" Cell cycle "
The sequence of events by which a cell duplicates its genome, synthesis the other constitutents of the cell and eventually divides into two daughter cells.
" Phases of cell cycle "
Interphase :
•G Phase : Cell metabolically active and grows continuously.
• S Phase : DNA synthesis occurs, DNA content increases from 2C to 4C.
1 but the number of chromosomes remains some (2N).
•G Phase : Proteins are synthesised in preparation for mitosis while cell growth continues.
2 M Phase (Mitosis Phase) : Starts with nuclear division, corresponding to separation of daughter chromosomes (karyokinesis) and usually ends with division of cytoplasm (cytokinesis).
Quiescent stage (G 0 ) : Cells that do not divide and exit G phase to enter an inactive stage called G 0 1 . Cells at this stage remain metabolically active but do not proliferate.
MITOSIS Prophase :
(i) Replicated chromosomes, each consisting of 2 chromatids, condense and become visible.
(ii) Microtubules are assembled into mitotic spindle.
(iii) Nucleolus and nuclear envelope disappear.
(iv) Centriole moves to opposite poles.
Metaphase :
(i) Spindle fibres attached to kinetochores (small disc-shaped structures at the surface of centromers) of chromosomes.
(ii) Chromosomes line up at the equator of the spindle to form metaphase plate.
Anaphase :
(i) Centromeres split and chromatids separate.
(ii) Chromatids move to opposite poles.
Telophase :
(i) Chromosomes cluster at opposite poles.
(ii) Nuclear envelope assembles around chromosome cluster.
(iii) Nucleolus, golgi complex, ER reform.
Cytokinesis : Is the divison of protoplast of a cell into two daughter cells after Karyokinesis (nuclear division).
Animal cytokinesis : Appearance of furrow in plasma membrane which deepens and joins in the centre dividing cell cytoplasm into two.
Plant cytokinesis : Formation of new cell wall begins with the formation of a simple precursor − cell plate which represents the middle lamella between the walls of two adjacent cells.
Significance of Mitosis :
1. Growth − addition of cells.
2. Maintenance of surface/volume ratio.
3. Maintenance of chromosome number.
4. Regeneration.
5. Reproduction in unicellular organism.
6. Repair and wound healing.
Meiosis :
• Specialised kind of cell division that reduces the chromosome num- ber by half, resulting in formation of 4 haploid daughter cells.
• Occurs during gametogenesis in plants and animals.
• Involves two sequential cycles of nuclear and cell division called Meiosis I and Meiosis II.
• Interphase occurs prior to meiosis which is similar to interphase of mitosis except the S phase is prolonged.
• 4 haploid daughter cells are formed. Meiosis I Prophase I : Subdivided into 5 phases.
Leptotene :
• Chromosomes make their as single stranded structures.
• Compaction of chromosomes continues.
Zygotene :
• Homologous chromosomes start pairing and this process of associa- tion is called synapsis.
• Chromosomal synapsis is accompanied by formation of synaptone- mal complex.
• Complex formed by a pair of synapsed homologous chromosomes is called bivalent or tetrad.
Pachytene : Crossing over occurs between non-sister chromatids of ho- mologous chromosomes.
Diplotene : Dissolution of synaptonemal complex occurs and the recom- bined chromosomes separate from each other except at the sites of crossing over. These X-shaped structures are called chaismata.
Diakinesis :
• Terminalisation of chaismata.
• Chromosomes are fully condensed and meiotic spindles assembled.
• Nucleolus disappear and nuclear envelope breaks down.
Metaphase I :
• Bivalent chromosomes align on the equatorial plate.
• Microtubules from opposite poles of the spindle attach to the pair of homologous chromosomes.
Anaphase I : Homologous chromosomes separate while chromatids re- main associated at their centromeres.
Telophase I :
• Nuclear membrane and nucleolus reappear.
• Cytokinesis follows (diad of cells).
Interkinesis : Stage between two meiotic divisions. (meiosis I and meiosis II) Meiosis II Prophase II
• Nuclear membrane disappears.
• Chromosomes become compact. Metaphase II
• Chromosomes align at the equator.
• Microtubules from opposite poles of spindle get attached to kineto- chores of sister chromatids. Anaphase II
• Simultaneous splitting of the centromere of each chromosome, al- lowing them to move towards opposite poles of the cell. Telophase II
• Two groups of chromosomes get enclosed by a nuclear envelope.
• Cytokinesis follows resulting in the formation of tetrad of cells i.e., 4 haploid cells. Significance of Meiosis
1. Formation of gametes : In sexually reproducing organisms.
2. Genetic variability
3. Maintenance of chromosomal number : By reducing the chromosome number in gametes. Chromosomal number is restored by fertilisation of gametes.
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