BASICS OF MEDICAL BIOLOGY - 2012

Cell Life Cycle. Cell Division. Temporal Organization of Cells

One of the fundamental tenets of Cell Theory is that Cells do not arise spontaneously, but are formed through the reproduction of pre-existing cells. This principle was formulated by the German pathologist R. Virchow in 1858: "omnis cellula e cellula" (every cell stems from another cell). Cell reproduction drives the growth and renewal of numerous structures in Multicellular Organisms. Furthermore, it underlies the mechanisms of hereditary trait transmission and the flow of biological information at the organismal level. Cell reproduction (proliferation) occurs via division. This process unfolds in two stages: first, The Nucleus divides (karyokinesis), followed by the Cytoplasm (cytokinesis). The primary types of Introduction/5.html">Eukaryotic Cell division are mitosis and Meiosis. Mitosis takes place in all somatic cells, whereas meiosis occurs during The formation of sex cells. In addition to Mitosis and Meiosis, there is amitosis—a direct division of somatic cells where the nucleus constricts without forming a mitotic apparatus. This type of division is believed to occur in malignant tumor cells.

Class="center">The Cell Cycle

The life of a cell encompasses both a life cycle and a cell cycle.

The life cycle is the period spanning from the formation of a cell via the division of a mother cell up to its subsequent division or death. Throughout their lifespan, cells grow, differentiate, and perform specific Functions. The life cycle is longer than The Cell cycle.

The cell (mitotic) cycle comprises the actual preparation for division (interphase) and the division process itself (mitosis). Such periodization (into life and mitotic cycles) is rather conventional, as cellular life is a continuous, indivisible process. Occasionally, the life and cell cycles coincide. This occurs in unicellular organisms, during the Embryonic period in multicellular organisms, and in postembryonic Tissues that undergo constant renewal (such as the epithelium, Bone Marrow, and Connective Tissue). Once cells undergo differentiation and assume specific functions, their life cycle exceeds the duration of the mitotic cycle.

The cell cycle is a complex, evolutionarily developed mechanism of temporal processes that alters the flow of substances, energy, and information within the cell, thereby ensuring cellular function and the continuous reproduction of cell generations.

Interphase is a period of cell growth, differentiation, and execution of specific functions, during which the biosynthetic activity of the cell changes. The analysis of these changes has allowed interphase to be divided into three distinct periods:

1. G₁ phase — (from English "grow") the presynthetic (postmitotic) phase.

2. S phase — (from English "synthesis") the synthetic phase.

3. G₂ phase — the postsynthetic (premitotic) phase.

Fig. 13. The cell cycle.

During the presynthetic G₁ phase, active cell growth takes place, the number of Organelles increases, and Proteins, ATP, and RNA are synthesized alongside cellular differentiation, enabling cells to perform their specific functions. Energy accumulates, organelle numbers increase, and the cell prepares for DNA Synthesis. Evidence shows that Proteins and Enzymes synthesized during this period determine the cell's transition to the subsequent S phase. The cell maintains a diploid chromosome set (2n) and a corresponding amount of DNA (2c). This demonstrates that the progression into the next S phase is governed by a system of specific genes. For instance, in prokaryotes, the transition to the S phase is regulated by more than 30 genes. Chromosomes are single-chromatid structures, and each contains a single DNA molecule.

The synthetic phase is the most critical stage of the cell cycle (its blockade by thymidine or significant radiation-induced DNA damage halts the cell cycle as a whole). The S phase involves DNA synthesis (Replication, doubling of the DNA molecule) and chromosome duplication. The amount of DNA doubles (2c→4c). The chromosome number (2n) remains unchanged, but each chromosome becomes duplicated, meaning it consists of two identical sister chromatids and, consequently, two identical DNA molecules. The quantity of histone proteins associated with the newly synthesized DNA also doubles. The onset of the S phase is known as the restriction point. DNA synthesis is triggered by the appearance of specialized signaling proteins (activator proteins) that degrade at the end of the S phase, allowing the cell to enter the postsynthetic phase. The duration of the S phase depends on The rate of METABOLISM/36.html">DNA replication, which varies across ontogenetic stages. For example, the replication rate during Cleavage takes about 30 minutes, whereas in embryonic cells, it extends to several hours. Specifically, in a 15-day rat embryo, the S phase lasts approximately 7 hours, while in an 18-day embryo, it drops to just 4.5 hours (owing to the recruitment of a larger number of replicons into replication). By the end of the S phase, the genetic constitution is 2n chromosomes and 4c DNA.

In the postsynthetic G₂ phase, DNA synthesis ceases, but the cell actively synthesizes RNA and proteins, including tubulins—the Building Blocks of the mitotic spindle. The cell volume doubles. Fully prepared for division, the cell enters mitosis. The genetic formula of the cell during this period is 2n4c. A key event triggering the transition to mitosis is the Activation of a specific protein kinase, which, upon binding with a cyclin protein, forms a catalytically active complex that phosphorylates numerous cellular proteins required to execute the mitotic process.

Thus, interphase accomplishes the synthesis of all molecular components necessary to drive the mitotic cycle through its completion via division, as well as the synthesis of RNA and proteins required for the G₁ phase of the subsequent cycle.

Modes of Cell Division

Reproduction (self-reproduction) is a fundamental property of living organisms. At THE CELLULAR LEVEL, self-reproduction is driven by cell division processes that underlie asexual reproduction, as well as the GROWTH AND DEVELOPMENT of organisms.

Prokaryotic Cells divide without forming a specialized apparatus through direct binary fission, which nevertheless involves DNA Replication and the precise distribution of DNA molecules to the daughter cells.

Eukaryotic cells divide via Two main mechanisms:

1. Direct division, or amitosis.

2. Indirect division, known as mitosis or karyokinesis.

3. During gametogenesis, a specialized form of indirect division occurs—meiosis—which leads to the formation of haploid cells (n/c).

Mitosis

Mitosis (karyokinesis, indirect division) is the primary type of eukaryotic cell division, resulting in the formation of two daughter cells from a single mother cell, which are genetically identical to the parent. The foundation of mitosis is The process of chromosome coiling and uncoiling. Mitosis was first described in plants in 1874 by the Russian botanist I.D. Chistyakov, and in animals in 1878 by the Ukrainian histologist P.I. Peremizhko. Mitosis takes a relatively short time, approximately 1/10 of the cell cycle duration. Four phases of mitosis are distinguished: prophase, metaphase, anaphase, and telophase, which lack sharp boundaries because mitosis itself is a continuous, gradual process. The duration of the phases varies, with prophase being the longest and anaphase the shortest (fig.).

Prophase. The Structure of the interphase nucleus disappears. At the beginning, thin threads—prophase chromosomes—become visible in The Nucleus as they begin to coil (dense tangle stage). The appearance of chromosomes is the most characteristic sign of the onset of mitosis, from which it derived its name (Greek mitos meaning thread). The chromosome number is 2n and DNA is 4c; prophase chromosomes are double, with their chromatids closely associated. In mid-prophase, it becomes noticeable that each chromosome consists of two coiled chromatids. The chromatids gradually thicken, shorten, and separate from each other (loose tangle stage), and a cleft appears between them.

The second major event of prophase is the Formation of the mitotic apparatus: in animals and lower plants, this occurs with the participation of centrioles, while in higher plants it occurs without them (centrioles are absent). Double centrioles (diplosomes) separate and, by the time the nuclear envelope breaks down, are already located at opposite poles of the cell. A radiating figure (centrosphere) forms around each of them. As the centrioles move apart, spindle fibers arise between them. Kinetochores are identified in the centromeric regions of prophase chromosomes, to which the microtubules of the mitotic spindle attach. Each prophase chromosome has two kinetochores located on its two chromatids. Kinetochores stimulate the polymerization of tubulins and induce the formation of microtubules directed toward the centrioles. Two centrospheres with double centrioles inside, along with the spindle between them, constitute the mitotic apparatus, which ensures the orderly movement of chromosomes in anaphase. All of its structures are built from microtubules through the polymerization of tubulin and other proteins.

Concurrently, nucleoli disappear, the nuclear envelope breaks down into fragments and then into small membrane vesicles, and Protein Synthesis structures change (the granular ER breaks down into cisternae and vacuoles, and the number of Polysomes decreases). Prophase concludes with The breakdown of the nucleus and the mixing of karyoplasm with cytoplasm.

Metaphase begins with the movement of chromosomes toward the cell equator (metakinesis) and typically accounts for about 1/3 of the duration of mitosis. The formation of the mitotic spindle is completed during metaphase, and chromosomes align in the equatorial plane of the spindle. Early metaphase is also called prometaphase; at this time, chromosomes are scattered in the central part of the cell, moving chaotically due to interactions with microtubules, but remaining within the zone of the ruptured nucleus.

Over time, the movement of chromosomes causes them to gather in the central equatorial region of the spindle, forming the metaphase plate. Chromosome movement ceases, their centromeric regions directed toward the center of the spindle, and their arms toward the periphery (this arrangement of chromosomes is typical of animal cells and is known as the "mother star"). Chromosome Condensation reaches its maximum. All chromosomes are clearly visible, which is why karyotyping (analyzing the number, size, and shape of chromosomes) is performed at this stage.

By the end of metaphase, sister chromatids separate within the chromosomes (a cleft is visible between them), and contact between chromatids remains only in the centromeric regions. Chromosomes take on an X- or V-shaped appearance. The genetic formula for this stage is 2n4c.

Anaphase is the shortest, most sharply defined phase of mitosis. The centromeres of the chromosomes simultaneously split, losing their centromeric connections, and the chromatids—now no longer bound to each other (and referred to as daughter chromosomes)—synchronously and "as if on command" move toward opposite poles at a relatively high speed (0.2–5 µm/min). From each chromosome, one chromatid goes to one pole, and the other to the opposite pole. Microtubules of the mitotic spindle drive chromosome movement. Two identical diploid sets of chromosomes (46 each) form at the poles of the cell. In a human cell at this stage, there are 92 single-chromatid chromosomes. The genetic formula of this stage is 4n4c (2n2c + 2n2c). Should the cell enter the next cell cycle, the chromosomes will become two-chromatid. Anaphase is a critical phase of mitosis; non-disjunction of chromosomes leads to the formation of cells with abnormal karyotypes (Mutations).

Telophase, the final phase of mitosis, begins when the two diploid sets of chromosomes stop at the cell poles and concludes with the formation of new interphase nuclei and the division of the mother cell into two daughter cells (cytokinesis). The processes of telophase reverse those of prophase. The STRUCTURE OF THE interphase nucleus is restored; chromosomes uncoil and elongate, becoming indistinguishable as individual structures. A new nuclear envelope begins to form at sites of contact with membrane vesicles. Following the completion of the nuclear envelope, nucleoli begin to form. Spindle microtubules break down and disappear. This marks the completion of mother nucleus division, followed by cytoplasmic division. The mitotic apparatus persists longest in the central equatorial region of the spindle, where the movement of microtubules and cytokinesis microfilaments leads to the formation of a constriction by the nuclei (contractile ring) and the subsequent splitting of the cell in two. Two daughter cells are formed, each with a genetic formula of 2n2c.

Mitosis is a whole-cell division process, meaning all cellular components participate in it. Membrane-bound organelles—the ER and Golgi apparatus—break down into small elements (The Endoplasmic reticulum into vacuoles and small cisternae, the Golgi apparatus into individual dictyosomes) and are pushed to the cell periphery and poles. The same occurs with Mitochondria and Lysosomes. A significant number of small vesicles as well as Ribosomes are found in the spindle zone among the microtubules. During cell division, organelles are passively distributed to the daughter cells.

In mammals, mitosis lasts 1–1.5 hours, G₂ lasts 2–5 hours, S lasts 6–10 hours, and the duration of G₁ varies considerably. The cell cycle also includes the G₀ period, when cells cease proliferation, differentiate, and transition to specialized functions. In some cases, specialized cells undergo irreversible differentiation and lose their capacity to divide (Neurons); in others, cells exit the CELL CYCLE AND differentiate, but under specific conditions can re-enter the cycle. For example, most Liver cells reside in the G₀ period, but if part of the liver is removed, many cells enter the G₁ period and begin dividing.

Mitosis is a complex, evolutionarily established, genetically determined process of Cell Nucleus division (karyokinesis) and cytoplasm division (cytokinesis) that ensures the precise, identical distribution and transmission of hereditary material to new cell generations. Each of the two newly formed daughter cells receives the same number of chromosomes as the mother cell. These chromosomes are exact copies of the parental ones and contain the full Complement of Genetic information characteristic of the mother cell. Daughter cells are genetically identical to the mother cell and to one another. Consequently, it becomes clear why a single plant cell can, under certain conditions, give rise to an entire plant. Mitosis maintains a constant chromosome number in somatic cells and their continuity across cell generations. Through mitosis, cell number increases, driving growth and regeneration, while in many species it serves as a form of asexual reproduction, increasing population numbers (e.g., Protozoa).

The Biological Significance of mitosis can be illustrated by the following diagram:

Amitosis (direct division) is the division of a nucleus through constriction into two relatively equal parts. Unlike mitosis, amitosis preserves the interphase structure of the nucleus and chromosomes, and no achromatic spindle is formed. Amitosis is not a fully effective method of eukaryotic nuclear reproduction because it lacks a precise, uniform distribution of hereditary information.

Fig. 14. Mitotic cycle

1 - interphase; 2 - early prophase; 3,4 - late prophase; 5 - metaphase; 6,7 - early and late anaphase; 8 - telophase.

It can lead to the formation of binucleated cells and is found in virtually all eukaryotes: protozoa, animals, and plants.

Several forms of amitosis are distinguished: uniform—yielding two equal nuclei; non-uniform—yielding unequal nuclei; and fragmentation—where the nucleus breaks down into many small nuclei of uniform or varying size.

Amitotic divisions occur in cells completing their life cycles or undergoing degeneration. For example, amitosis is found in the cells of animal embryonic membranes, ovarian follicular cells, and giant trophoblast cells. Amitotic divisions are also frequently observed during various pathological processes (inflammation, regeneration, malignant growth). In some instances, such as the constantly shedding epithelium of the Urinary Bladder, amitosis ensures rapid restoration of the cellular surface.

Endomitosis is the process of doubling the chromosome number without nuclear division. In this process, chromosomes replicate during interphase, but chromosome autoreplication is not followed by the dissolution of the nuclear envelope. Sometimes the nuclear envelope dissolves, but cell division still does not occur. As a result, the number of chromosomes within the cell increases. Endomitosis is frequently the cause of polyploidy and is found in intensively functioning cells of various tissues, such as liver cells.

Polyteny is the replication of fine subchromosomal structures—chromonemats—whose numbers increase manifold (1,000 or more). Replicated DNA strands do not separate, but remain closely apposed to one another, forming giant or polytene chromosomes. Due to uneven coiling, polytene chromosomes display bands (dark transverse lines) revealed upon staining. The number, size, and pattern of these bands are species-specific. Polyteny is utilized in constructing chromosome maps and identifying chromosomal rearrangements; comparing cytological maps of polytene chromosomes helps determine the taxonomic identity of individuals from different populations and sheds light on microevolution and speciation. Polyteny is observed in certain specialized cells, such as the Salivary Glands of dipterans.

Cell growth and growth factors. Upon completing mitosis, cells enter a growth phase. Cell growth is accompanied by an increase in the volume of the nucleus and cytoplasm. Cells intensively accumulate energy and synthesize proteins to build organelles and Cytoplasmic membranes. Growth is controlled by growth factors. These include: 1) protein growth factors; 2) specific CELLULAR RECEPTORS FOR them; and 3) proteins that regulate the influx of growth factors into cells. Protein growth factors belong to the group of growth Hormones.

Mitotic activity (the rate of cell proliferation) depends on both environmental and internal factors. Various Factors influencing the Organism lead to profound Changes in the cell proliferation regime. Specifically, a daily rhythm of mitotic activity has been established. In nocturnal animals, the peak of mitosis in most Organs occurs in the morning and the minimum at night, whereas in diurnal animals, the maximum is in the evening and the minimum during the day. This is linked to both activity rhythms and fluctuations in external (light, Temperature) and internal environmental factors. Internal factors regulating mitosis include neurohumoral mechanisms mediated by The Nervous system and hormones from the Adrenal Glands, pituitary, thyroid, and Gonads. Tissue breakdown products also exert a stimulatory effect on mitosis, which is most notably observed during regenerative processes.

Studying mitotic activity throughout the daily cycle is of great theoretical and practical significance. Tracking the number of mitotic events over twenty-four hours is crucial for accurately assessing the intensity of an organism's mitotic regime and physiological regeneration. Certain pathological conditions of the organism exhibit localized or generalized alterations in cell division. Knowing the times of maximum and minimum mitotic activity in various tissues throughout the day allows for a proper evaluation of the therapeutic effects of administered drugs. The mitotic activity index of a tissue or tissue culture is the number of cells undergoing mitosis per 1,000 examined cells in a histological specimen. Different cells in our body exhibit varying degrees of mitotic activity; therefore, all tissues are divided into three categories of cell populations:

1) stable cell populations – mitoses are not observed, and the DNA content remains constant. Neurons belong to this category; they persist throughout life, undergoing only age-related changes;

2) growing cell populations – groups of homogeneous cells where individual cells at various stages of mitosis are consistently present. In such populations, cells divide throughout the organism's lifespan, and the formation of new cells drives organ growth (Kidneys, certain glands, Muscles);

3) renewing cell populations – groups of homogeneous cells with a high rate of mitosis that replace dead cells, such as Cells of the digestive tract, Skin epidermis, Testes, and Hematopoietic organs.

Mitotic Abnormalities

Mitotic abnormalities are associated with damage to: 1) the Chromosome structure: various types of chromosomal damage exist, including fragmentation, stickiness, chromosome bridge formation, breaks, and loss of segments; 2) the mitotic apparatus, which is particularly sensitive to external factors such as radiation, chemicals (including medications), alcohol, viral infections, high temperatures, and plant poisons (e.g., colchicine). This leads to mitosis arrest, nondisjunction, and altered chromosome numbers in the karyotype of daughter cells; 3) cytokinesis: its disruption results in the formation of binucleated and multinucleated cells. Disruptions occurring during mitosis give rise to cells with aberrant karyotypes, a phenomenon known as pathological mitosis. Pathological mitoses are frequently observed in carcinogenesis, radiation sickness, and viral infections. Somatic mutations are also linked to mitotic pathologies. If mitosis is disrupted during gametogenesis, germline mutations occur.

Chromosomal Disorders resulting from the loss or gain of extra chromosomes originate from aberrant chromosome segregation. Disruptions in the mitotic process can lead to chromosome bridges, centromere damage, impaired chromosome movement, micronuclei formation, chromosome stickiness or breakage, and more. Significant alterations in the mitotic process are observed in tumor cells, and it is believed that The Emergence of pathological mitoses is one of the causes of malignant cellular transformation.

Cells with an abnormal chromosome number predominate in elderly and senile individuals. For instance, the loss of the Y chromosome is more frequently observed in the bone marrow cells of older adults.

Life of Cells Outside the Organism, Cell Cloning

A population of cells grown under controlled conditions in a nutrient medium outside the living organism (in vitro) is called a cell culture. A population of genetically identical cells originating from a single cell through successive mitotic divisions is termed a cell clone, and the process of producing such clones is known as cloning. A distinction is made between primary and continuous (established) cell lines.

Primary cultures. Cells can be derived from various human tissues and organs (skin, bone marrow, Blood, embryonic tissues). Connective tissue cells (fibroblasts) and peripheral Blood Leukocytes are most commonly used. Cultured cells rapidly multiply via division; however, the number of divisions is limited. For embryonic fibroblasts, this limit is around 50 divisions, after which the culture dies. Cells harvested from adult tissues are capable of fewer divisions. Many scientists attribute this phenomenon—named the Hayflick limit after its discoverer—to cellular Aging.

Continuous (established) cultures are malignant tumor cells capable of an unlimited number of divisions that can be maintained in vitro for years. Since 1951, the HeLa strain—derived from human Cervical Cancer tissue—has been cultivated in numerous laboratories. The strain is named after the initials of the patient from whom the tumor was taken.

Cell cultures are utilized for diagnosing viral diseases, studying the Human Karyotype, diagnosing inherited chromosomal disorders, investigating cellular aging mechanisms, determining the mutagenicity of chemical compounds, and producing pharmaceutical drugs. For example, Influenza, polio, and tick-borne encephalitis Viruses can be propagated in cell cultures to produce prophylactic sera. Plant-derived medicinal products are obtained from plant cell cultures that are difficult to cultivate on plantations (e.g., ginseng). Cell cultures form The basis of the somatic cell Hybridization method used for chromosome mapping.

The cloning potential of sexually reproducing organisms is illustrated by experiments conducted by English embryologist J. Gurdon between 1964 and 1966 using the African clawed frog (*Xenopus laevis*). He transplanted intestinal epithelial cell nuclei from tadpoles into frog eggs whose own nuclei had been inactivated by ultraviolet irradiation, and stimulated them to undergo cleavage. In this way, he successfully produced normal adult frogs. Each offspring organism obtained through this method can be considered a genetic clone of the original donor organism. A clone is defined as an organism or a group of genetically uniform organisms derived from a single common ancestor through Vegetative Reproduction.

Dolly the sheep was born in 1997 through cloning (without the participation of a ram's Gametes). To achieve this, scientists transferred the nucleus of a somatic cell from an adult sheep into an unfertilized egg harvested from another ewe. The resulting embryo was subsequently implanted into the Uterus of a surrogate (non-biological) mother sheep. Dolly lived for 6 years (the average lifespan for this animal species is 12 years). The mechanism used to "create" Dolly can, in principle, be applied to clone other mammals, including humans. The possibility of human cloning has raised A number of moral and ethical issues. Currently, research related to human cloning is legally prohibited in many countries.