Cell Cycle
* All organisms, even the largest, begin life from a single cell.
* The series of events by which a cell duplicates its genome, synthesises its constituents and eventually divides to form two daughter cells is known as the cell cycle.
* The cell cycle is divided into two basic phases-the interphase and the M phase or the mitosis phase.
* The interphase is the resting phase during which the cell prepares to divide by undergoing cell growth and DNA replication in an orderly manner.
* The interphase is sub-divided into three phases-the G1 phase, S phase and G2 phase.
* In the M phase, actual cell division or mitosis takes place.
Mitosis
* Mitosis results in the parent cell dividing into two daughter cells, which are not only identical to each other but also to the parent cell.
* Prophase is characterised by the formation of compact mitotic chromosomes and the initiation of mitotic spindles.
* Metaphase is characterised y spindle fibres attaching to the kinetochores of the chromosomes and the chromosomes moving to the spindle equator.
* Anaphase is characterised by the splitting of the centromeres and the separation of the chromatids, which then move to opposite poles.
* Telophase is characterised by the chromosomes clustering at opposite spindle poles, decondensation of chromosomes, formation of nuclear envelope and reappearance of the nucleolus, endoplasmic reticulum and golgi complex.
Cytokinesis and the Significance of Mitosis
* Mitosis along with cytokinesis defines the M phase of the cell cycle.
* Mitosis or nuclear division is usually followed by cytokinesis or the division of the cytoplasm.
* In animal cells, cytokinesis is initiated by the formation of a furrow in the plasma membrane, which gradually deepens to join the centre of the cell.
* In plant cells, cytokinesis is initiated by the formation of a cell-plate in the centre of the cell, which gradually grows outward to meet the existing lateral cell walls.
Meiosis-I
* In sexually reproducing organisms, meiosis results in the formation of sperm and egg cells.
* Diploid cells, after meiosis, produce haploid cells, which during fertilisation fuse to form a diploid offspring.
* Meiosis I begins with prophase I, during which crossing over occurs between non-sister chromatids of homologous chromosomes.
* In anaphase I, chromosomes reach the poles, spindle fibres disintegrate and the nuclear envelope and nucleolus reappear.
Meiosis-II
* Meiosis II follows meiosis I, and resembles mitosis as the same number of chromosomes is retained in the daughter cells.
* Prophase II is characterised by chromosomes getting thicker, shorter and distinct, with each chromosome consisting of two sister chromatids.
* Metaphase II is characterised by chromosomes aligning along the equatorial plane and microtubules from opposite poles of the spindle attaching themselves to the kinetochores of sister chromatids.
* Anaphase II is characterised by chromosomes dividing into two and the daughter chromosomes being pulled to the opposite poles of the cell.
* Telophase II is characterised by chromosomes reaching the opposite poles and a new nucleus getting organised at each pole.