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 here 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 by 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, decondenstaion of chromosomes , formation of nuclear envelope and reappearance of the nucleolus , endoplasmic reticulum and golgi complex.
Cytokinesis and the Significance of Mitosis
* Mitosis along the cytokinesis defines the M phase of the cell cycle.
* Mitosis or nuclear 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 diplod offspring.
* Meiosis I begins with prophase I, during which crossing over occurs between nonsister chromatids of homologous chromosomes.
* In metaphase I, chromosomes line up along the equatorial plate and the spindle fibre get attached to the kinetochores of the homologous chromosomes.
* In anaphase I, chromosome pull apart from each other along the spindle fibres and move to the opposite ends of the cell.
In telophase I, chromatids 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.
