Cell Cycle and Cell Division (Class 11 Biology) PDF

### Cell Cycle & Cell Division **Cell Cycle** - Sequence by which cells duplicate the genome, synthesize other constituents of cell and divide into two. - Human cell divide once in 24 hours, yeast divides every 30 minutes. **M-phase** * M can be mitosis or meiosis: * **Mitosis** - Equational...

### Cell Cycle & Cell Division **Cell Cycle** - Sequence by which cells duplicate the genome, synthesize other constituents of cell and divide into two. - Human cell divide once in 24 hours, yeast divides every 30 minutes. **M-phase** * M can be mitosis or meiosis: * **Mitosis** - Equational division. Number of chromosomes remains the same. * **Meiosis** - Reduction division. Chromosome number is halved. **Interphase** * Phase between two M-phases. * 95% of time. **Karyokinesis** - Nuclear division. **Cytokinesis** - Cell division. #### **Stages of Mitosis:** - **Prophase** - Chromatin condenses to form chromosomes. - Centromeres start moving to opposite poles. - Mitotic apparatus (Aster + Spindle fibres) appears. - **Metaphase** - Best stage to study morphology of chromosomes. - Centromere fibres (Spindle) attached to centromere of the chromosomes. - **Anaphase** - Centromeres split. - Chromatids separate. - Chromatids move to opposite poles. - **Telophase** - Chromosomes cluster at opposite ends. - Nuclear membrane reappears. #### **Cytokinesis:** * **Animal cells** - Furrow formation. * **Plant cells** - Cell plate formation. * Cell plate represents middle lamellae. **G0 phase:** Quiescent stage. Cell is metabolically active but don’t divide. Ex- RBC, heart cells. **Significance of Mitosis:** - Repair, growth of tissue - Restore nucleo-cytoplasmic ration **Syncytium:** Multinucleate condition. Karyokinesis X Cytokinesis. Example: Liquid endosperm of coconut. ### **Meiosis** - Reduction division - Four phases: #### **Meiosis I** - **Prophase I** - **Leptotene** - Chromosome compaction - **Zygotene** - Synapsis - Synaptonemal complex is formed - Bivalent is formed - **Pachytene** - Crossing over - Bivalent is clearly visible - Recombination nodule appears - **Diplotene** - Dissolution of synaptonemal complex - Chaismata formation - **Diakinesis** - Terminalization of Chaismata - **Metaphase I** - - **Anaphase I** - - **Telophase I** - - Nuclear membrane, nucleolus reappear. - **Interkinesis** - - Stage between M-I & M-II - Crossing over enzyme - recombinase - Exchange of genetic material between non-sister chromatid of homologous chromosome. - Chaismata - X shaped. - In oocyte of some vertebrates, Diplotene can last for months or years. #### **Meiosis II** - Everything is the same as Mitosis. **Significance of Meiosis:** - Conservation of the chromosome number through different generations. - Increases variation.