expected to find in a scientific doctrine. It shares more with the poetic and mystic perspective than with the explanatory determinism of concatenated natural events. The explanation had to be searched somewhere else, namely in the strict application of the rigors of scientific method. Definitely, it had to be postponed until biology, through accumulation of research, was able to formulate a massive set of coordinated hypotheses containing innumerable minute and variegated details relevant to the problem. Technological limitations made that impossible until the last decades of the 20th century.
An alternative hypothesis –dormant in the back of the scientists’ minds– was the idea, apparently valid, that the genes of each species could contain enough determinants for the construction of each organism, according to a set of specifications for the respective species. The discovery of the genetic code, in the second half of the 20th century, rendered some credibility to this line of thought. The reluctance to embrace it had to do with the vast number of cellular-mechanics details needed to understand, in an objective manner, the mechanics of the translation from the genetic information to the materials, structures and processes required to build an individual organism. It was also necessary to clarify how the action of genes made its appearance at each one of the organic parts in a differentiated way, and how and why they functioned differently, at each stage of the organism's life. Such huge collection of problems of detail and their progressive solutions by contemporary scientific thought –through the method of accumulation of steps– constituted a veritable monument to the program of reasoning and experimentation inaugurated in the Renaissance and still flourishing in our civilization after half a millennium. And yet, it was then when the difficulties for a purely genetic explanation of ontogenesis arose. Comparative studies from molecular biology came to suggest with high probability that the genes from the chimpanzee and human species178 differed only by less than 2% of their DNA. It was difficult to assert that such small number of genes was, on its own, responsible for the vast difference between the organisms of these two species. More recently, the sequencing of the human genome lowered to a third the estimated number of our genes, making it comparable to that of many other –much less complex– species. It began to seem necessary to postulate other factors acting as amplifiers of the discriminating effect of such a low number of genes.
A set of different hypotheses of greater specificity came to the rescue. Let us recall that for a gene to express itself implies its production, through transcription, of a particular RNA sequence which acts as instruction for the ribosome machinery to build a certain protein. It was discovered that some genes are not directly involved in the production of construction materials or agents for the organism's operation, but rather in the way in which other genes express themselves, that is, in the regulation of genetic expression. More specifically, some genes order the production of proteins, usually small, whose function is restricted to attaching to other particular genes and deactivate them. Adding to complexity, other genes order the production of repressive agents to deactivate the repressors, which actually means reactivating the genes originally repressed. This elaborated system of regulation by certain genes of the expression of others, generates great flexibility and power, worthy of the most advanced human computer programs. It implies an impressive amplification of our genes' capacity for diversified expression.
Furthermore, the discovery of the intercellular matrix and CAMs (cell adhesion molecules), created an unexpected intra organismic panorama: a true ecological landscape where it is possible for cells to move across the body in addition to their sending signals179 to each other to mutually influence their behavior. The cell adhesion molecules are, by themselves, a remarkable multiplicative factor of the constructive effectiveness of genes. CAMs belong to the globulin family. As such, they show domains180 similar to the constant parts of antibodies. They associate themselves via homophilic (amongst equals) or heterophilic (amongst different) unions. These molecules usually settle in pores of the cellular membrane, with one of their domains outside –and the rest of the molecule inside– the cell, in such a way that phosphorylation181 of a cytoplasmic domain can originate changes in the domain outside the cellular membrane, producing signals' expedition or absorption. Hence, CAMs are capable of originating and supporting signal cascades of large projection. The signaling occurs by means of the emission of simple peptides (small proteins, amino acids, or nucleotides)182. The same signaling mechanisms which reign generally in the entire organism are those which, with little adaptation, operate in the neuronal circuits of the brain.
Within that landscape, the extracellular matrix turns out to be extraordinarily important as a constructive support for exploration. Built by contribution from several cells, it constitutes a world of fibers between cell and cell, a true swarm of intermediate filaments. Cells can travel, literally, between those filaments, following the trail of substances emitted by other cells, like traffic signals marking a road. These journeys and inquiries produce effects independent from genes' action, caused by fortuitous encounters with diverse elements, affecting organic conformation beyond what coded in the genetic material. The most outstanding migrations in that intra ecological world are those of embryo cells, which start to move along their under-construction body very early on, by means of amoeboid movements, until they reach the locations assigned to them by natural selection. Especially important among those migrations, enduring long beyond birth, is that of nervous cells and their axons, whose growth cones are capable of tracing chemical signals far-off from their cellular nuclei. Speed and obstacle hazards, basically random, strongly contribute to individual differences between organisms of the same species, even in the case of identical twins, endowed as they are with the same genetic lot.
Let the reader visualize the ghostly picture of this gigantic microscopic migration, which builds up our insides during the first four years of our lives. Any number of marvelous adventures can occur in the surrealistic landscape of our growing organism, worthy of the imagination of a fairytale writer. In such an environment, all kinds of incidents can occur, maximally diversifying the unique traits of each specimen of the species. At the same time, the global story will be basically the same. The heroes will fulfill their mission, although gropingly, in an effective way and within the indispensable terms needed to win their prize. Not because of any conscious attribute in the constitutive elements of the developing being, but because –how could it not be so?– in this internal ecological landscape the principles of external ecology also rule: Events are determined by the same natural constraints, channeling and pressures which determined the species selection since times immemorial.
The central nervous system is an enormous constellation. The brain cortex is estimated to contain more than 1011 neurons, each one connected to approximately 104 others, for an astronomical total of 1015 different connections. Obviously, human DNA could not contain all the necessary information to directly determine each one of those neurons and establish each one of their connections. The quantity of information needed to specify even a small percentage of them, would demand an incredible mass of genetic information, which would not fit in the thirty-five thousand genes at the most that we currently know are part of the human genome. Furthermore and generally, genomes maintain relative constancy, notwithstanding vast differences in brain sizes. Although the human one possesses thousands of times the number of neurons possessed by that of the smallest vertebrate, and millions more connections, these differences do not correlate with any meaningful increase in genome size. On the other hand, although it would seem reasonable that brain wiring in different species was guided by different principles, today we know that this is not the case. Take some cerebral tissue from a member of a species; implant it in that of another: no connections will go astray. If a set of pig neurons is implanted in a rat brain, the result will be a perfectly decent rat brain (Deacon, 1997).
At this point the ecological landscape we have postulated comes to good use. Neuron migration and axon growth are notable by their being exploratory and reactive, naturally adaptable to the physical constraints offered by the rest of the nervous system and other body parts already assembled. The axon moves forward along the intercellular matrix, following signals which its growth cone probes out and chases, drifting apart in the presence of obstacles or repulsive substances. For this daring enterprise, fetal axons lack explicit instructions about in what direction to growth or to which particular cell to arrive. They simply follow the lead of intercellular filaments, responding to adhesiveness differences in the surface of the cells they bump into, and to the tissue mechanistic properties and the specific growth factors,183 emitted by cells in target areas. The moment the growing of an axon begins can set a bias. Development seems to be more affected by physical closeness and random constraints with the neighbors than by the specificity of signals. The large number of axons makes their intercrosses in different directions difficult; they tend to diverge in parallel beams. This tendency for spatial organization will be, at the beginning, only approximate, compatible with diverging connections. As development progresses, though, most aberrant projections will be pruned by competition from those that obtain higher convergence reinforcing each other.
Mechanisms of this sort are enough to guide the development of the nervous system in a general way, with scant genetic instructions. Better-determined connectivity will only be achieved later, by selective attrition. Neurons tend to overproduce branches in growing axons. These branches tend to probe large number of potential targets during the early stages. Just a fraction of early connections will survive the competition among axons of different neurons over the same synaptic targets. Development, lacking enough information, invests heavily in redundancy: all neurons tend to connect with all the others! Only later will each particular mind sculpt out the excessive material with the chisel of apoptosis,184 subject which we will momentarily turn to. Of the vast number of synaptic connections and neurons, reaching its maximum at age four, only the most efficient will remain when the organism and its resulting personality reach maturity, around age eighteen. (Changeux, 1986) Nature, a liberal ruler, prefers to overproduce first and to prune afterwards, rather than supervise and coordinate the development of countless and varied cell connections, in a socialist fashion.185
Apoptosis, the programmed death of the cell, has its evolutionary origin in the endosymbiotic absorption186 of mitochondria carried out by the eukaryote cell. This primitive bacterium had within itself the mechanism to produce energy through carbohydrate oxidation, releasing carbon dioxide and water, what endowed cells with highly efficient power stations. However, as it happens in the larger scale of human civilization, energy generation produces contamination, which in the microscopic case is represented by free radicals,187 ultimately lethal for cells. When enzymatic controls fail, free radicals can attack the cells' lipids, proteins and nucleic acids. If these accidents accumulate, they can be fatal to the organism as a whole, giving place to several degenerative diseases.
This death through enzymatic-control failure has been recruited by natural selection as an essential tool in the important task of constructing the individual out of the germinal cell and, subsequently, of supervising the quality of cellular division. The program is effective in the attainment of very different objectives: molding of a particular physical form, adjusting the relative size among different organs, the elimination of cells with genetic endowment badly duplicated or with deviant conduct, among many others.
Apoptosis mechanisms appeared early in the history of living beings, not once but many times. In higher organisms, apoptosis takes part in embryo development. In general, it participates –in association with cellular division, differentiation and migration– in the concrete construction of the species individual (morphogenesis). It plays, for instance, an essential role in the metamorphic processes characteristic of insects and amphibians. It also performs a direct morphological role in vertebrates, being responsible, for example, for the toe and finger separation in primates during embryonic development. Its most universal morphogenic role, however, consists in mutually and precisely adapting the number of cells in each body part during the organism construction (Raff, 1993).
Countless apoptosis events occur during the formation of the nervous system. It does happen to isolated neurons, fired by the absence of signals from other similar cells, what normally ensures their survival. It constitutes a sort of “death by omission” that guarantees the precise number of cells in each zone of the nervous system, eliminating those wrongly located and favoring those which, through their having amassed a maximum of contacts, are better located to fulfill their functions. Finally, programmed cellular death is used by the organism in a systematic way to eliminate cells whose genetic endowment has been accidentally modified. Modifications that alter the cellular-division cycle and give origin, in principle, to the production of cancer, deserve special mention. In these cases, apoptosis eliminates the cells that have gone astray, avoiding their transformation –through uncontrolled multiplication– into malignant tumors.
Apoptosis is, of course, regulated by the timely expression of appropriate genes. The existence in cells of “suicidal genes” certainly sounds paradoxical. It is so only when one judges it in relation to the cell that performs its own destruction. In contrast, for multicellular organisms, the existence of an intrinsic cellular death program is truly advantageous, as the existence of altruistic conducts on the part of their members is for bees or ants swarms or for human societies (Morange, 1998). An apoptosis disorder can have disastrous consequences for the organism. We know today that it is involved in the emergence of AIDS and different forms of cancer. The AIDS virus has the capacity to trigger –through mechanisms not yet wholly understood– the suicide of certain white corpuscles (T4 lymphocytes), weakening thus the immune system. Likewise, an apoptosis inhibition provoked by mutations leads cells normally destined to die, to proliferate in an anarchic manner, forming tumors.
The apoptosis process is destructive in a sophisticated and clean way. It proceeds with an orderly DNA fragmentation that culminates with the stopping of all metabolic activities for lack of genetic instructions. It culminates with an implosion which completely disintegrates the cell. Apoptosis is clearly distinct from death by necrosis (equivalent to putrefaction) and from the traumatic death produced by crude murderous agents (bacteria, virus, or chemical toxic substances). Apoptosis, in contrast, is death without trauma, during which the genetic material destroys itself by cutting the DNA in discrete sections, ironically, following its own instructions. As the magnetic tapes of espionage movies, genetic material contains the mechanism for its own emergency elimination (Le trésor, 1995).
What is most surprising in this phenomenon is that, even when unleashed by the scarcity of signals from other cells, its decisive cause is intrinsic to the cell itself. In the previous section, we referred to the death of neurons during nervous system development. This attrition can reach even 50% or more in many kinds of neurons. Such massive death is considered to be a consequence of the corresponding neurons' failure to obtain, from the target cells, specific and sufficient signals for them to survive. Favorable molecular signals prevent the process by repressing an intrinsic suicidal program. In other words and amazingly, the default value in cell conduct is causing its own death! Only while satisfactory operating perspectives are present (enough reasons to keep on living, so to speak) the default suicidal program remains pending (Raff, 1993).188
One of the most outstanding, elaborated and useful features of the eukaryote cell organization is its quality control system. This control is exercised at several points of the cellular cycle, especially during cell division and immediately before, thus covering two peremptory flanks: on the one hand, the transmission of genetic material to a new cellular generation; on the other, the delicate operation of routine integral copying of the genetic material, a very dangerous circumstance for the genes, at risk of falling victims of transcription errors fatal for their survival in the individual organism.
In this crucial situation, life shows off its resourcefulness, always in its characteristic style: proliferation and subsequent selection. The proliferation occurs by means of an abundant cellular reproduction; the selection, through the application of apoptosis to cells committing irregularities in the duplication of their genetic material. Just as in the development of the nervous system, the same algorithm of natural selection that produced the species applies itself to the maintenance of the individual: generate by means of abundant replication and sift afterwards the specimens maladjusted to the environment. In this case, the role of environment is played by a series of check points, solidly entrenched by natural selection into the life cycle of the eukaryote cell.
The cellular cycle is a tidy set of events during which the cell performs its metabolic functions, increases its size and, finally, proceeds to divide itself into two daughter cells. These daughter cells will repeat the cycle all over again. The cycle stages are usually referred to with the names G1, S, G2, and M. “G” stands for “gap.” “S,” for "synthesis," representing the phase during which DNA replicates itself. “M” stands for "mitosis," the phase wherein, through nucleus duplication and cytosol partition, two new identical cells replace the original one. The G1, S and G2 phases are usually grouped into a single super stage, under the name 'interphase.' Inversely, M (mitosis) is usually subdivided into six sub stages: prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis. The first five mitotic sub stages correspond to the cell's nuclear duplication, whereas the last one to the cytosol partitioning.
What follows is a summary of the events that occur in each one of those steps.
Interphase. Cells prepare for mitosis by duplicating their genetic material during S, thus acquiring their maximum size. Chromosomes are not perceptible in this phase, since the genetic material was partially unpacked at the end of the last cellular division, preserving only its two first levels of wrapping.189 DNA duplication produced during the synthesis stage has created chromatid pairs190 –main protagonists of cell replication– to which we will be referring in the sequel.
Prophase. The cell detaches itself from the intercellular matrix to which it was stuck during interphase. Genetic material begins to condense and takes the typical form of chromosomes under the optic microscope. DNA stops producing RNA. Centriols191 start moving toward opposite poles of the cell. Fibers extending from centromers192 to centriols appear, forming a spindle.
Prometaphase. The cellular membrane dissolves. Proteins are pasted to centromers, thus creating the kinetochores, to which microtubules adhere themselves, and which complete the spindle structure needed for chromosome migration.
Metaphase. Spindle fibers adhere to the chromatids near the centromers and align them along the cellular-nucleus equator. Such organization ensures that, in the next stage, when chromatides separate, each new nucleus will receive a copy from each chromosome.
Anaphase. Centromers break up and chromatids separate when they reach the kinetochores, being impelled by diverse mechanisms to move along the tubular meridians toward opposite ends of the cell.193
Telophase. The two chromatid sets reach opposite cell poles, forming nuclear membranes around each one. The spindle fibers dissolve and the new chromosomes begin to uncoil, until they completely disappear from sight through the optic microscope.
Cytokinesis. In the case of animal cells, a fibrous ring, composed of a protein called actin, contracts the mother cell in its equator and squeezes it until its cytosol splits into two independent cells, each with its own nucleus.194
A large regulation apparatus exists in this cycle, with many levels and great redundancy, which insures a strong system of quality control, totally automatic.195 Mitosis is the shortest part of the cycle; it only covers a 10% of its duration. Interphase is the longest: it covers the other 90%. It is also the most varied, where the most important regulating events or checkpoints occur. At the end of G1, moments before DNA synthesis begins, an evaluation is produced, which decides whether the cell must proceed to prepare itself for mitosis. If the internal and external signs are unfavorable, the cell leaves the cycle and enters the stage of non division, wherein most human specialized cells exist (such as neurons or muscular tissue). However, if the signs are favorable, the cell begins to increase its volume while entering the S stage. Eukaryote cells cannot begin to split themselves unless they have doubled their size through replication of their genetic material. That is why, at the end of synthesis, a second checkpoint is encountered, which evaluates the cell’s volume as well as any deficit incurred during replication (persistence of not duplicated DNA). The control maintains a blockage not to be raised until there is no single unduplicated strand left in the nucleus. Once this control is passed, the cell enters mitosis, starting to unpack the DNA in preparation for the splitting of the already-duplicated genetic material. We know today that, besides these two main checkpoints, several others exist, at least one in each mitosis step. Detection of a single error will unleash apoptosis.
The discovery of apoptosis has opened many arenas for reflection regarding life as one finds it in the human species. A theoretical question is whether programmed death transcends the cellular level, the concept being also applicable to the organism as a whole. Another –more practical– question is whether such a mechanism, either at cellular or organism levels could be somehow deactivated or it should be considered an inescapable attribute of all living matter. Letting these reflections run their course, one may consider the perennity of unicellular beings, whose cellular division implies double perpetuation, both as cells and as independent creatures, since for them the concepts coincide. Apoptosis can only benefit multicellular beings, among whom the sacrifice of an internal component might in some cases be convenient for the survival of the organism, assuming a surplus of cellular duplications.196
We should take into account –in the case of multicellular beings– the distinction between germinal and somatic cells. This transcendental invention of life, related to sexual reproduction, seems to indicate that there is nothing inherent in the multicellular-organism cell that compels it to be programmed to die. Germinal cells are, on principle, immortal. Apart from the genetic material exchanges that occur through recombination197 between homologous chromosome pairs during meiosis, their integrity is as lasting as that of unicellular beings No preinscribed program forces them to die at a specific date or occasion.198
To answer the question whether it is possible to deactivate the suicide program in a somatic cell (part of a multicellular organism), we must refer to cancer, one of the remaining scourges of humankind. It carries precisely the deactivation of that program with it. It implies the indefinite multiplication of affected cells, bound –if not treated– to cause the death of the organism. You might wonder what would happen if those cells with their suicide program deactivated were to be kept alive apart from the organism. A researcher at The University of Chicago had the same curiosity and decided to settle the issue by means of an experiment started more than half a century ago. After thirty years of failed attempts, he was finally able to preserve in culture some cancerous cells from a patient called Henrietta Lack.199 These cells have reproduced countless times and turned out distributed in molecular-biology laboratories all around the world, having reached a total weight of several tons. These HeLa cells –as they are called in memory of the patient– constitute the first human cells that have not perished outside of a body. They have helped the eradication of poliomyelitis, flown in space shuttles, taken part in nuclear-blast tests. They have become the favorite tool in the study of radiation effects, virus growth, and drug testing. The Physiology or Medicine 2001 Nobel Prize was awarded for research programs wherein HeLa cells played a pivotal role. They are also expected to contribute to the finding of a cancer vaccine and the deciphering of many mysteries of aging.
Up to here, we have considered a single type of programmed cellular death, the one associated with cellular mechanisms of quality control: apoptosis. The difference between germinal and somatic cells, as well as the mere existence of cancer, open up views toward a quite different type of suicide program: that at the organism scale. It is unleashed by development detention and culminates with the exhaustion of a term inscribed in the genetic material of each species. As it is well known, embryo cells grow and reproduce at a far higher rate than that of infants. Infant cells do it at a rate far higher than that of adolescents, who do it much faster than adult cells. Starting from there, and throughout a long period of time, cells go on softening their reproduction rate as old age progresses and “natural death” approaches, i.e., the converging failure –due to simple weakening– of multiple organic systems. It is a merit of biological researches of recent years to have begun to explain these phenomena as the expression of particular regulatory genes. These genes are themselves subject to a regulation processes of filigree daintiness, within a hyper complex control system. In all this symphony of converging actions, an underlying one stands out, which seems to have the category of orchestra director: the progressive shortening of the chromosome ends, the telomeres. Unlike germinal and embryo cells, whose chromosomes are not shortened in cellular division, mature somatic cells all lose a few nucleotides at each cellular division. Consequences are fatal. Let us ponder this matter in some detail.
Eukaryote chromosomes have the particularity of ending in repetitive sequences of non coding nucleotides which receive the name of telomeres. It is as if each group of species had, at the end of its chromosomes, a “trade mark” made up of several consecutive repetitive letters. In vertebrates, the repeating sequence is TTAGGG. This trade mark occurs many times and can reach the hundreds or thousands.
Telomeres were identified by H.J. Muller, already in the thirties. These structures are essential to cells. They perform an important role in preserving chromosomal stability during both meiosis and mitosis, by preventing chromosomes from joining to each other at their ends, thus losing their independence. However, their most evident function is that of serving as genetic clocks, setting a limit to the number of possible divisions for the somatic cells.
Telomeres are not separate units within the chromosome. They simply are the last nucleotides of the two DNA strands that form the double helix, in total continuity with the rest of them. These DNA regions get shorter with each cellular division, limiting the cell to go only through a restricted number of divisions. The shortening would eventually eliminate vital genes contiguous to the telomere, due to exhaustion of the non coding nucleotide repetitions which telomeres consist of (Hernandez, 1999).
Why the shortening? To understand it, let us recall some details of how the polymerase works over the DNA strands in order to duplicate them. Polymerase sits at the DNA 3’ terminal on the advanced or sense strand, in order to begin its work. Since it cannot replicate the section of DNA where it sits, each newly created strand will lack some initial nucleotides. The duplication of the antisense strand does not present such a problem, since the polymerase starting point is different; it performs the synthesis in a discontinuous way, by the Okazaki-fragments technique.200
The lost sequence is replaceable thanks to another enzyme, telomerase, a ribonucleoprotein.201 This enzyme works differently from polymerase: it uses as a guide for the DNA synthesis an RNA sequence carried within, specific for each species, instead of the external guide used by polymerase.202 The enzyme's RNA is the complement of the telomere repetitive sequence and, because of that, able to recognize it and attach itself to it. The enzyme lengthens the telomere using that pattern as mold. When the lengthening is complete, it moves on and repeats the cycle. Not being too rigorous, variations from cell to cell in the sequence may occur. Such errors, however, do not affect its essential function: compensating the length loss of the chromosome. The crucial point is this: genes that produce telomerase are preferably expressed during embryo stages. Consequently, adult somatic cells turn out having a deficit in telomerase, losing from fifty to two thousand nucleotides at each cellular division.203 The final result of this strange dance of the telomere with its dodgy companion is that, at the end of a large but finite number of divisions, certain proteins attached to the telomere dissolve in the plasma and finally trigger apoptosis. They avoid a traumatic death for the cell only at the price of inducing the execution of its programmed suicide.204
What does all this mean for the organism? Nothing less than that our days are counted. Our somatic cells are constituted in such a way that they carry in themselves the possibility for just a limited –although large– number of mitotic divisions. Once the term expires, they begin to eliminate themselves one by one, causing the decadence and death of the organism. Germinal cells, on their part, not submitted to this fate, always reproduce identically and, in what concerns their genetic program, are destined to live forever. A perverse mimicking of such immortality is the one that improvises the macabre dance of cancer, to which we now turn.
The term “oncogenic paradigm” denotes a certain set of hypotheses which, around 1985, scientists began to consider as a better explanation for the nature and origin of cancer than the viewpoints prevailing during the 70’s, slacker and often contradictory to each other.205 Development, considered as a regulatory process, implies a differential repression of genes at different moments of the organism life. Against such background, cancerous cells would be a case of –local and incorrect– regression of differentiated cells toward the embryo stage of genetic expression. Under the new vision, often referred to as the oncogenic revolution, cancer is considered, much more specifically, as a change in the expression of a limited number of genes, called oncogenes, which produce an exceptional course of the cellular cycle resulting from alterations in the intracellular signaling web.206 What best distinguishes this paradigm from the previous opinions is its limited and clear-cut number of hypotheses. In a scientific environment wherein cancer had been over explained, it introduces a model with standardized methods (recombining DNA techniques) and clearly defined objects (oncogenes). Included hypotheses, as well as omitted ones, are equally important in this reorganization. Thus, it moves away from a vague idea of genetic malfunctioning towards a rigorous conception of specific biochemical interactions. The result is that researchers had acquired the conviction that the cancer molecular base had finally been discovered (Morange, 1997).
In its more general sense, the oncogenes which the new model speaks of are those genes which codify the proteins involved in cellular-cycle control. Their name comes from the Greek and means “tumor generators.” However, perfectly normal genes that do not generate tumors, and even repress their growth, are also covered by this general denomination. With that term, in its general sense, scientists refer globally to three very specific types of genes linked with cancer. All have in common the fact that they participate in the regulation of the cellular cycle. This sophisticated word has, nevertheless, another more specific meaning and –to laymen confusion– both senses usually occur together in one and the same scientific article, even in the same paragraph. Let us try to clarify this mess a little bit.
Oncogenes –as the term is used in a general careless sense– correspond in scientific exact language to either of these three distinct large categories:
Protooncogenes are a family of normal genes which codify proteins involved in growth pathways207 and cell-division control. They can mutate and turn into one of the other –here listed– categories. Protooncogenes are much conserved in evolution, i.e., they present large-sequence coincidences along diverse species.
Oncogenes stricto senso are protooncogenes which have suffered damage, through mutation208 or virus attack. As result of this damage, they codify an altered version of the same proteins codified by protooncogenes, or an excessive quantity of them, provoking a pathway imbalance toward excessive cellular growth and duplication. If, in the same locus, an allele209 is protooncogene and the other oncogene stricto senso, the latter predominates (referred to as “dominant” and the former, “recessive”).
Tumor repressor genes are normal genes whose absence can bring about cancer, if there are oncogenes (stricto senso) in the corresponding cell. People predisposed to cancer are usually so because they were born with imperfect copies of this kind of gene. Since genes of all cells (except sexual cells) occur in pairs –one member of each pair coming from each parent–, it is enough that one of the copies is in good shape to perform the repressing function. However, if the second copy is also defective, by heredity or mutation, the repression cannot occur.
Each tumor originates –as life itself– in a single parent cell, damaged in one or other specific oncogene. It represents an individual experiment in cellular evolution based in genetic instability. In each case, the damage releases the gene or its protein product from normal regulation, taking the cell to never-ending proliferation. It has been proved that cancerous cells come short in the functions necessary to regulate cellular duplication. Compensatory incorporation of normal genetic material can eventually replace the deficit and suppress the cancerous growth. The genes in charge of this repairing action are the tumor repressor ones. Damage in oncogenes may come from simple copy errors at cell replication, environmental or cultural pernicious influences (such as smoking), or other traumata. As to viruses, we know that they are capable of firing a cancer in virtually all vertebrate species already examined. Their action is an integral ecological part of the millenarian planet
The idea that tumor formation could result from the affectation of one or several genes was not new. The surprise laid on the fact that oncogenes turned out to be so limited in number (approximately one hundred), and on that they all participated in one and the same global cellular function: codifying the proteins in charge of intracellular signaling pathways and webs which work for the adaptation of the cellular cycle to the needs of the organism at each moment. A simple mutation in one of these genes, the replacement of a single amino acid by another, is enough to transform completely normal in vitro cells into cancerous ones. However, it is possible to say –in general terms– that cancer only occurs in practice as consequence of two malignant mutation rounds: the first produces the regression of the cell to an embryo tempo of growth and reproduction; the second harms the cellular cycle in its quality control mechanisms in charge of repairing those damages or –as a last resort– firing apoptosis. This explains why cancer actually is a rather rare disease, for although we begin to be affected by it comparatively frequently (the first mutation) in most cases we are safeguarded by the cellular-cycle quality controls. The illness only progresses and manifests itself if such a defensive mechanism fails to work by damage in still other gene (the second mutation).
In short, according to the new vision, the apparently countless causes of cancer seem all to work in the same way, namely, damaging a few keys of our genetic keyboard (Bishop, 1995). We have found the enemy and are beginning to understand its attack lines. Nevertheless and for the time being, we understand cancer much better than we can fight against it. The incessant work of an international army of researchers in a multitude of world laboratories will surely –and hopefully soon– complete this urgent task.
The extraordinary biological discoveries of the last decade, including the sequencing of the human genome, with its concomitant biotechnological advances, mean that a deep revolution has occurred in medicine, truly even a redefinition of the traditional borders between life and death. The most important feature of this revolution involves the passage from a secular medical tradition, based in the accumulation of experimental results (more or less successful recipes), toward a systematic program of anomaly prediction –and possible counterattack– based on the knowledge of the machine itself, farther from a recipe tradition and closer to mechanical engineering. Familiarity with the human genome –and, eventually, with the corresponding proteome211– is placing the molecular biologist and the physician more and more in the novel possibility of “consulting the handbook” of the concrete machine which is the patient organism. This new approach will undoubtedly put the disease diagnosis and prognosis on a firmer ground regarding the means conducive to “repairing” the affected "part." Pharmacology is in the process of changing from being an empirical discipline to becoming an accurate rational technique. Here we are, admiring the apotheosis of
biology as reverse engineering,212 and entering a period in the
history of medicine we could very well brand its age of reason.
As confirmation of the above mention trend, I find relevant a recently-held pharmaco-genetic convention, whose main presentations were posted in Internet by Science. One of the presenters, Dr. Allen David Roses, put forward in general terms the current method, “not of five or ten years from now but of this very moment,” to validate drugs. Two different situations are to be distinguished:
The testing of the new drug itself, during which massive experiments are performed with volunteer patients whose genetic profiles are collected (through sequencing of their genomes) in order to study the statistical relation they may keep with the experimental results.
The ulterior application of the tested drug, where the genome of the patient is also used, not as part of the statistical study but as a means to determine which of the drugs in the market is best suited to treat his particular illness.
Continuing the comparison, we could consider things in the following manner. At the research stage for the creation of the drug, the patients’ genomes are used for writing the drug's handbook. During the application of the medication to an individual
patient, at the other extreme, alternative drugs handbooks are consulted on the basis of the genetic characteristics of the
The lecturer strongly emphasized the secure character of the patient genetic profile, to which he attributed “forensic” reliability, i.e., a degree of safety comparable to the one demanded from justices to convict a criminal whose DNA coincide with vestiges found in the crime scene. It is also important to note that current experiments for drug validation are expected to establish both its effectiveness and its harmful side effects. The effectiveness degrees in traditional experiments vary between 10 and 40%, wheras the harmful side effects usually appear in about 4% of the cases.
Two considerations are here in point. In the first place, drug selection is now done disqualifying those that provoke harmful effects in patients with a specific profile. In the previous situation, on the other hand, patients had to accept a small probability of being affected, or else take the drug and stop using it as soon as side effects begin to appear. In the second place, and this is what most marks the difference between the traditional and the new methods, once the validation has been performed, tests can be done again separating the patients according to genetic profiles, what produces higher percentages of effectiveness, to marks between 80 and 90%, and a considerable reduction of harmful effects. In conclusion, the safety of a drug prescription based on genetic profiles has passed from being the shot in the dark of traditional empirical medicine to a level of safety comparable to having a computer-assisted repair of our car in a modern garage. Splendid difference!
Another aspect of this new rational approach, opened up by stem cells and their extraordinary characteristics, is the research regarding the possibility to regenerate organs, or even to produce them as “spare parts.” Unlike human genome sequencing, this other chapter of the new medicine is technically simple, since it does not require any complex state-of-the-art bioelectronic technology. The difficulties it must overcome are of a different character: ethico-political. Such cells are not yet differentiated and, therefore, can acquire –in suitable contexts– the specific configuration appropriate for functioning in any body organ. In other words, they are multipotent. They are found in the embryo, but also in the fetus and in certain slits of the adult organism. In the embryo, as Arlene J. Klotzko (2002) highlights, they appear near the fifth day of gestation with the blastocyst, a hollow ball made up of approximately one hundred cells, none of which identifiable yet as destined to any specific part of the human organism.
Research on the way to obtain their transformation into specific cells –in order to treat up-to-now incurable diseases such as Parkinson and diabetes– has been delayed in most countries due to the opposition of fundamentalist groups that consider the human embryo sacred, as a "potential" person.213 Nevertheless, thanks to the more liberal attitude adopted by the English Parliament,214 some human life saving applications may well be achieved in the near future in the U. K. Due to the fundamentalist prejudices prevailing in the United States, it would seem that pure science and medical applications in this field will fall behind with regard to countries with more modern ethical standards. On the other hand, it might be that on this issue, as in many others, technical and scientific advances arrive to make idle the ethical question with some revolutionary discovery or invention which supersedes or dissolves the problem.
Every mature science holds “residual categories,”215 namely concepts not positively defined within the corresponding discipline but rather left unexamined in the subject of study once the science has completely exhausted the categorization capacity determined by its own paradigm. “Old age” is one such category in the biological sciences. The methods derived from its key concept, evolution through natural selection, are not able to explain –except by residue– the aging phenomena. Natural selection is ill fit to filter old-age attributes since its products depend crucially on the capacity of individuals to reproduce themselves, and such capacity is terminated (in females) or severely tempered (in males) beyond half age. Thus, genetic alterations favoring death or decadence, generated in advanced age, have little or no chance of being filtered by natural selection. Moreover, genic forms selected to ensure effective reproduction during the fertile period can very well have ill effects for the quality of life or the life duration of organisms exceeding that age (Morange, 1998).
We all know the symptoms of aging. Its more general description is that of a progressive inability to confront environmental challenges. Aging consists of a deterioration in the functioning of all organs, the most affected being those of the senses and cognitive faculties, especially memory. In a less palpable but also fundamental way, the immune system suffers deep alterations, which make it less reactive to external aggression. Even chromosomes are modified. As we have seen, its extremities –the telomeres– progressively shrink, leading to the eventual arrest of cellular division. Cancer, on its part, whose genetic base has been established, is a disease predominant in old age. In short, aging turns the organism fragile and, consequently, increases its propensity to die. There is neither a gene of aging, nor a disease to which we can attribute the aging symptoms or which could be branded as cause of the “natural death” typical of old people. Senescence, and the death which it inevitably leads to, are the normal results of the weakening and final collapse concomitant to all vital functions. Such is the teaching of the received wisdom; we will be reviewing whether there are reasons to qualify this conclusion before the end of this chapter.
The dream of overcoming mortality belongs in the general heritage of human cultures. Ours, an eminently Faustian216 one, indulged during the Middle Ages in different fantasies related to “the fountain of the eternal youth.” By inspiring the old alchemists, this legend served as the sting that gave birth to modern chemistry and the consequent exuberance of biochemistry and genetics. On the other hand, and in spite of their remarkable accomplishments, contemporary medical sciences have done very little to transform these fantasies in reality, as it was to be expected. Let us enumerate three attempts, from most to least successful and from least to most important.
The possible evolutionary consequences of the drug sildenafil and similar ones. By contributing to equate the old with the young male in sexual prowess, it might have secular effects in the fixation of mutations favorable to longevity. However, they would not be too large, taking into account that most old men do not couple with fertile women. In addition, there is no social pressure to medically expand the reproductive age of females.
It has been determined that a nutrition lacking in calories lengthens the life of rats and mice. Given the similarity of the respective genomes, it is likely that the same is also true for human beings. The biochemical explanation of the benefits of this temperance relates to the already-mentioned effects of free radicals217 derived from oxygen, extremely reactive and dangerous for the organism. Since these substances are an unavoidable sub-product of food combustion, metabolic softening can diminish their production and part of the damages leading to aging (Weindruch, 1996).
Finally, we can concoct the promise of a “remedial telomerase,” an hypothetical new drug that could, in principle, increase longevity. As we have seen,218 telomerase producing genes are differentially expressed during the embryo stage and in adult age. It is possible to imagine that a supplement of telomerase added to the diet, starting at a certain age, could be absorbed by the cells and produce a delay in the shortening of the telomeres and the consequent cellular death. However, telomere length, this ultimate limit of life duration, is not the major contributor to aging, as we will see in the sequel. In any case, this hypothetical drug is solely a theoretical possibility, with a feasibility still to be explored.219
Much clearer is the prospect of a different borderline between life and death, seemingly being delineated by the new medicine. It is obvious, since decades ago, the paradoxical effect of the advance of public hygiene and medicine in the increase of mortality through cancer and cardiovascular diseases. Since other causes of death are better controlled, these factors have started to look stronger, becoming the heaviest mortality causes in parallel with a general increase in life expectancy. Something similar is about to happen with other causes of disease, as the fight against cardiovascular problems and cancer progresses, and the imminent invention of a vaccine against the latter, bound to occur thanks to knowledge already acquired concerning its biomolecular basis. Once these current major enemies are defeated, it is inevitable for other terrible scourges to rise as dominants, perhaps harder to beat than those of the 20th century. It is ease to guess that the next one to appear in the horizon will be a host of mitochondrial malfunctions, better known as degenerative diseases. In fact, many authors maintain that cancer and cardiovascular diseases are already their vanguard, since their common base is the damage caused to DNA by oxidizing agents generated by mitochondria220.
The common bases for mitochondrial diseases are two fundamental facts of the mitochondrion genetics:221
The importance for the organism of everything referring to cellular energy production, where mitochondria are key pieces.
The rudimentary system of quality control peculiar to the mitochondria cellular cycle, much less evolved than that of the eukaryote cell that houses them.
Late in evolution, it seems that we must begin to pay a high price for one of the most productive agreements of our biological history: the feudal contract222 performed by an ancient precursor of ours with a humble energy-producing bacterium, welcomed into its fold and hence resulted protected from external enemies. This protectionism, similar to the political-economical one, halted the bacterium evolution for lack of challenge, leaving this rudimentary power plant deprived of the self-improvement incentives inherent to a regime of open competition.
Mitochondrial DNA mutates ten times more frequently than nuclear DNA. A good part of those mutations is induced by the oxidation resulting from its proximity to the power plant of the respiration chain. Notwithstanding, the presence of hundreds of mitochondria in the cell, and thousands of copies of their genome, considerably dilutes the deleterious effect of mutations. A 15% of normal mitochondria in a cell is estimated enough to fulfill properly their expected function. As the organism ages, however, the threshold begins to be reached and the cells degenerate and die due to insufficient energy production. It has been theorized that this process is at the root of many degenerative diseases, among them cardiovascular ones, late diabetes, arthritis, cataracts, and finally Alzheimer and Parkinson.
Since these illnesses start to show up at the beginning of old age, when mitochondria begin to lose their capacity to produce ATP, it is possible to argue that they are preventable in principle, even curable. A possible strategy should be some kind of technological reinforcement for the mitochondrion capacity to set up a workable defense against the attack of free radicals to their DNA. On the other hand, it has been demonstrated that ROS agents play an essential role in cellular processes, even in memory functions; hence, their total elimination would not only be harmful but even impossible. The body must maintain some sort of equilibrium between their production and the defense against their destructive effects. Taking into account that such equilibrium was created by natural selection, the preponderance of those agents in old age would have to be searched for in other causes. Holistic-inclined authors have proposed, for instance, the hardening of cellular membranes which hinders the free motion of messages maintaining physiological harmony among the different cell conglomerates (Clark, 2001). In support of this position we could add that, just as intercellular signaling pathways223 play a decisive role in the construction of individual organisms, they most probably should be playing it also during its decline.
Finally, if all degenerative diseases turn out to be, as they seem, mitochondrial illnesses indissolubly linked to aging processes, the concept of aging as a non illness, as a simple “general weakening” that opens the door to a host of diseases, should be revised. It would be more appropriate to consider it, along with degenerative diseases, a manifestation of the most radical disease of all, the mother of all diseases: the attack to our genetic material by byproducts of our internal and external energy generators. If such would turn out to be the case, and biological sciences could finally find a way to prevent and cure this mother illness, the quality of human life could improve immensely. In addition, the possibility that non accidental death would always occur without trauma224 would open up, delayed until the supreme terminus: the exhaustion of our telomeres. Unless, of course, telomerase turns out to be, after all, the long searched-for medieval fountain of eternal youth!
Note 178: See Chapter 2, OUR POOR RELATION.
Note 179: See Appendix K: INTERCELLULAR SIGNALS.
Note 180: Protein areas with a specific discernible function.
Note 181: See Chapter 4, ENERGY GENERGY GENERATION IN THE CELL.
Note 182: The acetylcholine neurotransmitter is an exception, with a more complex composition.
Note 183: Small proteins that promote the growth of specific cells. By finding them on their way, axons reinforce their will to move forward (danger: metaphor!).
Note 184: The Greek word “apoptosis” means "fall of leaves," the familiar phenomenon that occurs every year during autumn in temperate-climate regions. The term means the genetically programmed death which affects all cells.
Note 185: See Chapter 8, THE OVERFLOWING OF THE HUMAN BRAIN.
Note 186: Symbiosis consists of a phenomenon of mutual dependence among individuals from two different species. It is called endosymbiosis when one of these two individuals resides inside the other. The most outstanding cases of endosymbiosis are those that resulted from the absorption of the proteobacterium which gave rise to mitochondria in animals and plants, and that of the cyanobacterium which gave origin to chloroplasts in plants.
Note 187: See Appendix M: FREE RADICALS.
Note 188: See ON HOW DEATH SCULPTS LIFE.
Note 189: The double-helix DNA of the human cell, if expanded, would reach one meter and a half in length. If we joined the DNA of all 1014 cells of the organism, we would get a strand eleven amstrongs wide per several million kilometers long. In order to accommodate all this in our body dimensions, four levels of packaging are required. A first level reduces the length eight times, using four protein molecules –called histons– as spools. A second level of packaging, in solenoid shape, reduces the length another eight times, supported by a fifth histon. At a third level the package folds in bundles of irregular frequency, each supported by a small protein scaffolding, producing a reduction of about twenty times. At a final level, still unknown mechanisms reduce the length another twenty times. In the latency state, DNA is strongly coiled around histons. When a gene is activated, its DNA relaxes, allowing transcription and protein production. These coiling and loosening occur thanks to a group of enzymes that modifies the histons’ shape. Similar phenomena occur during the synthesis stage, allowing DNA replication.
Note 190: Each of the four threads making up a chromosome pair during cell division.
Note 191: Two cylindrical structures composed of short microtubules, surrounded by a cloud of undefined material whose function is still unknown. Centriols control microtubules' arrangement in the cellular skeleton. They contain their own DNA and duplicate independently from the cell they inhabit, in a similar way as mitochondria do.
Note 192: Points where the chromatids, identical copies of the genetic material, are linked.
Note 193: Chromatids, linked among them since their creation in the interphase, are destined to split during mitosis in order to migrate toward opposing centriols. The cellular-biology textbook reader is prone to certain confusion derived from the bad habit of many authors of calling "chromosome" both the two linked identical copies at the beginning of mitosis and the separate copies that have already migrated to the cell poles. More careful authors solve the problem speaking, in the first case, of "double chromosomes" and, in the second, of "single chromosomes." The patient reader must take into account that such duality differs from the one represented by the fact that chromosomes (singly or in tandem) exist simultaneously as homologous pairs, corresponding to the contribution of paternal and maternal heredities in the fertilized egg. Enough to mislead the alertest student!
Note 194: In the case of plants, the need for a rigid intercellular wall requires the synthesis of a plaque between the two new cells.
Note 195: The automation of the whole process is insured by certain regulatory proteins –especially cyclins, expressed at each stage of the cycle–, determining which particular substrates must successively be submitted to phosphorylation.
Note 196: The reader knowledgeable in sociobiology (Wilson, 1975) would bet that a unicellular being could be sacrificed for the greater benefit of its genes present as identical copies in a colony of members of their species. Surprisingly, natural selection has not thus far exerted itself on this enterprise, or perhaps researchers have not been, up to now, successful in discovering it. Apoptosis has only been found in multicellular beings, not in cell colonies (Maugras, 1996).
Note 197: See Chapter 5, THE MACHINERY OF DIVERSITY.
Note 198: Of course, a suicide program is inscribed in its genetic material; however, it can only be effective after the germ has jointed with the sexually complementary germinal cell, reaching the character of a somatic cell.
Note 199: Henrietta Lack, an African-American woman who suffered from uterine cancer, was treated with radium in 1951 at the John Hopkins Hospital in Baltimore. Her gynecologist, consulting neither her nor any member of her family, sent a small sample of her cancerous tissue to Dr. George Gey, for research purposes. Henrietta died a few months later; however, a large quantity of copies of her cells lives on today. The family has stated that, had they been consulted, they would have given authorization for the research. Since then, medical ethics has advanced enormously. And an abuse of this kind would be much less likely to happen today (Skloot, 2001).
Note 200: See Appendix E: SENSE AND ANTISENSE IN DNA STRANDS.
Note 201: An enzyme formed by a protein and an RNA sequence. Both components are essential to its activity.
Note 202: The DNA strand to be complemented.
Note 203: Telomerase activity varies in different stages of life. It has been detected in ovaries and testicles of fetuses, newborns and adults, but not in mature ova or spermatozoa. Most embryo somatic tissues, from sixteen to twenty weeks of development, show a high level that disappears after birth. It is also high in adult tissues with an intense cell proliferation, such as the endothelium and endometrium. In other cells it can be induced in certain stages of life, as occurs with T and B lymphocytes. In one of the first works on the subject it was found that, in cultured cells taken from 18 different human tissues, 98 out of each 100 immortal cells and none out of 22 mortal cells showed telomerase activity. Likewise, 90 out of 101 biopsies of 12 types of tumors, and none out of 50 normal somatic tissues, showed activity of the enzyme. More recent studies have confirmed an increased telomerase activity in breast, prostate, astrocitomes and other cancers (Hernandez, 1999).
Note 204: The non eukaryote cell does not present a shortening problem of its single chromosome; due to its circular character, it does not even have a telomere.
Note 205: Up to that date, only preparadigmatic views of cancer existed in oncology, in the terms used by the historian of science Thomas Kuhn (1962). Many unclear ideas concerning the origin and nature of cancer existed, none of them able to convince most scientists and worthy of prevailing as the oncology paradigm; nevertheless, all coincided already in vaguely looking at cancer as a development disease.
Note 206: Note that, in this case, we are dealing with "intra" cellular signals and not with "inter" ones. They are molecular processes similar to those of intercellular signaling, but which occur within the cell.
Note 207: In molecular biology, the sets of participants –proteins and genes– in a process, together with its sequence order and ramifications.
Note 208: See Appendix C: MUTATIONS AND GENETIC DRIFT.
Note 209: See Appendix F: ALLELES.
Note 210: Several of these viruses have been "domesticated" in the course of millennia and are peacefully integrated into our genome.
Note 211: Proteome is the systematized inventory of all the proteins resulting from the expression of a specific genome.
Note 212: See Appendix B: REVERSE ENGINEERING.
Note 213: In accordance with the outdated doctrine of act and potency conceived by Aristotle in ancient times. It teaches that the organism is already present, although in some kind of virtual or recondite form, in the species seeds. It is basically a magical doctrine, in as much as it does not include any constructive mechanism spanning the bridge between what is "potential" and what is "actual." To quote this theory today in support of a scientific doctrine is tantamount to go back more than two millennia in the history of science.
Note 214: Human Fertilization and Embryology Act of 1990. According to this important decision, manipulation of embryos not older than fourteen days is legally allowed. The age limit is important. Before then, the embryo is undefined with respect of its getting to be a single person or two or more people (in the case of identical twins).
Note 215: A key epistemological concept which I owe to Talcott Parsons and have put to significant use ever since my doctoral dissertation (Gutierrez, 1970).
Note 216: By reference to the German legend of Dr. Faust, who made a pact with the devil to regain his youth and win Marguerite's love, a story immortalized by Goethe.
Note 217: See Appendix M: FREE RADICALS.
Note 218: See this chapter, THE DANCE OF TELOMERES WITH TELOMERASE.
Note 219: By some molecular biologist reading this book. Ideas are not copy-writable.
Note 220: These agents are called ROS, for Reactive Oxygen Species, and include oxygenated free radicals and other powerful oxidants, byproducts of the respiratory chain.
Note 221: See Appendix L: MITOCHONDRIA.
Note 222: See Chapter 5, THE CELL'S POWER PLANTS.
Note 223: See Appendix K: INTERCELLULAR SIGNALS.
Note 224: See this chapter, ON HOW DEATH SCULPTS LIFE and THE DANCE OF TELOMERES WITH TELOMERASE.