CELL DIVISION

the process by which cells reproduce. See meiosis; mitosis. Internal membranes The presence of internal membranes distinguishes eukaryotic cells (cells with a nucleus) from prokaryotic cells (those without a nucleus). Prokaryotic cells are small (one to five micrometres in length) and contain only a single plasma membrane; metabolic functions are often confined to different patches of the membrane rather than to areas in the body of the cell. Typical eukaryotic cells, by contrast, are much larger, the plasma membrane constituting only 10 percent or less of the total cellular membrane. Metabolic functions in these cells are carried out in the organelles, compartments sequestered from the cell body, or cytoplasm, by internal membranes. This section discusses internal membranes as structural and functional components in the organelles and vesicles of eukaryotic cells. The principal organellesthe nucleus, mitochondrion, and (in plants) chloroplastare discussed elsewhere (see below The nucleus and The mitochondrion and the chloroplast). Of the remaining organelles, the lysosomes, peroxisomes, and (in plants) glyoxysomes enclose extremely reactive by-products and enzymes. Internal membranes form the mazelike endoplasmic reticulum, where cell membrane proteins and lipids are synthesized, and they also form the stacks of flattened sacs called the Golgi apparatus, which is associated with the transport and modification of lipids, proteins, and carbohydrates. Finally, internal cell membranes can form storage and transport vesicles and the vacuoles of plant cells. Each membrane structure has its own distinct composition of proteins and lipids enabling it to carry out unique functions. General functions and characteristics Like the plasma membrane, membranes of some organelles contain transport proteins, or permeases, that allow chemical communication between organelles. Permeases in the lysosomal membrane, for example, allow amino acids generated inside the lysosome to cross into the cytoplasm, where they can be used for the synthesis of new proteins. Communication between organelles is also achieved by the membrane budding processes of endocytosis and exocytosis, which are essentially the same as in the plasma membrane (see above The plasma membrane: Transport across the membrane). On the other hand, the biosynthetic and degradative processes taking place in different organelles may require conditions greatly different from those of other organelles or of the cytosol (the fluid part of the cell surrounding the organelles). Internal membranes maintain these different conditions by isolating them from one another. For example, the internal space of lysosomes is much more acidic than that of the cytosolpH 5 as opposed to pH 7and is maintained by specific proton-pumping transport proteins in the lysosome membrane. Another function of organelles is to prevent competing enzymatic reactions from interfering with one another. For instance, essential proteins are synthesized on the rough endoplasmic reticulum and in the cytosol, while unwanted proteins are broken down in the lysosomes and also, to some extent, in the cytosol. Similarly, fatty acids are made in the cytosol and then either broken down in the mitochondria for the synthesis of ATP or degraded in the peroxisomes with concomitant generation of heat. These processes must be kept isolated. Organelle membranes also prevent potentially lethal by-products or enzymes from attacking sensitive molecules in other regions of the cell by sequestering such degradative activities in their respective membrane-bounded compartments. The internal membranes of eukaryotic cells differ both structurally and chemically from the plasma membrane. Like the plasma membrane, they are constructed of a phospholipid bilayer into which are embedded, or bound, specific membrane proteins (see above The plasma membrane: Chemical composition and structure of the membrane). The three major lipids forming the plasma membranephospholipids, cholesterol, and glycolipidsare also found in the internal membranes, but in different concentrations. Phospholipid is the primary lipid forming all cellular membranes. Cholesterol, which contributes to the fluidity and stability of all membranes, is found in internal membranes at about 25 percent of the concentration in the plasma membrane. Glycolipids are found only as trace components of internal membranes, whereas they constitute approximately 5 percent of the plasma membrane lipid. The cell matrix and cell-to-cell communication The development of single cells into multicellular organisms involves a number of adaptations. The cells become specialized, acquiring distinct functions that contribute to the survival of the organism. The behaviour of individual cells is also integrated with that of similar cells, so that they act together in a regulated fashion. To achieve this integration, cells assemble into specialized tissues, each tissue being composed of cells and the spaces outside of the cells. The surface of cells is important in coordinating their activities within tissues. Embedded in the plasma membrane of each cell are a number of proteins that interact with the surface or secretions of other cells. These proteins enable cells to recognize and adhere to the extracellular matrix and one another and to form populations distinct from surrounding cells. These interactions are keys to the organizational behaviour of cell populations. Each contributes to the formation of embryonic tissues, and each is necessary for normal tissue function in the adult organism. The extracellular matrix A substantial part of tissues is the space outside of the cells, called the extracellular space. This is filled with a composite material, known as the extracellular matrix, composed of a gel in which are suspended a number of fibrous proteins. The gel consists of large polysaccharide (complex sugar) molecules in a water solution of inorganic salts, nutrients, and waste products known as the interstitial fluid. The major types of protein in the matrix are structural proteins and adhesive proteins. There are two general types of tissue distinct not only in their cellular organization but also in the composition of their extracellular matrix. The first type, mesenchymal tissue, is made up of clusters of cells grouped together but not closely adherent to one another. They synthesize a highly hydrated gel, rich in salts, fluid, and fibres, known as the interstitial matrix. Connective tissue is a mesenchyme that fastens together other more highly organized tissues. The solidity of various connective tissues varies according to the consistency of their extracellular matrix, which in turn depends on the water content of the gels, the amount and type of polysaccharides and structural proteins, and the presence of other salts. For example, bone is rich in calcium phosphate, giving this tissue its rigidity; tendons are mostly fibrous structural proteins, yielding a ropelike consistency; and joint spaces are filled with a lubricating fluid of mostly polysaccharide and interstitial fluid. Epithelial tissues, the second type, are sheets of cells adhering at their side, or lateral, surfaces. They synthesize and deposit at their bottom, or basal, surfaces an organized complex of matrix materials known as the basal lamina or basement membrane. This thin layer serves as a boundary with connective tissue and as a substrate to which epithelial cells are attached. The cytoskeleton The cytoskeleton is the name given to a fibrous network formed by different types of long protein filaments present throughout the cytoplasm of eukaryotic cells (cells containing a nucleus). As the term cytoskeleton implies, these filaments create a scaffold or framework that organizes other cell constituents and maintains the shape of the cell. In addition, some filaments cause coherent movements, both of the cell itself and of its internal organelles. Prokaryotic (non-nucleated) cells, which are much smaller than eukaryotic cells, seem not to require an internal skeleton of this type. Three major types of cytoskeletal filaments are commonly recognized: actin filaments, microtubules, and intermediate filaments. Actin filaments and microtubules are dynamic structures that continuously assemble and disassemble in most cells. Intermediate filaments are stabler and seem to be involved mainly in reinforcing cell structures. A wide variety of accessory proteins work in concert with each type of filament, linking filaments to one another and to the plasma membrane and helping to form the networks that endow the cytoskeleton with its unique functions. Many of these accessory proteins have not yet been characterized, limiting understanding of the cytoskeleton. Actin filaments Actin is a small globular protein molecule that polymerizes to form long filaments. Because each actin subunit faces in the same direction, the actin filament is polar, with different plus and minus ends. An abundant protein in all eukaryotic cells, actin is most easily studied in muscle cells, where it assembles into unusually stable filaments. In muscle the actin filaments are organized into regular arrays, and they interdigitate with a set of thicker filaments formed from a second protein called myosin. These two proteins create the force responsible for muscle contraction. When the signal to contract is sent along a nerve to the muscle, the myosin molecules are activated to hydrolyze adenosine triphosphate (ATP), releasing energy in such a way that a myosin filament moves along an actin filament, causing the two filaments to slide past each other. Large muscles are composed of bundles of many long muscle cells; when the actinmyosin assemblies slide, each of these giant muscle cells shortens, and the overall effect is the contraction of the entire muscle. Actin is also present in non-muscle cells, where it forms less highly ordered arrays of filaments responsible for certain types of cellular movement. Actin is intimately involved in linking the plasma membrane to the underlying cytoplasm. In some cells, these filaments are stabilized by the binding of accessory proteins so as to form microvilli, stable protrusions of the plasma membrane that resemble tiny bristles. Microvilli on the surface of epithelial cells function to increase the cell's surface area, facilitating the absorption of vital molecules through the membrane. Other types of microvilli are involved in the detection of sound in the ear, where their movement, caused by sound waves, sends an electrical signal to the brain. Many actin filaments in non-muscle cells have only a transient existence, polymerizing and depolymerizing in controlled ways that create movement. For example, many cells continually send out and retract tiny microspikes, long, needlelike projections of the plasma membrane that are thought to enable cells to taste their environment. Like microvilli, microspikes are formed when actin filaments push out the membrane, but because these actin filaments are less organized and less stable, microspikes have only a brief existence. Another actin structure only transiently associated with the plasma membrane is the contractile ring, which is composed of actin filaments running around the circumference of the cell during cell division. As its name implies, this ring pulls in the plasma membrane by a myosin-dependent process, thereby pinching the cell in half. How actin is specifically organized into these different transient arrays is unknown, but actin-binding proteins on the plasma membrane probably play a central role. The evolution of cells It is highly unlikely that scientists will ever re-create the crucial experiment that led to the origin of life. Billions of unsuccessful experiments must have been carried out in countless ponds and marshes before life first evolved, and these experiments lasted for hundreds of millions of years. During this period, conditions on Earth were different from those today. There was little oxygen in the atmosphere, and it is thought that the environment was rich in hydrogen, methane, and ammonia. Laboratory experiments show that, under these conditions, lightning charges could have created a large variety of small organic molecules, including the nucleotides, amino acids, and other raw materials of which cells are made. It is thought that life first arose after thick soups of such organic material had accumulated in local regions on Earth. The development of genetic information Life could not exist until a collection of specific catalysts appeared that could promote the synthesis of more catalysts of the same kind. Early stages in the evolutionary pathway presumably centred on RNA molecules, which not only present specific catalytic surfaces but also contain the potential for their own duplication through the formation of a complementary RNA molecule. It is assumed that a small RNA molecule eventually appeared that was able to catalyze its own duplication. Such an autocatalytic RNA molecule would have multiplied faster than its neighbours, usurping the RNA precursor molecules in the primeval soup. Primitive RNA replication would have been imperfect, so that many variant autocatalytic RNA molecules would have arisen. Any variations that increased the speed or the fidelity of self-replication would have enabled those variant RNA molecules to outmultiply their neighbour RNA. Simultaneously, there would have been the natural selection of other small RNA molecules existing in symbiosis with autocatalytic RNA molecules, being replicated in return for catalyzing a useful secondary reaction such as the production of better precursor molecules. In this way, sophisticated families of RNA catalysts would eventually have evolved, in which cooperation between different molecules produced a system that was much more effective at self-replication than a collection of individual RNA catalysts. The next major step in the evolution of the cell would have been the development, in one family of self-replicating RNA, of a primitive mechanism of protein synthesis. Protein molecules cannot provide the information for the synthesis of other protein molecules like themselves. This information must ultimately be derived from a nucleic acid sequence. Protein synthesis is much more complex than RNA synthesis, and it could not have arisen before a group of powerful RNA catalysts evolved. Each of these catalysts presumably has its counterpart among the RNA molecules that function in the current cell: (1) There was an information RNA molecule, much like messenger RNA (mRNA), whose nucleotide sequence was read to create an amino acid sequence; (2) there was a group of adaptor RNA molecules, much like transfer RNA (tRNA), that could bind to both mRNA and a specific activated amino acid; and (3) finally, there was an RNA catalyst, much like ribosomal RNA (rRNA), that facilitated the joining together of the amino acids aligned on the mRNA by the adaptor RNA. At some point in the evolution of biologic catalysts the first cell was formed. This would have required the partitioning of the primitive soup of biologic catalysts into individual units, each surrounded by a membrane. Membrane formation might have occurred quite simply, since many amphiphilic moleculeshalf hydrophobic (water-hating) and half hydrophilic (water-loving)aggregate to form bilayer sheets in which the hydrophobic portions of the molecules line up in rows to form the interior of the sheet and leave the hydrophilic portions to face the water. Such bilayer sheets can spontaneously close up to form the walls of small, spherical vesicles, as do the phospholipid bilayer membranes of present-day cells. As soon as the biologic catalysts became compartmentalized into small individual units, or cells, the units would have begun to compete with one another for the same ingredients in the surrounding soup. Now the development of variant, but efficient, catalysts would have served only the cell itself and its progeny, rather than being dissipated throughout a much larger volume. The active competition that ensued must have greatly speeded up evolutionary change, serving as a powerful force for the development of more efficient cells. In this way, cells eventually arose that contained new catalysts, enabling them to use simpler, more abundant precursor molecules for their growth. Because these cells were no longer dependent on a rich soup of preformed ingredients for their survival, they were able to spread far beyond the limited environments where the first primitive cells arose. It is often assumed that the first cells appeared only after the development of a primitive form of protein synthesis. However, it is by no means certain that cells cannot exist without proteins, and it has been suggested as an alternative that the first cells contained only RNA catalysts. In either case, protein molecules, with their chemically varied side chains, are more powerful catalysts than RNA molecules; therefore, as time passed, cells arose in which RNA served primarily as genetic material, being directly replicated in each generation and inherited by all progeny cells in order to specify proteins. As cells became more complex, a need would have arisen for a stabler form of genetic information storage than that provided by RNA. DNA, related to RNA yet chemically stabler, probably appeared rather late in the evolutionary history of cells. Over a period of time, the genetic information in RNA sequences was transferred to DNA sequences, and the ability of RNA molecules to replicate directly was lost. It was only at this point that the central process of biologythe synthesis, one after the other, of DNA, RNA, and proteinappeared. The history of cell theory Formulation of the theory Early observations The history of cell theory is a history of the actual observation of cells, because early prediction and speculation about the nature of the cell were generally unsuccessful. The decisive event that allowed the observation of cells was the invention of the microscope in the 17th century, after which interest in the invisible world was stimulated. Robert Hooke, who described cork and other plant tissues in 1665, introduced the term cell because the cellulose walls of dead cork cells reminded him of the blocks of cells occupied by monks. Even after the publication in 1672 of excellent pictures of plant tissues, no significance was attached to the contents within the cell walls. The magnifying powers of the microscope and the inadequacy of techniques for preparing cells for observation precluded a study of the intimate details of the cell contents. The inspired amateur of early microscopy Antonie van Leeuwenhoek, beginning in 1673, discovered blood cells, spermatozoa, and a lively world of animalcules. A new world of unicellular organisms was opened up. Such discoveries extended the known variety of living things but did not bring insight into their basic uniformity. Moreover, when Leeuwenhoek observed the swarming of his animalcules but failed to observe their division, he could only reinforce the idea that they arose spontaneously. Cell theory was not formulated for nearly 200 years after the introduction of microscopy. Explanations for this delay range from the poor quality of the microscopes to the persistence of ancient ideas concerning the definition of a fundamental living unit. Many observations of cells were made, but apparently none of the observers was able to assert forcefully that cells were the units of biologic structure and function. Three critical discoveries made during the 1830s, when improved microscopes with suitable lenses, higher powers of magnification without aberration, and more satisfactory illumination became available, were decisive events in the early development of cell theory. First, the nucleus was observed by Robert Brown in 1833 as a constant component of plant cells. Next, nuclei were also observed and recognized as such in some animal cells. Finally, a living substance called protoplasm was recognized within cells, its vitality made evident by its active streaming, or flowing, movements, especially in plant cells. After these three discoveries, cells, previously considered as mere pores in plant tissue, could no longer be thought of as empty, because they contained living material. Two German biologists, Theodore Schwann and Matthias Schleiden, clearly stated in 1839 that cells are the elementary particles of organisms in both plants and animals and recognized that some organisms are unicellular and others multicellular. This statement was made in Schwann's Mikroskopische Untersuchungen ber die bereinstimmung in der Struktur und dem Wachstume der Tiere und Pflanzen (1839; Microscopical Researches into the Accordance in the Structure and Growth of Animals and Plants). Schleiden's contributions on plants were acknowledged by Schwann as the basis for his comparison of animal and plant structure. Schleiden and Schwann's descriptive statements concerning the cellular basis of biologic structure are straightforward and acceptable to modern thought. They recognized the common features of cells to be membrane, nucleus, and cell body and described them in comparisons of various animal and plant tissues. A statement by Schleiden pointed toward the future direction of cell studies: The problem of the origin of cells Schwann and Schleiden were not alone in contributing to this great generalization of natural science, for strong intimations of the cell theory occur in the work of their predecessors. Recognizing that the basic problem was the origin of cells, these early investigators invented a hypothesis of free cell formation, according to which cells developed de novo out of an unformed substance, a cytoblastema, by a sequence of events in which first the nucleolus develops, followed by the nucleus, the cell body, and finally the cell membrane. The best physical model of the generation of formed bodies then available was crystallization, and their theory was inspired by that model. In retrospect, the hypothesis of free cell formation would not seem to have been justified, however, since cell division, a feature not characteristic of crystallization processes, had frequently been observed by earlier microscopists, especially among single-celled organisms. Even though cell division was observed repeatedly in the following decades, the theory of free cell formation lingered throughout most of the 19th century; however, it came to be thought of more and more as a possible exception to the general principle of the reproduction of cells by division. The correct general principle was affirmed in 1855 by a German biologist of great prestige, Rudolph Virchow, who asserted that omnis cellula e cellula (all cells come from cells). Doubt remained, however. The inherently complex events of cell division prevented a quick resolution of the complete sequence of changes that occur during the process. First, it was noted that a cell with a nucleus divides into two cells, each having a nucleus; hence, it was concluded that the nucleus must divide, and direct division of nuclei was duly described by some. Better techniques served to create perplexity, because it was found that, during cell division, the nucleus as such disappears. Moreover, at the time of division, dimly discerned masses, now recognized as chromosomes, were seen to appear temporarily. Observations in the 1870s culminated in the highly accurate description and interpretation of cell division by Walther Flemming in 1882. His advanced techniques of fixing and staining cells enabled him to see that cell reproduction involves the transmission of chromosomes from the parent to daughter cells by the process of mitosis and that the division of the cell body is the terminal event of that reproduction. The discovery that the number of chromosomes remains constant from one generation to the next resulted in the full description of the process of meiosis. The description of meiosis, combined with the observation that fertilization is fundamentally the union of maternal and paternal sets of chromosomes, culminated in the understanding of the physical basis of reproduction and heredity. Meiosis and fertilization therefore came to be understood as the complementary events in the life cycle of organisms: meiosis halves the number of chromosomes in the formation of spores (plants) or gametes (animals), while fertilization restores the number through the union of gametes. By the 1890s life in all of its manifestations could be thought of as an expression of cells. The mitochondrion and the chloroplast Mitochondria and chloroplasts are the powerhouses of the cell. Mitochondria appear in both plant and animal cells as elongated, cylindrical bodies, roughly one micrometre in length and closely packed in regions actively using metabolic energy. Oxidizing the products of cytoplasmic metabolism, they convert the energy so liberated into adenosine triphosphate (ATP), the energy currency of the cell. Chloroplasts are the photosynthetic organelles in plant and plantlike bacterial cells. They trap light energy and convert it partly into ATP but mainly into certain chemically reduced molecules that, together with ATP, are used in the first steps of carbohydrate production. Mitochondria and chloroplasts share a certain structural resemblance, and both have a somewhat independent existence within the cell, synthesizing some proteins and dividing according to their own genetic instructions. Mitochondrial and chloroplastic structure Both organelles are bounded by an external porous membrane, which allows passage of small metabolites but which is an effective barrier, in general, to the movement of large cytoplasmic proteins. An inner membrane, concentric to the outer, is impermeable even to small ions, such as protons. The membranes of both organelles have a lipid bilayer construction, which has been shown to be a selective barrier against the passage of charged, water-soluble molecules and ions (see above The plasma membrane: Chemical composition and structure of the membrane). Between the inner and outer layers is the intermembrane space. In mitochondria the inner membrane is thrown into a vast number of folds called cristae, enormously expanding the surface area of the membrane. In contrast, the inner membrane of chloroplasts is relatively smooth, but within this membrane is yet another series of membranes, also enormously folded, that form a set of flattened, disklike sacs called the thylakoids. The organellar space enclosed by the inner membrane is called the matrix in mitochondria and the stroma in chloroplasts. Both spaces are filled with a fluid containing a rich mixture of metabolic products, enzymes, and ions. Enclosed by the thylakoid membrane of the chloroplast is the thylakoid space. The extraordinary chemical capabilities of the two organelles lie in the cristae and the thylakoids. Both membranes are studded with enzymatic proteins either traversing the bilayer or dissolved in the lipid. These proteins contribute to the production of energy by transporting material across the membranes and by serving as electron carriers in the important oxidationreduction reaction. The nucleus The nucleus is the information centre of the cell in all higher organisms. It is separated from the cytoplasm by the nuclear envelope, and it houses the double-stranded, spiral-shaped deoxyribonucleic acid (DNA) molecules, which contain the genetic information necessary for the cell to retain its unique character as it grows and divides. The presence of a nucleus distinguishes the eukaryotic cells of multicellular organisms from the prokaryotic, one-celled organisms, such as bacteria. In contrast to the higher organisms, bacteria do not have nuclei, so that their DNA is maintained in the same compartment as the other cellular components. The primary function of the nucleus is the expression of selected subsets of the genetic information encoded in the DNA double helix. Each subset of a DNA chain, called a gene, codes for the construction of a specific protein out of a chain of amino acids. Information in DNA is not decoded directly into proteins, however. First it is transcribed, or copied, into a range of messenger ribonucleic acid (mRNA) molecules, each of which encodes the information for one protein (or more than one protein in bacteria). The mRNA molecules are then transported through the nuclear envelope into the cytoplasm, where they are translated, serving as templates for the synthesis of specific proteins. The nucleus must not only synthesize the mRNA for many thousands of proteins, but it must also regulate the amounts synthesized and supplied to the cytoplasm. Furthermore, the amounts of each type of mRNA supplied to the cytoplasm must be regulated differently in each type of cell. In addition to mRNA, the nucleus synthesizes and exports other classes of RNA involved in the mechanisms of protein synthesis. Structural organization of the nucleus DNA packaging The nucleus of the average human cell is only six micrometres (6 10-6 metre) in diameter, yet it contains about 1.8 metres of DNA. This is distributed among 46 chromosomes, each consisting of a single DNA molecule about 40 millimetres (1 1/2 inches) long. The extraordinary packaging problem this poses can be envisaged by a scale model enlarged a million times. On this scale a DNA molecule would be a thin string two millimetres thick, and the average chromosome would contain 40 kilometres (25 miles) of DNA. With a diameter of only six metres, the nucleus would contain 1,800 kilometres of DNA. These contents must be organized in such a way that they can be copied into RNA accurately and selectively. DNA is not simply crammed or wound into the nucleus like a ball of string; rather, it is organized, by molecular interaction with specific nuclear proteins, into a precisely packaged structure. This combination of DNA with proteins creates a dense, compact fibre called chromatin. An extreme example of the ordered folding and compaction that chromatin can undergo is seen during cell division, when the chromatin of each chromosome condenses and is divided between two daughter cells (see below Cell division and growth). The plasma membrane A thin membrane, some .005 micrometre across, surrounds every living cell, delimiting the cell from the environment around it. Enclosed by this plasma membrane are the cell's constituents, often large, water-soluble, highly charged molecules such as proteins, nucleic acids, carbohydrates, and substances involved in cellular metabolism. Outside the cell, in the surrounding water-based environment, are ions, acids, and alkalis that are toxic to the cell, as well as nutrients that the cell must absorb in order to live and grow. The plasma membrane, therefore, has two functions: first, to be a barrier keeping the constituents of the cell in and unwanted substances out; and second, to be a gate allowing transport into the cell of essential nutrients and movement from the cell of waste products. Chemical composition and structure of the membrane Most current knowledge about the biochemical constituents of plasma membranes originates in studies of red blood cells. The chief advantage of these cells for experimental purposes is that they may be obtained easily in large amounts and that they have no typical membrane structure, other than the plasma membrane itself, to interfere with study of that structure. Careful studies of these and other cell types have shown that all plasma membranes are composed of proteins and fatty-acid-based lipids. Cell membranes actively involved in metabolism contain a higher proportion of protein; thus, the membrane of the mitochondrion, the most rapidly metabolizing organelle of the cell, contains as much as 75 percent protein, while the membrane of the Schwann cell, which forms an insulating sheath around many nerve cells, has as little as 20 percent protein.