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CELL CYCLE AND CELL DIVISION:
Cell Cycle:
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a series of events during cell division. These events are themselves under genetic control.
Phases of Cell Cycle:
The cell cycle is divided into two basic phases: (1) Interphase and (2) M-Phase (Mitosis phase).
(1) Interphase:
The interphase is divided into three further phases: (i) G1 phase (Gap 1), (ii) S phase (Synthesis) and (iii) G2 phase (Gap 2).
- G1 phase: During G1 phase the cell is metabolically active and continuously grows but does not replicate its DNA.
- S phase: DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm. The amount of DNA per cell doubles but the number of chromosome remains the same (2n) even after S phase.
- G2 phase: Proteins are synthesised in preparation for mitosis while cell growth continues.
- The interphase lasts more than 95% of the duration of cell cycle.
- The interphase is also called the resting phase.
- During interphase cell is preparing for division by undergoing both cell growth and DNA replication in an orderly manner.
- After the interphase, diploid somatic cells will undergo mitosis while diploid germ cells undergo meiosis.
- Quiescent stage (G0) : stage in which cells (such as nerve and heart muscle cells) don't divide after exiting G1 stage.
(2) M-Phase (Mitosis phase):
M Phase (Mitosis phase):
- In animals, mitosis is only seen in the diploid somatic cells. Plants can show mitotic divisions in both haploid and diploid cells.
- Mitosis is also called as equational division since the number of chromosomes in the parent and progeny cells is the same after the cell division is complete.
- Mitosis is divided into the following four stages: (a) Prophase, (b) Metaphase, (c) Anaphase and (d) Telophase.
- In animals, mitosis is only seen in the diploid somatic cells. Plants can show mitotic divisions in both haploid and diploid cells.
- Mitosis is also called as equational division since the number of chromosomes in the parent and progeny cells is the same after the cell division is complete.
(a) Prophase:
Prophase which is the first stage of mitosis occurs after G2 phases of interphase.
The key features of prophase are:
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(b) Metaphase:
At this stage, metaphase chromosome is made up of two sister chromatids, which are held together by the centromere.
Small disc-shaped structures at the surface of the centromeres are called kinetochores. The key features of metaphase are:
The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate. |
- Cytokinesis:
- Cytokinesis: division of cytoplasm of a cell at the end of mitosis or meiosis leading to two daughter cells.
- Karyokinesis: division of cell nucleus during mitosis.
Animal and plant cell undergo cytokinesis by different mechanism:
- Animal cell: Furrow appears in the plasma membrane. The furrow gradually deepens and ultimately joins in the centre dividing the cell cytoplasm into two.
- Plant cell: In plant cells, wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls. The formation of the new cell wall begins with the formation of a simple precursor, called the cell-plate that represents the middle lamella between the walls of two adjacent cells.
Significance of Mitosis:
- Cell division restores the nucleo-cytoplasmic ratio.
- A very significant contribution of mitosis is cell repair.
- Growth of plants and animal.
Meiosis:
The production of offspring by sexual reproduction includes the fusion of two gametes, each with a complete haploid set of chromosomes.
Gametes are formed from specialised diploid cells. This specialised kind of cell division that reduces the chromosome number by half results in the production of haploid daughter cells. This kind of division is called meiosis.
The key features of meiosis are as follows:
Gametes are formed from specialised diploid cells. This specialised kind of cell division that reduces the chromosome number by half results in the production of haploid daughter cells. This kind of division is called meiosis.
The key features of meiosis are as follows:
- Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but only a single cycle of DNA replication.
- Meiosis 1 is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase.
- Meiosis involves pairing of homologous chromosomes and recombination between them.
- Four haploid cells are formed at the end of meiosis II.
- Meiotic events can be grouped under the following phases:
Meiosis I:
Prophase I:
Prophase I has been further subdivided into the following five phases based on chromosomal behaviour, i.e., Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.
leptotene:
diakinesis:
leptotene:
- The chromosomes start to condense (become dense).
- During this stage chromosomes start pairing together and this process of association is called synapsis. Such paired chromosomes are called homologous chromosomes.
- During this stage bivalent chromosomes now clearly appears as tetrads.
- Crossing over occurs between non-sister chromatids of the homologous chromosomes.
- Crossing over is the exchange of genetic material between two homologous chromosomes.
- Homologous chromosomes start to separate from each other except at the sites of crossovers. These X-shaped structures formed by crossovers are called chiasmata.
diakinesis:
- This is marked by terminalisation of chiasmata.
- Chromosomes are fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation.
Metaphase I:
- Homologous pairs become aligned in the center of the cell.
- The microtubules from the opposite poles of the spindle attach to the pair of homologous chromosomes.
Anaphase I:
- The homologous chromosomes separate, while sister chromatids remain associated at their centromeres.
Telophase I:
- The nuclear membrane and nucleolus reappear, cytokinesis follows.
- Interkinesis: The stage between the two meiotic divisions is called interkinesis.
- Interkinesis is followed by prophase II.
Meiosis II:
Prophase II:
- Meiosis II resembles a normal mitosis.
- The nuclear membrane disappears by the end of prophase II and the chromosomes again become compact.
Metaphase II:
- Chromosomes align at the equator and the microtubules get attached to the kinetochores of sister chromatids.
Anaphase II:
- Centromeres split and the sister chromatids separate.
Telophase II:
- Chromosomes once again get enclosed by a nuclear envelope; cytokinesis follows resulting in the formation of tetrad of cells i.e., four haploid daughter cells
- Meiosis ends with telophase II.
Significance of Meiosis:
- Conserves specific chromosome number of each species across generations in sexually reproducing organisms.
- Increases the genetic variability (important for evolution) in the population of organisms from one generation to the next.