Cell Cycle:

• Cell division is a very important process in all living organisms.

• During the division of a cell, DNA replication, and cell growth also take place.

• All these processes, i.e., cell division, DNA replication, and cell growth, hence, have to take place in a coordinated way to ensure the correct division and formation of progeny cells containing intact genomes.

• The sequence of events by which a cell duplicates its genome synthesizes the other constituents of the cell, and eventually, divides into two daughter cells is termed the cell cycle.

• Although cell growth (in terms of cytoplasmic increase) is a continuous process, DNA synthesis occurs only during one specific stage in the cell cycle.

• The replicated chromosomes (DNA) are then distributed to daughter nuclei by a complex series of events during cell division.

• These events are themselves under genetic control.

Phases of Cell Cycle:

• A typical eukaryotic cell cycle is illustrated by human cells in culture.

• These cells divide once approximately every 24 hours.

• However, this duration of the 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:

• It represents the phase between two successive M phases.

  • It is significant to note that in the 24-hour average duration of the cell cycle of a human cell, cell division properly lasts for only about an hour.

• The interphase lasts more than 95% of the duration of the cell cycle.

  • 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:

• G1 Phase:

  • It corresponds to the interval between mitosis and initiation of DNA replication.

  • During the G1 phase, the cell is metabolically active and continuously grows but does not replicate its DNA.

• S Phase:

  • S or synthesis phase marks the period during which DNA synthesis or replication takes place.

  • During this time the amount of DNA per cell doubles. If the initial amount of DNA is denoted as 2C then it increases to 4C.

  • However, there is no increase in the chromosome number; if the cell had a diploid or 2n number of chromosomes at G1, even after the S phase the number of chromosomes remains the same, i.e., 2n.

  • In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm

• G2 Phase:

  • During the G2 phase, proteins are synthesized in preparation for mitosis while cell growth continues.

• G0 Stage:

  • Some cells in adult animals do not appear to exhibit division (e.g., heart cells) and many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death.

  • These cells that do not divide further exit the G1 phase to enter an inactive stage called the quiescent stage (G0 ) of the cell cycle.

  • Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism.

Mitosis Phase:

• The M Phase represents the phase when the actual cell division or mitosis occurs.

• The M Phase starts with the nuclear division, corresponding to the separation of daughter chromosomes (karyokinesis), and usually ends with the division of cytoplasm (cytokinesis).

  • In animals, mitotic cell division is only seen in the diploid somatic cells.

• However, there are a few exceptions to this where haploid cells divide by mitosis, for example, male honey bees.

  • Against this, the plants can show mitotic divisions in both haploid and diploid cells.

• It is the most dramatic period of the cell cycle, involving a major reorganization of virtually all components of the cell.

  • Since the number of chromosomes in the parent and progeny cells is the same, it is also called equational division.

  • Though for convenience mitosis has been divided into four stages of nuclear division (karyokinesis), it is very essential to understand that cell division is a progressive process and very clear-cut lines cannot be drawn between various stages.

• Karyokinesis involves four stages:

  • Prophase

  • Metaphase

  • Anaphase

  • Telophase

Prophase:

• Prophase which is the first stage of karyokinesis of mitosis follows the S and G2 phases of interphase.

  • In the S and G2 phases, the new DNA molecules formed are not distinct but intertwined.

• Prophase is marked by the initiation of condensation of chromosomal material.

  • The chromosomal material becomes untangled during the process of chromatin condensation.

  • The centrosome, which had undergone duplication during the S phase of interphase, now begins to move towards opposite poles of the cell.

• The completion of the prophase can thus be marked by the following characteristic events:

  • Chromosomal material condenses to form compact mitotic chromosomes.

  • Chromosomes are seen to be composed of two chromatids attached together at the centromere.

  • Centrosome which had undergone duplication during interphase begins to move towards opposite poles of the cell.

  • Each centrosome radiates out microtubules called asters.

  • The two asters together with spindle fibers form mitotic apparatus.

• Cells at the end of prophase, when viewed under the microscope, do not show Golgi complexes, endoplasmic reticulum, nucleolus, and the nuclear envelope.

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, the condensation of chromosomes is completed and they can be observed clearly under the microscope.

  • This then is the stage at which the morphology of chromosomes is most easily studied.

  • At this stage, the metaphase chromosome is made up of two sister chromatids, which are held together by the centromere.

    • Small disc-shaped structures at the surface of the centromeres are called kinetochores.

    • These structures serve as the sites of attachment of spindle fibers (formed by the spindle fibers) to the chromosomes that are moved into position at the center of the cell.

  • Hence, the metaphase is characterized by all the chromosomes coming to lie at the equator with one chromatid of each chromosome connected by its kinetochore to spindle fibers from one pole and its sister chromatid connected by its kinetochore to spindle fibers from the opposite pole.

• The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate.

• The key features of metaphase are:

  • Spindle fibers attach to the kinetochores of chromosomes.

  • Chromosomes are moved to the spindle equator and get aligned along the metaphase plate through spindle fibers to both poles**.**

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 daughter 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 remains directed towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind.

• Thus, the anaphase stage is characterized by the following key events:

  • Centromeres split and chromatids separate.

  • Chromatids move to opposite poles.

Telophase:

• At the beginning of the final stage of karyokinesis, 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 each set of chromatin material tends to collect at each of the two poles.

• This is the stage that shows the following key events:

  • Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements.

  • A nuclear envelope develops around the chromosome clusters at each pole forming two daughter nuclei.

  • The nucleolus, Golgi complex, and ER reform.

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 the separation of cytoplasm called cytokinesis at the end of which cell division gets completed.

• 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 center dividing the cell cytoplasm into two.

• Plant cells however are enclosed by a relatively inextensible cell wall, therefore they undergo cytokinesis by a different mechanism.

• In plant cells, wall formation starts in the center 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).

Significance of Mitosis:

• Mitosis or the equational division is usually restricted to the diploid cells only.

• However, in some lower plants and in some social insects haploid cells also divide by mitosis.

• It is very essential to understand the significance of this division in the life of an organism.

• Mitosis usually results in the production of diploid daughter cells with identical genetic complements.

• The growth of multicellular organisms is due to mitosis.

• Cell growth results in disturbing the ratio between the nucleus and the cytoplasm.

  • It, therefore, becomes essential for the cell to divide to restore the nucleo-cytoplasmic ratio.

• A very significant contribution of mitosis is cell repair.

  • The cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are being constantly replaced.

• Mitotic divisions in the meristematic tissues – the apical and the lateral cambium, result in the continuous growth of plants throughout their life.

Meiosis:

• The production of offspring by sexual reproduction includes the fusion of two gametes, each with a complete haploid set of chromosomes.

• Gametes are formed from specialized diploid cells.

• This specialized kind of cell division that reduces the chromosome number by half results in the production of haploid daughter cells.

  • This kind of division is called meiosis.

• Meiosis ensures the production of the haploid phase in the life cycle of sexually reproducing organisms whereas fertilization restores the diploid phase.

• We come across meiosis during gametogenesis in plants and animals.

• This leads to the formation of haploid gametes.

• The key features of meiosis are as follows:

  • Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but only a single cycle of DNA replication.

  • Meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase.

  • Meiosis involves the pairing of homologous chromosomes and recombination between non-sister chromatids of homologous chromosomes.

  • Four haploid cells are formed at the end of meiosis II.

Meiosis I:

Prophase I:

• The Prophase of the first meiotic division is typically longer and more complex when compared to the prophase of mitosis.

• It has been further subdivided into the following five phases based on chromosomal behavior:

• Leptotene:

  • During the leptotene stage, the chromosomes become gradually visible under the light microscope.

  • ^^The compaction of chromosomes continues throughout leptotene. ^^

  • This is followed by the second stage of prophase I

• Zygotene:

  • During this stage, chromosomes start pairing together and this process of association is called synapsis.

    • Such paired chromosomes are called homologous chromosomes.

  • Electron micrographs of this stage indicate that chromosome synapsis is accompanied by the formation of a complex structure called synaptonemal complex.

    • The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad.

    • However, these are more clearly visible in the next stage.

  • The first two stages of prophase I are relatively short-lived compared to the next stage

• Pachytene:

  • During this stage, the four chromatids of each bivalent chromosome become distinct and clearly appear as tetrads.

  • This stage is characterized by the appearance of recombination nodules, the sites at which crossing over occur between non-sister chromatids of the homologous chromosomes.

  • Crossing over is the exchange of genetic material between two homologous chromosomes.

    • Crossing over is also an enzyme-mediated process and the enzyme involved is called recombinase.

    • Crossing over leads to the recombination of genetic material on the two chromosomes.

  • Recombination between homologous chromosomes is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing over.

• Diplotene:

  • The beginning of diplotene is recognized by the dissolution of the synaptonemal complex and the tendency of the recombined homologous chromosomes of the bivalents to separate from each other except at the sites of crossovers.

    • These X-shaped structures are called chiasmata. In oocytes of some vertebrates, diplotene can last for months or years.

• Diakinesis.

  • The final stage of meiotic prophase I is diakinesis.

    • This is marked by the terminalisation of chiasmata.

  • During this phase, the chromosomes are fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation.

  • By the end of diakinesis, the nucleolus disappears and the nuclear envelope also breaks down.

  • Diakinesis represents the transition to metaphase

Metaphase I:

• The bivalent chromosomes align on the equatorial plate.

• The microtubules from the opposite poles of the spindle attach to the kinetochore of homologous chromosomes.

Anaphase I:

• The homologous chromosomes separate, while sister chromatids remain associated at their centromeres.

Telophase I:

• The nuclear membrane and nucleolus reappear, and cytokinesis follows and this is called a dyad of cells.

• 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.

• There is no replication of DNA during interkinesis.

• Interkinesis is followed by prophase II, much simpler prophase than prophase I.

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.

• 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 of sister chromatids.

Anaphase II:

• At this stage, the chromosomes align at the equator and the microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids.

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 the tetrad of cells i.e., four haploid daughter cells.

Significance of Meiosis:

• Meiosis is the mechanism by which conservation of a specific chromosome number of each species is achieved across generations in sexually reproducing organisms, even though the process, per se, paradoxically, results in a 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