all organisms even the largest begin life from a single cell the cell then grows by dividing into two wherein each parent cell forms two daughter cells which mature and further divide to form new cells process of cell division is vital to all living organisms in this process cell growth takes place followed by DNA replication you and cell division you however the process must take place in a coordinated manner so that each cell divides correctly and the genomes in the progeny cells remain intact the series of events by which a cell duplicates its genome synthesizes its constituents and eventually divides to form two daughter cells is known as the cell cycle you although the process of cell growth is a continuous one DNA synthesis takes place only during one particular stage in the cell cycle after DNA synthesis the replicated chromosomes or the DNA are distributed to the daughter nuclei through a complex series of genetically controlled events during cell division the duration of a cell cycle varies from one organism to another a typical eukaryotic cell like those in human beings divides once in about every 24 hours whereas in yeast division takes place in about 90 minutes also the cell cycle is divided into two basic phases the interface and the M phase or the mitosis phase the interface is the resting phase during which the cell prepares to divide by undergoing cell growth and DNA replication in an orderly manner the interface is subdivided into three phases the g1 phase or gap 1s phase or synthesis phase and g2 phase or gap 2 during the g1 phase or the first growth phase normal cell functions occur the cell continues to grow and remains metabolically active but the DNA doesn't replicate in the s-phase or synthesis phase the DNA synthesizes or replicates and the amount of DNA in the cell doubles without any increase in the number of chromosomes also the centriole begins to duplicate in the cytoplasm finally in the g2 phase or gap to the cell prepares for mitosis and cell division by synthesizing proteins while the cell continues to grow the next phase of the cell cycle is the M phase we're actual cell division or mitosis takes place the N phase begins with nuclear division which is the division of daughter chromosomes known as mitosis or karyokinesis it ends with a division of the cytoplasm which is known as cytokinesis did you know that in the 24-hour average cell cycle of a human cell proper cell division lasts for only about an hour while more than 95% of the cell cycle duration is the interface in adult animals for example in humans some cells like those in the heart do not appear to display cell division animals also have many cells that divide occasionally only to replace cells lost due to cell death or injury such cells that do not display further cell division exit the g1 phase and enter an inactive stage called the g0 phase or the quiescent stage selves in this stage remain metabolically active but proliferate only when the need arises over in animal cells mitotic cell division occurs only in diploid somatic cells whereas in plants it is seen in both haploid and diploid cells in living organisms the cell cycle is a series of events that leads to cell duplication and division resulting in the growth of organisms mitosis or karyokinesis along with cytokinesis defines the M phase or the mitosis phase of the cell cycle during mitosis parent cell divides into two daughter cells which are not only identical to each other but also to the parent cell also known as equation all division as both the parent and the daughter cells have the same number of chromosomes mitosis is a continuous process however convenience it is divided into four stages of nuclear division namely prophase metaphase anaphase and telophase prophase is the first phase of mitosis and follows the s and the g2 phases of interphase s-phase the sizes or replicates in the g2 phase the cell continues to grow but the DNA molecules formed are intertwined and not distinct during prophase chromatin condensation or the untangling of the chromosomal material takes place and the central that had duplicated during the S phase starts to move toward the opposite poles of the cell certain characteristic events mark the completion of trophies compact mitotic chromosomes are formed due to the condensation of chromosomal material each chromosome is made up of two chromatids attached together at the centromere moreover the centrioles that had moved to it ends of the cell initiate the formation of the mitotic spindles with the help of the microtubules and the proteinaceous constituents of the cell cytoplasm at the end of prophase ulis endoplasmic reticulum bulgy complexes and nuclear envelope are not visible when viewed under a microscope prophase is followed by metaphase the onset of which is marked by the disintegration of the nuclear membrane in the scattering of the chromosomes in the cytoplasm during this stage really seen and studied under the microscope as their condensation is complete moreover chromosome is made up of two sister chromatids bound together by a centromere Mira has a small disc shaped structure on its surface called the kinetochore spindle fibers attach to the kinetochores of the chromosomes and the chromosomes move towards the spindle equator the plane of alignment of the chromosomes during this phase is known as the metaphase plate metaphase is followed by anaphase start of this stage centromere of each chromosome splits simultaneously and the chromatids separate these daughter chromatids are the chromosomes of the future daughter nuclei the chromatids then slowly moved towards opposite poles centromeres of the chromatids face the pole and are at the leading edge while their arms trailed behind telophase this stage is characterized by the chromosomes clustering at opposite spindle poles chromosomes D condense or lose their individuality as discrete elements and a mass of chromatin material tends to collect at the two poles for over a nuclear envelope is formed around each chromosome cluster and the nucleolus Golgi complex and endoplasmic reticulum reappear at both poles mitosis is divided into four stages at the end of which a cell segregates its chromosomes into two identical sets in two daughter nuclei mitosis or karyokinesis along with cytokinesis defines the M phase or the mitosis phase of the cell cycle mitosis results in the segregation of duplicated chromosomes into two daughter nuclei and is followed by the division of cytoplasm that is cytokinesis to produce two daughter cells however cytokinesis is different in animal and plant cells in animals cytokinesis is initiated by the formation of a furrow in the plasma membrane which gradually deepens to join the center of the cell fighting the cytoplasm into two other hand have a relatively in extensible cell wall and so cell division is initiated by the formation of a cell plate in the center of the cell gradually this plate grows outward to meet the existing lateraled cell walls the cell plate represents the middle lamella of the cell wall also during cytokinesis organelles like plastids and mitochondria are equally distributed between the daughter cells did you know that in some organisms karyokinesis is not followed by cytokinesis which results in the formation of a syncytium a cell with multiple nuclei such a syncytium is found in the early embryo of Drosophila and the liquid endosperm of coconut mitosis is one of the most important cellular process also called equational division as the daughter cells inherit the same diploid number of chromosomes as in the mother cell it is generally seen in diploid cells however in certain lower plants like bryophytes and social insects like honeybees mitosis occurs in haploid cells mitosis or mitotic division in somatic cells is responsible for the growth of multicellular organisms in plants meristematic tissues namely the apical and the lateral cambium divide actively by mitosis leading to the continuous growth of the plant throughout its life it also restores the nuclear cytoplasmic ratio which is disturbed as a cell grows moreover mitosis results in the formation of new cells which replaced dead cells for example blood cells and cells in the upper layers of the epidermis in the skin and the lining of the gut are continually replaced thus mitosis is usually accompanied by cytokinesis and plays a vital role in the growth of organisms in sexually reproducing organisms meiosis results in the formation of sperm cells and egg cells cells that undergo meiosis are diploid with two sets of chromosomes however these diploid cells meiosis produce haploid cells with one set of chromosomes during the process of fertilization these haploid cells fuse to form a diploid offspring does maintain the chromosome number in a species meiosis like mitosis is also preceded by an interphase stage during which DNA replicates and each chromosome double-d's to consist of two identical strands of DNA rose's consists of two cell divisions namely meiosis one and two meiosis one is divided into four stages prophase 1 metaphase 1 anaphase 1 and telophase 1 similarly meiosis 2 is divided into four phases prophase 2 metaphase to anaphase to and telophase do meiosis one haves the number of chromosomes and meiosis two splits the sister chromatids of each chromosome resulting in four haploid cells meiosis one begins with prophase one typically more complex and lasts longer compared to the prophase of mitosis based on chromosomal behavior prophase 1 is divided into five phases namely leptin Zeiger teen bakit team dip lateen and dye Kinesis all through the lab to teen stage compaction of chromosomes continues and they gradually become visible under a microscope as the DNA has already duplicated each chromosome consists of two chromatids in the next stage that is the psycho team stage homologous chromosomes the paternal set and one from the maternal set get attracted to each other and form pairs is called synapses during this process a complex structure known as but synapse enamel complex is formed this complex structure consists of a pair of synapsed homologous chromosomes called a tetrad or a bivalent the zygote in stage is followed by the packet in stage during which the bivalent chromosomes are clearly visible as tetrads moreover large protein complexes on the synapse animal complex called recombination nodules begin to appear these nodules are believed to be the sites where crossing over occurs crossing over is an event unique to meiosis we're in an exchange of genetic material occurs between the non sister chromatids of homologous chromosomes the process of crossing over is initiated by the enzyme recombinase and results in the recombination of generic material between the two chromosomes crossing over is very significant during the formation of the meats as it results in new combinations of genes leading to genetic variation in the offspring by the end of the packet in stage recombination between homologous chromosomes is complete and the chromosomes are linked at the sites of crossing over the next stage in prophase one is the de platino stage the beginning of this stage is marked by the dissolution of the synapse animal complex moreover the recombined homologous chromosomes of the by balance display a tendency to separate from each other except at the crossover sites the egg-shaped part on the homologous chromosomes that holds the two together once they have crossed over is called a chiasmata the Diplo teen stage can last for several months or years in our sites of certain vertebrates the final stage and prophase one is Dionysos during which the chiasmata terminates moreover the chromosomes fully condense and become clearly visible while the mitotic spindle assembles and prepares the homologous chromosomes for separation this stage represents the transition to metaphase 1 and ends with the disappearance of the nucleolus and disintegration of the nuclear envelope now in metaphase one the chromosomes line up along the equatorial plate of the cell and the spindle fibers get attached to the kinetochores of the homologous chromosomes metaphase 1 is followed by anaphase 1 in this phase zooms pull apart from each other along the spindle fibers and moved to opposite ends of the cell the chromosomes are still double-stranded with two sister chromatids connected at their centromeres this marks the end of anaphase 1 which is then followed by telophase 1 in this phase the chromosomes are at the poles while the spindle fibers disintegrate and the nuclear envelope and the nucleolus reappear cell division or cytokinesis follows telophase 1 and results in two cells referred to as a dyad therefore Oh sis one which progresses from prophase one to telophase one results in the formation of two cells with half the number of chromosomes compared to the original cell meiosis consists of two cell divisions namely meiosis one and two one is divided into four stages prophase one metaphase 1 anaphase 1 and telophase 1 similarly meiosis 2 is divided into four phases prophase 2 metaphase 2 anaphase two and telophase 2 meiosis one results in two haploid daughter cells due to the separation of the homologous chromosomes in the parent cell roses too follows meiosis one and results in four haploid cells meiosis to resembles mitosis as the same number of chromosomes is retained in the daughter cells between the two mitotic stages there is a short-lived resting stage known as inter Kinesis during which the DNA doesn't replicate inter Kinesis is followed by prophase to a stage that is much simpler than prophase one during this phase the chromosomes get thicker shorter and distinct with each chromosome consisting of two chromatids by the end of this phase the nucleolus disappears and the nuclear envelope disintegrates prophase 2 is followed by metaphase 2 during this phase the chromosomes move towards the equator and aligned themselves along the equatorial plane by polar spindles are formed and the microtubules from the opposite poles of each spindle attach themselves to the kinetic cause of the sister chromatids the next phase of meiosis 2 is anaphase 2 during early anaphase the centromere of each chromosome divides longitudinally into two and splits the chromosome into two daughter chromosomes these daughter chromosomes are then pulled to the opposite poles of the cell during the late anaphase stage the final phase of meiosis 2 is telophase 2 during this phase the chromosomes reach the opposite poles and a new nucleus with a nuclear membrane is organized at each Pole did you know that all four nuclei differ from each other in genetic aspects due to the crossing over in prophase 1 telophase 2 is generally followed by cytokinesis during which the cytoplasm in each cell is equally divided resulting in the formation of four haploid daughter cells meiosis is a significant process in sexually reproducing organisms meiosis in germ cells followed by the fertilization of gametes restores the chromosome number in the offspring of sexually reproducing organisms another significant feature of meiosis is that it leads to variation in offspring due to the crossing over in prophase one during which genetic materials recombine variation plays a very important role in the process of organic evolution meiosis plays a vital role in sexually reproducing organisms the process takes place only in reproductive cells or germ cells and is also known as reduction division as the number of chromosomes is reduced to exactly half in the daughter cells