explain cell division full chapter of 11
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Cell cycle
It is a series of events that takes place in a cell, leading to the formation of two daughter cells from a single mother cell.
Cell cycle is divided into two basic phases: Interphase and M phase Phases of cell cycle.
· Interphase
· M phase (mitosis phase) karyokinesis and cytokinesis
Interphase
o G1 phase
o S phase
o G2 phase
o Go phase-quiescent stage
Mitotic phase
· Karyokinesis (nuclear division): – Prophase, Metaphase, Anaphase and Telophase.
· Cytokinesis (division of cytoplasm)
Interphase
Interphase involves a series of changes that prepares the cell for division. It involves the period of cell growth and cell division in an orderly manner.
It is divided into three phases:
G1 phase – It involves growth of cell and preparation of DNA for replication.S phase – It involves DNA synthesis. The amount of DNA doubles, but the chromosome number remains the same.G2 phase – It involves protein synthesis and further growth of cell, which prepares it for division.G0 phase or Quiescent phase – It is the stage when metabolically active cell remains quiescent for long period of time.
I Mitosis
It is a process of cell division where chromosomes replicate and get equally distributed into two daughter cells. Hence, it is also called equational division.The process of mitosis keeps the chromosome number equal in daughter as well as parental cell.Mitosis usually takes place in somatic cells.Mitosis involves four stages:
Prophase
It involves initiation and condensation of chromosomes.Nucleolus and nuclear membrane disappears.Metaphase
Chromosomal material condenses to form compact chromosomes that get aligned in the middle of nucleus at equatorial plate.Anaphase
Centromere splits and chromosomes move apart towards two opposite poles due to shortening of spindle fibres.Telophase
Chromosomes finally reach their respective poles.Nuclear envelope assembles around each chromosome clusters.Nucleolus and other organelles reform.
Karyokinesis and Cytokinesis
Karyokinesis is the division of nucleus during mitosis or meiosis which is followed by cytokinesis.Cytokinesis involves the division of cytoplasm of a cell.Cytokinesis is achieved in animal cell by cleavage, which deepens and divides the cell into two.It is achieved in plant cell by cell plate formation.When karyokinesis is not followed by cytokinesis, a multinucleated condition arises. This is called Syncytium.
Significance of mitosis
Results in formation of diploid genetically identical daughter cellsGrowth of the body takes place by mitosis.Cell repair and replacement of worn out tissuesMaintenance of nucleo-cytoplasmic ratioVegetative reproduction in plants takes place by mitosis.
II Meiosis
It is the process which involves the reduction in the amount of genetic material.It mainly occurs in germ cells.At the end of meiosis II, four haploid cells are formed.It is comprised of two successive nuclear and cell division with a single cycle of DNA replication.The phases of meiosis are as shown below-Meiosis I
1. Prophase I – It comprises of 5 stages:
i. Leptotene
Chromosomes start condensing.ii. Zygotene
Pairing of chromosomes called synapsis occurs.A pair of synapsed homologous chromosomes is called bivalent or tetrad.iii. Pachytene
Exchange of genetic material (crossing over) between non-sister chromatids occurs.Chiasmata formationiv. Diplotene
Bivalents formed during pachytene separate from each other (except at chiasmata) due to dissolution of synaptonemal complex.v. Diakinesis
Terminalisation of chiasmata can be observed.By the end of this stage, the nucleolus disappears and the nuclear envelope breaks.2. Metaphase I
Bivalents (tetrad) get aligned along metaphase plate through spindle fibres.3. Anaphase I
Homologous chromosomes separate while chromatids remain attached at their centromere.4. Telophase
Nucleolus and nuclear membrane reappear around chromosome clusters at each pole.Inter-kinesis – It is the stage between two meiotic divisions.Meiosis II
1. Prophase II
Chromosomes become compact.Nuclear membrane disappears.2. Metaphase II
Chromosomes align at the equator.Kinetochores of sister chromatids attach to spindle fibres at each pole.3. Anaphase II
Chromatids separate by splitting of centromere.As a result, chromatids move towards their respective poles in the cell.4. Telophase II
Nuclear envelope and nucleolus reform around the chromosome clusters.Cytokinesis:
After meiosis II, the process of cytokinesis results in the formation of four haploid cells.Significance of meiosis:
It results in reduction of chromosome number by half in gametes, which again doubles during fertilization. Therefore, it helps to conserve the chromosome number of species from generation to generation.Crossing-over, occurring in pachytene stage of meiosis I, is a source of genetic variation in sexually reproducing organisms.The variation thus formed helps in evolution.
It is a series of events that takes place in a cell, leading to the formation of two daughter cells from a single mother cell.
Cell cycle is divided into two basic phases: Interphase and M phase Phases of cell cycle.
· Interphase
· M phase (mitosis phase) karyokinesis and cytokinesis
Interphase
o G1 phase
o S phase
o G2 phase
o Go phase-quiescent stage
Mitotic phase
· Karyokinesis (nuclear division): – Prophase, Metaphase, Anaphase and Telophase.
· Cytokinesis (division of cytoplasm)
Interphase
Interphase involves a series of changes that prepares the cell for division. It involves the period of cell growth and cell division in an orderly manner.
It is divided into three phases:
G1 phase – It involves growth of cell and preparation of DNA for replication.S phase – It involves DNA synthesis. The amount of DNA doubles, but the chromosome number remains the same.G2 phase – It involves protein synthesis and further growth of cell, which prepares it for division.G0 phase or Quiescent phase – It is the stage when metabolically active cell remains quiescent for long period of time.
I Mitosis
It is a process of cell division where chromosomes replicate and get equally distributed into two daughter cells. Hence, it is also called equational division.The process of mitosis keeps the chromosome number equal in daughter as well as parental cell.Mitosis usually takes place in somatic cells.Mitosis involves four stages:
Prophase
It involves initiation and condensation of chromosomes.Nucleolus and nuclear membrane disappears.Metaphase
Chromosomal material condenses to form compact chromosomes that get aligned in the middle of nucleus at equatorial plate.Anaphase
Centromere splits and chromosomes move apart towards two opposite poles due to shortening of spindle fibres.Telophase
Chromosomes finally reach their respective poles.Nuclear envelope assembles around each chromosome clusters.Nucleolus and other organelles reform.
Karyokinesis and Cytokinesis
Karyokinesis is the division of nucleus during mitosis or meiosis which is followed by cytokinesis.Cytokinesis involves the division of cytoplasm of a cell.Cytokinesis is achieved in animal cell by cleavage, which deepens and divides the cell into two.It is achieved in plant cell by cell plate formation.When karyokinesis is not followed by cytokinesis, a multinucleated condition arises. This is called Syncytium.
Significance of mitosis
Results in formation of diploid genetically identical daughter cellsGrowth of the body takes place by mitosis.Cell repair and replacement of worn out tissuesMaintenance of nucleo-cytoplasmic ratioVegetative reproduction in plants takes place by mitosis.
II Meiosis
It is the process which involves the reduction in the amount of genetic material.It mainly occurs in germ cells.At the end of meiosis II, four haploid cells are formed.It is comprised of two successive nuclear and cell division with a single cycle of DNA replication.The phases of meiosis are as shown below-Meiosis I
1. Prophase I – It comprises of 5 stages:
i. Leptotene
Chromosomes start condensing.ii. Zygotene
Pairing of chromosomes called synapsis occurs.A pair of synapsed homologous chromosomes is called bivalent or tetrad.iii. Pachytene
Exchange of genetic material (crossing over) between non-sister chromatids occurs.Chiasmata formationiv. Diplotene
Bivalents formed during pachytene separate from each other (except at chiasmata) due to dissolution of synaptonemal complex.v. Diakinesis
Terminalisation of chiasmata can be observed.By the end of this stage, the nucleolus disappears and the nuclear envelope breaks.2. Metaphase I
Bivalents (tetrad) get aligned along metaphase plate through spindle fibres.3. Anaphase I
Homologous chromosomes separate while chromatids remain attached at their centromere.4. Telophase
Nucleolus and nuclear membrane reappear around chromosome clusters at each pole.Inter-kinesis – It is the stage between two meiotic divisions.Meiosis II
1. Prophase II
Chromosomes become compact.Nuclear membrane disappears.2. Metaphase II
Chromosomes align at the equator.Kinetochores of sister chromatids attach to spindle fibres at each pole.3. Anaphase II
Chromatids separate by splitting of centromere.As a result, chromatids move towards their respective poles in the cell.4. Telophase II
Nuclear envelope and nucleolus reform around the chromosome clusters.Cytokinesis:
After meiosis II, the process of cytokinesis results in the formation of four haploid cells.Significance of meiosis:
It results in reduction of chromosome number by half in gametes, which again doubles during fertilization. Therefore, it helps to conserve the chromosome number of species from generation to generation.Crossing-over, occurring in pachytene stage of meiosis I, is a source of genetic variation in sexually reproducing organisms.The variation thus formed helps in evolution.
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in significance of mitosis in unicellular organisms, mitosis helps in asexual reproduction also can add
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