Mitosis and Meiosis PDF

CELL CYCLE AND CELL DIVISION  Cell Cycle The sequence of events by which a cell duplicates its genome, synthesizes the other constituents of the cell and then divides into two cells is called as cell cycle. An average duration of cell cycle in human cells is 24 ho...

CELL CYCLE AND CELL DIVISION  Cell Cycle The sequence of events by which a cell duplicates its genome, synthesizes the other constituents of the cell and then divides into two cells is called as cell cycle. An average duration of cell cycle in human cells is 24 hours, while that of yeast is 90 minutes.  Phases of Cell Cycle Two phases – Interphase and M-phase. During M-phase, the cell actually divides. The M-phase starts with nuclear division, i.e. karyokinesis and ends with the division of cytoplasm, i.e. cytokinesis. Interphase is the long, undividing phase between two successive M-phase. It was initially known as resting phase. During interphase, the cell prepares itself for cell division by undergoing both cell growth and DNA replication.  Mitosis A type of cell division that occurs in almost all cells in the body, allowing the body to grow, develop, and repair itself. It is called equational division as chromosomes are equally divided between two identical daughter cells. It starts with the division of nucleus (karyokinesis) followed by the separation of daughter chromosomes and terminates with cytoplasmic division (cytokinesis).  Stages/phases of Mitosis 1. Karyokinesis: Involves the division of the nucleus. Mitosis is further divided into four phases: a. Prophase: Longest phase of division. Chromosomal material condenses to form compact mitotic chromosomes. Chromosomes are seen to be composed of two chromatids attached together at the centromere. Centrioles move to opposite poles of the cell. Nucleolus and nuclear envelope disappear. Microtubules are assembled to form mitotic spindles. b. Metaphase Arrangement of chromosomes at equator and aligns along the metaphase plate through spindle fibres. Attachment of spindle fibres to kinetochores of 2 sisters chromatids chromosomes. The best stage to observe the shape, size and number of chromosomes. c. Anaphase Splitting of centromere. Movement of chromatids towards the opposite poles. Shortest duration phase, i.e. only of 2-3 min and it is very rapid. d. Telophase Chromosomes decondense into long fine filaments. Spindle fibres disappear and centriole replicate. Nuclear envelope reforms around the chromosomes. Reappearance of nucleolus, Golgi complex and ER.  2. Cytokinesis Separation of cytoplasm into each daughter cell is called cytokinesis. Mitosis is accomplished by the division of the parent cell into two daughter cells. It occurs by the appearance of a furrow in plasma membrane. The furrow deepens and joins in the centre thus dividing the cell cytoplasm into two. Significance of Mitosis Helps in growth. Helps in repair and regeneration of body. Helps in maintaining genetic stability as it produce two identical daughter cells.. Helps n asexual reproduction in plants and animals.  Meiosis It is the reductional cell division which results in the production of a haploid set of chromosomes. For example, haploid gametes (n) formed from specialized diploid (2n) cells. Diploid status of gametes is then restored at the time of fertilization. Meiosis involves two sequential cycles of nuclear and cell division called meiosis-I and meiosis-II, but only a single cycle of DNA replication. During meiosis, pairing of homologous chromosomes and recombination between non-sister chromatids occurs, so as to produce four haploid cells.  Meiosis- I Subdivided into four phases, i.e., prophase - I, metaphase - I, anaphase - I and telophase - I. 1. Prophase – I: It is the longest phase, and it is subdivided into the following five phases. a) Leptotene: Condense of chromosomes  b) Zygotene Chromosomes start pairing together and this process of association is called synapsis. Such paired chromosomes are called homologous chromosomes. Chromosome synapsis is accompanied by the formation of complex structure called synaptonemal complex. The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad. c) Pachytene Crossing over occurs between non-sister chromatids of homologous chromosomes. Crossing over leads to recombination of genetic material on the two chromosomes. d) Diplotene Homologous chromosomes separate except at the sites of cross over, so as to form X- shaped structure chiasmata. Synaptonemal complex begins to dissolve and disappears e) Diakinesis Terminalization of chiasmata, chromosomes are fully condensed, nucleolus disappears and nuclear membrane breaks down. 2. During metaphase I, the bivalent chromosomes align on the equatorial plate. 3. In anaphase I, the homologous chromosomes separate, while sister chromatids remain associated at their centromeres. 4. Telophase-I is marked by the appearance of nuclear membrane and nucleolus, cytokinesis follows, and dyad of cells is formed.  The stage between two meiotic divisions DNA replication does is called interkinesis not occur during this and is generally stage. short-lived. Meiosis II It is similar to mitosis as they both involve equational division. The parental and progeny cells involved in meiosis - II are haploid. It also involves four substages, i.e. prophase-II, metaphase-II, anaphase - II and telophase- II, Similar to mitosis except that these processes occur in two haploid cells. At the end of meiosis, tetrad of cells, i.e., four haploid cells, are formed. Significance of Meiosis It is the mechanism of conversion of specific chromosome number of each species in sexually reproducing organisms is achieved across generations. It leads to the formation of gametes which is important for sexual reproduction. It provides a chance for the appearance of new gene combinations, owing to crossing over. It increases the genetic variability in the population of organisms from one generation to the next. Variations help in evolution.