Base pairing does not only account for DNA’s duplex structure, but also for its replication. The chemical system that assembles a new DNA chain using an old one as template automatically inserts T in front of A in the template, and vice versa, G and C likewise calling for each other (fig. 5.2).
A very similar base-pairing code rules the replication of RNA, which closely resembles DNA, with the only chemical differences being that deoxyribose is replaced in the common thread by ribose (hence the name of ribo nucleic acid, or RNA) and the base T is replaced by U, a close relative that, like it, pairs with A. In addition, in nature, RNA rarely exists in double-helix form. It is usually made of a single thread, most often folded into a tangle of loops closed by short double-helical joints linking complementary stretches situated at distinct sites of the same thread.
Fig. 5.2. DNA replication. The top half shows part of a DNA double helix, unwound so as to expose the joining of the two complementary strands by base pairing: A joins with T by means of two hydrogen bonds; G joins with C by means of three hydrogen bonds. The bottom half pictures synthesis, guided by the same base-pairing mechanism, of two new complementary strands on the separated strands of the original DNA. (Knowledgeable readers will note that this schematic diagram by the discoverer of DNA replication leaves out the fact, discovered after the picture was drawn, that the two strands are replicated in reverse directions.) After A. Kornberg, in Molecules to Living Cells: Readings from “Scientific American” (New York: W. H. Freeman, 1980), 270–280.
It is very probable, as we have seen in chapter 2, that, in the origin of life, RNA preceded DNA as replicable bearer of information and, therefore, that replication developed first for RNA. Today, this ancestral phenomenon takes place only in cells infected by certain viruses (the polio virus, for example) that possess an RNA genome. Everywhere else, replication concerns DNA. Historically, however, RNA replication was probably the first manifestation of base pairing, inaugurating what may well be the most fundamental process in the whole history of life on Earth.
Indeed, base pairing has turned out to be the dominant mechanism for information transfer throughout the living world, from the origin of life to the present day. It does not just rule DNA and RNA replication, but also the transcription of DNA into RNA and the opposed process of reverse transcription, the synthesis of DNA on an RNA template, which is carried out by certain viruses, for example the causal agent of AIDS. Base pairing also plays a fundamental role in the many interactions between RNA molecules that take place in the translation from RNA into proteins and in many other processes. It is the key mechanism in the universal language of life.
With the appearance of cells, cell division was added to molecular replication in biological reproduction
Reproduction remained molecular until the appearance of the first cells. After that, DNA replication had to be followed by doubling of the cells that contained the DNA, so that each daughter cell would be left with one of the two DNA copies. This doubling first occurred by simple division and, later, in eukaryotic cells, by a much more complex process called mitosis. We won’t go into the details of this process, except to note that it involves rodlike structures, called chromosomes, bearing the DNA molecules that make up the cell’s genome. Each cell division is preceded by duplication of the chromosomes, itself intimately linked with replication of their DNA content.
Multicellular beings reproduce by way of single mother cells
Such mechanisms sufficed as long as organisms remained unicellular. Once the first multicellular organisms appeared, a new reproduction mechanism evolved. Barring some rare exceptions, such as the reproduction of certain plants by budding, all multicellular organisms originate from a single mother cell that, by division and differentiation, gives rise to all the cells of the organism, and is called totipotential for that reason. It might be assumed, a priori, that this mother cell would arise in a parental organism, either from a differentiated cell returning to the totipotential state by “dedifferentiation” (fig. 5.3) or, as proposed by the German biologist August Weismann (1834–1914), from a continuous line of totipotential cells dividing asymmetrically to give rise, on one hand, to a totipotential cell that perpetuates the line, called “germ plasm” by Weismann, and, on the other, to a cell committed toward the formation of differentiated cells and eventually leading to the new organism (fig. 5.4).
Such mechanisms are not involved in the reproduction of organisms, but they play a role in other phenomena of considerable interest. Thus, the Weismann hypothesis accounts for many cases of cell renewal. In the bone marrow, for example, the various blood cells arise from a continuous line of so-called stem cells, which divide asymmetrically to give one daughter cell destined to differentiate further into a red blood cell or one of the various types of white blood cells, while the other daughter cell remains a stem cell. Similar processes take place in most other organs, thereby replacing damaged cells. Even brain cells, which had long been seen as irreplaceable, can be generated by this mechanism. The possible therapeutic use of such “somatic” stem cells (from the Greek soma, body) for tissue repair has evoked enormous interest in recent years, especially because their use does not encounter the same ethical objections as does the use of embryonic cells, which is condemned by a number of religious groups because it involves the destruction of a potential human being.
Fig. 5.3. Hypothetical model of reproduction from a somatic differentiated cell that dedifferentiates into a totipotential cell leading to a new organism. This phenomenon is not involved in the reproduction of organisms, but it is, to a certain extent, in cancerous transformation and, especially, in artificial cloning (see chapter 15).
As to dedifferentiation, it occurs, for example, in the conversion of normal cells into cancer cells, which are thereby almost returned to the status of rapidly dividing embryonic cells. Dedifferentiation has also become a subject of burning interest in relation with artificial cloning techniques. Recently, headlines were made by the announcement that certain differentiated cells can be induced by relatively simple means to return to stem cell status, another potential breakthrough in the production of stem cells for therapeutic purposes. We shall return to these important issues at the end of the book (see chapter 15).
Fig. 5.4. Weismann’s theory. August Weismann postulated a continuous germ line from which successive generations of organisms branch out laterally by asymmetric division. The model does not apply to the reproduction of organisms but accounts for the formation of somatic cells from pluripotential stem cells.
The mother cell of multicellular beings arises from two parental cells by sexual reproduction
The mechanism almost universally used for reproduction by multicellular organisms involves, not one, but two cells. It is sexual reproduction (fig. 5.5). In this process, the mother cell from which a new organism is destined to arise is the product of the fusion of two distinct cells, most often with very different properties. In technical jargon, these cells are called “gametes” or “germ cells,” their properties are distinguished by the terms “male” and “female,” their fusion is known as “fertilization,” and the product of this process is called a “fertilized egg cell.”
Fig. 5.5. Sexual reproduction. This diagram illustrates the maturation, by way of meiosis, of haploid male and female gametes from diploid mother cells, and the formation of a diploid fertilized egg by fertilization of the female oocyte by the male spermatozoon. Note that cytoplasmic organelles, including mitochondria, are eliminated in the course of sperm maturation but are conserved in the course of oocyte maturation. This phenomenon is taken advantage of in the phylogenetic procedure based on the comparative sequencing of mitochondrial DNA (see mitochondrial Eve, chapter 9).
Chromosome doubling caused by sexual reproduction is corrected by meiosis during gamete maturation
One wonders how sexual reproduction can ever have developed, as it implies a phenomenon that, according to every prediction, should have had a lethal effect, namely t
he multiplication of chromosomes, whose number doubles with every generation due to the fusion of two cells. This drawback was eluded by the development of a special kind of mitotic division, called meiosis, in which the double, or diploid, number of chromosomes inherited from the fertilized egg is reduced back to a single, or haploid, set in the course of germ-cell maturation. Thus, when two (haploid) gametes, male and female, join in fertilization, they generate a (diploid) fertilized egg, containing two sets of chromosomes. All the cells of the organism that arise through the development of the egg are likewise diploid, with the exception of the cells destined to become gametes. These cells undergo meiosis in the course of their maturation and become haploid, ready to repeat the cycle. This alternation between haploidy and diploidy is called alternation of generations.
Surprisingly, sexual reproduction, with its attendant passage through meiosis, occurs in the three multicellular lineages, plants, fungi, and animals. Development of such a complicated mechanism independently three times defies plausibility. One is thereby led to look for its origin in protists. Unicellular eukaryotes do indeed sometimes engage in this kind of reproductive fusion, especially under conditions of stress. Even prokaryotes occasionally practice what is known as conjugation, a process in the course of which two such cells exchange genetic material, thus creating new genetic combinations.
Sexual reproduction is the laboratory of evolution
Here probably lies the main advantage of cell fusion. It offers opportunities for testing new combinations of genes, which may be a vital asset when genetic innovation becomes a crucial condition of survival. This is all the more true because it is during meiosis that the process called crossing-over, or recombination, takes place. In this process, pieces are exchanged between chromosomes of the same pair, thereby creating unique combinations of genetic material that were not present before and offering evolution an almost infinite variety of genetic motifs to play with.
Sexual reproduction represents the veritable laboratory of evolution. Thanks to it, innumerable genetic variants have been continually subjected to screening by natural selection (see chapter 7). Genesis of this mechanism no doubt constitutes a key step in the development of multicellular organisms. The complexity of this step perhaps explains why multicellular life, as we know it, was so late in appearing.
Male and female gametes differ
A feature of sexual reproduction common to the vast majority of plants and animals is the participation of two distinct types of germ cells with very different properties and functional roles. The female germ cells, or oocytes, sometimes also called (unfertilized) egg cells, are large and immobile, fitted with a full complement of cytoplasmic structures and crammed with abundant nutrient reserves and other essential substances. The role of the female germ cell is to passively await fertilization and then provide all that will be needed to start development. In contrast, the male cells, called spermatozoa, or sperm cells, are small and motile, reduced to little more than a nucleus devoid of surrounding cytoplasm and propelled by an undulating tail, or flagellum. Their function is to seek a compatible egg cell and penetrate it, or, rather, insert their nucleus into it, which is all that is needed to convert the haploid egg cell into a diploid fertilized egg. A significant consequence of this mechanism is that the mitochondria of the fertilized egg are exclusively derived from the female germ cell (see fig. 5.5). This property is exploited in the analytical method used to trace descent by the female line (see chapter 9, mitochondrial Eve).
Relative to this difference in functions, male gametes are always produced in large numbers, and female gametes in very small numbers. This division of labor is energetically economical, as a female gamete is much costlier to make than a male gamete. This leaves to the sole male gametes the task, favored by their large number, of seeking a female gamete to fertilize. Many specializations of the corresponding organisms are related to the different functions of the gametes they produce (see below: sexual dimorphism).
Plant reproduction involves spores
Reproductive strategies have evolved very differently in plants and animals. In the latter, the haploid stage in the alternation of generations is invariably fleeting and transient, leading almost directly from meiosis to gametes through a short succession of cell divisions (maturation) that takes place in the sex glands of the male and female organisms, whereas the rest of the bodies of each sex consists entirely of diploid cells. In plants, the pathway from meiosis to gametes goes by way of an intermediate, haploid form, called a spore, which undergoes a variable degree of development, sometimes very complex, before giving rise to the gametes (fig. 5.6). This haploid stage may be up to dominant in certain cases, while the diplod stage plays only a brief, transient role.
Fig. 5.6. Alternation of generations in plants. This diagram shows how plant life alternates between haploid and diploid forms, by way of spores, on one hand, and of sexual reproduction, on the other. The relative importance of the two forms varies according to the type of organism. In animals, the haploid phase is reduced to the maturation of the gametes following meiosis.
The primitive seaweeds illustrate this situation in exemplary fashion. In some species, the haploid and diploid stages have the same importance and may even be almost identical in appearance. In others, either one or the other stage is dominant, with the other stage serving only a transitional role. Mosses, the first land-adapted plants, are largely haploid and rely on a brief diploid stage to move from one haploid generation to the other. In the more evolved ferns, the situation is reversed; the main form is diploid, and the haploid stage is only a short interlude occurring underground.
This difference between animals and plants is linked to the very different life-styles of the two types of organisms, which impose different strategies to allow the indispensable encounter between the gametes of the two sexes. Animals take advantage of their mobility to ensure this encounter. The immobile plants, on the other hand, rely mostly on external agents, such as wind or, alternatively, insects or other animals. Hence the need of a transportable form of gametes. Such is the role of spores. This function was greatly facilitated by the acquisition of a resistant, impermeable covering for the spores, allowing them to travel over considerable distances and to remain dormant for considerable times until encountering conditions favorable to germination and subsequent fertilization.
An important development, in both plants and animals, was separation between the sexes. This occurred very early in the animal line by what is known as sexual dimorphism, the division of reproductive functions between two distinct types of mature individuals, males and females, entrusted with the production of spermatozoa and oocytes, respectively, and endowed with appropriate specializations.
In plants, sexual separation was achieved mainly at the spore level. Male spores continued to act in dissemination, mostly in the form of pollen grains, whereas the female spores served to create a static favorable environment in which incoming male spores of the same species would selectively germinate and produce the spermatozoa needed to fertilize the locally produced oocytes. These specializations have reached an extraordinary degree of diversity in the flowering plants. Flowers are veritable traps for catching male spores, developed around the system that produces female gametes and endowed by evolution with myriad features—shapes, colors, scents—that cause our delight but, more relevant to the plants’ reproductive success, proved effective in attracting pollinators. Remarkably, most flowers also contain the male reproductive apparatus, but in a form that hinders local fertilization, thus avoiding inbreeding and the attendant perpetuation of the same genes, which is known to be genetically unfavorable.
Seeds and fruits harbor, until germination, the plant embryos issued from fertilized eggs
In all higher plants, the fertilized egg develops into an immature embryo, which soon becomes arrested in its development and enclosed within a resistant casing, together with a reserve of nutrients that are to be used, upon germination, to support the f
urther development of the embryo up to a state where it can exploit environmental resources on its own. Called seeds, these structures have a simple covering in the gymnosperms (gymnos means naked in Greek), which comprise mostly the pine trees and other conifers. In the angiosperms (from the Greek aggeion, covering), or flowering plants, a group that includes the majority of extant plants, the seeds are situated inside fruits, which are formations derived from the flowers and filled, under aspects of astonishing diversity, with rich nutrient stores, which serve for the nutrition of the embryo and have become, thanks to a fruit production that exceeds by far the requirements of reproduction, an abundant and succulent source of food for the animal world (including humans).
Fungi also reproduce by way of spores
Fungi, like plants, rely mostly on spores for their dispersal. The most spectacular manifestation of fungal reproduction is represented by the multifarious mushrooms, the spore-disseminating structures that suddenly shoot up into the open from hidden mycelia that spread their networks below the ground’s surface.
In animals, parent mobility favors union between spermatozoa and oocytes
Animals, taking advantage of their motility, developed a great diversity of reproductive strategies. As long as the animals kept to their aquatic birthplace, males often did little more than discharge a swarm of spermatozoa in the vicinity of females, leaving it mostly to the swimming ability of the cells and to their large number to ensure successful encounter with a female’s oocytes. Most of the time, females lay their unfertilized oocytes in the same site, so fertilization and subsequent development of the fertilized egg occur in water. Cases are known, however, in which the oocytes are not discharged, but are fertilized and develop inside the female body. Some fish, called viviparous for this reason, produce progeny in this way.
Genetics of Original Sin Page 7
