CELL CYCLEAND CELL DIVISION 10.1.1 Phases of Cell Cycle A typical eukaryotic cell cycle is illustrated by human cells in culture. These cells divide once in approximately every 24 hours (Figure 10.1). However, this duration of cell cycle can vary from organism to organism and also from cell type to cell type. Yeast for example, can progress through the cell cycle in only about 90 minutes. The cell cycle is divided into two basic phases: • Interphase • M Phase (Mitosis phase) The M Phase represents the phase when the actual cell division or mitosis occurs and the interphase represents the phase between two Figure 10.1 A diagrammatic view of cell cycle successive M phases. It is significant to note indicating formation of two cells that in the 24 hour average duration of cell from one cell cycle of a human cell, cell division proper lasts for only about an hour. The interphase lasts more than 95% of the duration of cell cycle. The M Phase starts with the nuclear division, corresponding to the separation of daughter chromosomes (karyokinesis) and usually ends with division of cytoplasm (cytokinesis).The interphase, though called the resting phase, is the time during which the cell is preparing for division by undergoing both cell growth and DNA replication in an orderly manner. The interphase is divided into three further phases: • G phase (Gap 1) • S phase (Synthesis) 1 How do plants and • G phase (Gap 2) 2 animals continue to G1 phase corresponds to the interval between mitosis and initiation grow all their lives? Do all cells in a plant of DNA replication. During G phase the cell is metabolically active and 1 divide all the time? continuously grows but does not replicate its DNA. S or synthesis phase Do you think all cells marks the period during which DNA synthesis or replication takes place. continue to divide in During this time the amount of DNA per cell doubles. If the initial amount all plants and of DNA is denoted as 2C then it increases to 4C. However, there is no animals? Can you increase in the chromosome number; if the cell had diploid or 2n number tell the name and the location of tissues of chromosomes at G , even after S phase the number of chromosomes 1 having cells that remains the same, i.e., 2n. divide all their life in In animal cells, during the S phase, DNA replication begins in the higher plants? Do nucleus, and the centriole duplicates in the cytoplasm. During the G2 animals have similar phase, proteins are synthesised in preparation for mitosis while cell growth meristematic tissues? continues. CELL CYCLEAND CELL DIVISION Cells at the end of prophase, when viewed under the microscope, do not show golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope. 10.2.2 Metaphase The complete disintegration of the nuclear envelope marks the start of the second phase of mitosis, hence the chromosomes are spread through the cytoplasm of the cell. By this stage, condensation of chromosomes is completed and they can be observed clearly under the microscope. This then, is the stage at which morphology of chromosomes is most easily studied. At this stage, metaphase chromosome is made up of two sister chromatids, which are held together by the centromere (Figure 10.2 b). Small disc-shaped structures at the surface of the centromeres are called kinetochores. These structures serve as the sites of attachment of spindle fibres (formed by the spindle fibres) to the chromosomes that are moved into position at the centre of the cell. Hence, the metaphase is characterised by all the chromosomes coming to lie at the equator with one chromatid of each chromosome connected by its kinetochore to spindle fibres from one pole and its sister chromatid connected by its kinetochore to spindle fibres from the opposite pole (Figure 10.2 b). The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate. The key features of metaphase are: • Spindle fibres attach to kinetochores of chromosomes. • Chromosomes are moved to spindle equator and get aligned along metaphase plate through spindle fibres to both poles. 10.2.3 Anaphase At the onset of anaphase, each chromosome arranged at the metaphase plate is split simultaneously and the two daughter chromatids, now referred to as chromosomes of the future daughter nuclei, begin their migration towards the two opposite poles. As each chromosome moves away from the equatorial plate, the centromere of each chromosome is towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind (Figure 10.2 c). Thus, anaphase stage is characterised by Figure 10.2 a and b : A diagrammatic view of stages in mitosis 166 BIOLOGY Figure 10.2 c to e : A diagrammatic view of stages in Mitosis the following key events: • Centromeres split and chromatids separate. • Chromatids move to opposite poles. 10.2.4 Telophase At the beginning of the final stage of mitosis, i.e., telophase, the chromosomes that have reached their respective poles decondense and lose their individuality. The individual chromosomes can no longer be seen and chromatin material tends to collect in a mass in the two poles (Figure 10.2 d). This is the stage which shows the following key events: • Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements. • Nuclear envelope assembles around the chromosome clusters. • Nucleolus, golgi complex and ER reform. 10.2.5 Cytokinesis Mitosis accomplishes not only the segregation of duplicated chromosomes into daughter nuclei (karyokinesis), but the cell itself is divided into two daughter cells by a separate process called cytokinesis at the end of which cell division is complete (Figure 10.2 e). In an animal cell, this is achieved by the appearance of a furrow in the plasma membrane. The furrow gradually deepens and ultimately joins in the centre dividing the cell cytoplasm into two. Plant cells however, are enclosed by a relatively inextensible cell wall, thererfore they undergo cytokinesis by a different mechanism. In plant cells, wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls. The formation of the new cell wall begins with the formation of a simple precursor, called the cell-plate that represents the middle lamella between the walls of two adjacent cells. At the time of cytoplasmic division, organelles like mitochondria and plastids get distributed between the two daughter cells. In some organisms karyokinesis is not followed by cytokinesis as a result of which multinucleate condition arises leading to the formation of syncytium (e.g., liquid endosperm in coconut). CELL CYCLEAND CELL DIVISION Figure 10.3 Stages of Meiosis I Anaphase I: The homologous chromosomes separate, while sister chromatids remain associated at their centromeres (Figure 10.3). Telophase I: The nuclear membrane and nucleolus reappear, cytokinesis follows and this is called as dyad of cells (Figure 10.3). Although in many cases the chromosomes do undergo some dispersion, they do not reach the extremely extended state of the interphase nucleus. The stage between the two meiotic divisions is called interkinesis and is generally short lived. Interkinesis is followed by prophase II, a much simpler prophase than prophase I. 10.4.2 Meiosis II Prophase II: Meiosis II is initiated immediately after cytokinesis, usually before the chromosomes have fully elongated. In contrast to meiosis I, meiosis II resembles a normal mitosis. The nuclear membrane disappears by the end of prophase II (Figure 10.4). The chromosomes again become compact. Metaphase II: At this stage the chromosomes align at the equator and the microtubules from opposite poles of the spindle get attached to the kinetochores (Figure 10.4) of sister chromatids. Anaphase II: It begins with the simultaneous splitting of the centromere of each chromosome (which was holding the sister chromatids together), allowing them to move toward opposite poles of the cell (Figure 10.4). 170 BIOLOGY Figure 10.4 Stages of Meiosis II Telophase II: Meiosis ends with telophase II, in which the two groups of chromosomes once again get enclosed by a nuclear envelope; cytokinesis follows resulting in the formation of tetrad of cells i.e., four haploid daughter cells (Figure 10.4). 10.5 SIGNIFICANCEOF MEIOSIS Meiosis is the mechanism by which conservation of specific chromosome number of each species is achieved across generations in sexually reproducing organisms, even though the process, per se, paradoxically, results in reduction of chromosome number by half. It also increases the genetic variability in the population of organisms from one generation to the next. Variations are very important for the process of evolution. SUMMARY According to the cell theory, cells arise from preexisting cells. The process by which this occurs is called cell division. Any sexually reproducing organism starts its life cycle from a single-celled zygote. Cell division does not stop with the formation of the mature organism but continues throughout its life cycle.