Mitosis and Meiosis Cell Division - Cell Biology | Grade XI

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Mitosis and Meiosis Cell Division - Cell Biology | Grade XI

Mitosis and Meiosis Cell Division

Mitosis cell division:

The process of cell division in which al the chromosomes replicate and separate into two groups having equal number of chromosomes as their parent cell. Thus two nuclei are formed. During the process

mitosis cell divisionInterphase:

The nucleus is metabolically active. This phase is divided into 3 sub-phases:

  • G1 phase: Protein and RNA synthesis occurs, nucleus appears to contain fine delicate threads, and chromosomes are fully extended.
  • S phase: DNA and protein synthesis occurs, chromosomes replicate, synthesis of RNA stops.
  • G2 phase: Synthesis of DNA stops, Enzymes are formed, cell organelles multiply.

Karyokinesis:

Prophase:

  • Chromosomes become slender elongated distinct and separate threads
  • Chromatids lie parallel to each other and are connected by Centro mere
  • Chromosomes become shorter and thicker and Matrix sheath appears around chromatids.
  • Nuclear membrane and nucleolus start to disappear
  • Centrioles appear and start to move towards pole.

Metaphase:

  • Spindle fibres appear and centromere also produced fine fibers called tractile fibers.
  • Spindle fibers connect centromere of each chromosome
  • All the chromosomes arrange at the equatorial plane of the cell.
  • The centromere of each chromosome divides into two centromeres in longitudinal way
  • One centromere of each chromosome is connected with spindle fiber of one pole and another Centro mere is connected with spindle fiber of another pole.

Anaphase:

  • A kind of repulsive force is developed between two centromeres of each chromosome
  • One of the two centromeres having its own chromatids moves towards one pole and another cnetromere move towards another pole.
  • Two groups of chromosomes are formed at each pole due to contraction of spindle fibers

Telophase:

  • Spindle fibers disappear
  • Chromosomes at each pole become elongated long coiled forming chromatin network
  • Nuclear membrane and nucleolus reappears
  • Thus, two nuclei are formed at each pole.

Cytokinesis:

The process of division of cytoplasm is called cytokinesis

a. Cell plate Method:

  • Number of vacuoles and vesicles fused together at the middle of cell between two nuclei forming a plate like structures. The plate like structures are called phragmoplast
  • Cell wall components accumulates at the each side of the cell plate
  • And single cell is divided into two daughter cells
  • This method takes place in plant cell.

b. Cleavage Method:

  • No cell plate is formed in this method.
  • The cytoplasm is constricted directly from the constriction of cell membrane and ultimately the cell is divided into two parts.
  • This method takes place in animal cell.

Significance of Mitosis:

  • It keeps the chromosome number constant and genetic stability in daughter cells, so the linear heredity of an organism is maintained.
  • It helps in growth and development of zygote into adult.
  • It plays a significant role in wound healing, replacement of old and decaying and dead cells and regeneration of lost parts.
  • It helps the cell in maintaining proper size
  • It also helps in asexual reproduction by fragmentation, budding, stem cutting etc.
  • Through the process, equilibrium is maintained in the amount of DNA and RNA

Meiosis Cell Division:

The process of cell division which takes place in a diploid cell and produces four haploid cell is meiosis cell division i.e. Each cell having half the number of chromosomes as that of parent cell.

Meiosis I:

Meiosis IThe meiosis I the division is called reductional division or heterotypic division because the cell divides into two daughter cells having chromosome number half in two daughter cells as that present in the mother cell.

Prophase I: It is very complex and ling phase. It is further divided into five successive sub phases

Leptotene:

  • The nucleus increases in size
  • Chromosomes become slender and separate and distinct threads
  • Chromosomes contain bead like structures called chromomeres.

Zygotene: 

  • The homologous chromosomes make pair
  • The process of making pair by homologous chromosomes is called synapses.
  • Each pair is called bivalent
  • The process of synapses is initiated by a kind of attraction force between homologous chromosomes.
  • The homologous chromosomes can be defined as morphologically and genetically similar one paternal and one maternal chromosome are called homologous chromosomes.

Pachytene:

  • The homologous pair coiled around each other, Chromosomes replicate,
  • Chromosomes become shorter and thicker
  • Each bivalent appears four stranded
Diplotene:
  • Uncoiling of chromosomes takes place from centromere due to a kind of repulsive force
  • Some parts however remain still attached at certain points
  • The attaching points are called chaismata.
  • At chiasmata chromosomal segments are broken and rejoined cross wise i.e. Exchange of maternal and paternal chromosomal segments.
  • This phenomenon is called crossing over
  • Some auther mention the crossing over takes place in pachytene stage.

Diakinesis:

  • Chiasmata move towards the end of chromosome. This process is called terminalization
  • The nuclear membrane starts to disappear
  • Nucleolus also disappear

Metaphase I:

  • Spindle fibers appears
  • Chromosomes arrange at the equator centromere of each bivalent face towards opposite pole.
  • One centromere of each bivalent becomes attached with spindle fibers of one pole and another centromere of each bivalent gets attached with spindle fibers of another pole.

Anaphase I:

  • Each bivalent separate together
  • Separated bivalent move towards opposite poles due to contraction of spindle fiber
  • Two groups of chromosomes are formed at each pole having haploid number of chromosomes

Telophase I:

  • Chromosomes arrange into two groups one at each pole
  • Nuclear membrane and nucleolus reappears
  • Spindle fibers disappear
  • Chromosomes become long elongated to form chromatin

Cytokinesis: It may or may not occurs

Meiosis II:

meiosis iiIt is called equational division or homotypical division because the cells produced after meiosis I undergo further division to produce four cells where chromosome number remains same as that of parent cells, similar to mitosis.

Karyokinesis:

Prophase II:

  • The Chromosome become shorter and thicker, Nuclear membrane and nucleolus disappear. Spindle fibers start to appear

Metaphase II:

  • Chromosomes arranged at the equator
  • The centromere of each chromosome divides into two centromeres in longitudinal way.
  • One centromere of each chromosome is connected with spindle fiber of one pole and another centromere is connected with spindle fiber of another pole.

Anaphase II:

  • A kind of repulsive force is developed between two daughter centromeres
  • One of the two centromeres having its own chromatids move towards one pole and another centromere move towards another pole
  • Two groups of chromosomes are formed at each pole due to contraction of spindle fibers.

Telophase II:

  • Chromosomes organize into nucleus at each pole
  • Chromosomes become long, elongated forming chromatin
  • Nuclear membrane and nucleolus reappears
  • Spindle fibers disappears

Cytokinesis:

  • Successive method: The first cytokinesis takes place after meiosis first to form two daughter cells and second cytokinesis takes place after meiosis II to form four daughter cells. The process is called successive Method.
  • Simultaneous Method: In this method, cytokinesis takes place only after Meiosis II to produce four daughter cells.

Significance of Meiosis cell division:

  • Gametes and spores are formed by meiosis divisions that are essential for sexual reproduction
  • Meiosis helps to maintain fix number of chromosomes in organisms
  • Crossing over causes the genetic variation among the species
  • It helps in alternation of generations of haploid and diploid generations of plants and animals
  • The segregation of paired chromosome resulting in different combination of character daughter cells.

Differences between mitosis and meiosis:

Mitosis

Meiosis

  1. It occurs in somatic cells
  2. Mitosis division completed only in one division

Prophase

  1. It is very short Synapses is absent
  2. No coiling and no replication takes place
  3. No chiasmata formation takes place No crossing over takes place
  4. No Variation occurs

Metaphase

  1. One metaphasic plate is formed Contromere divides longitudinally

Anaphase

  1. Two chromatids of each chromosome separate and move towards opposite pole
  2. Each chromosome is single stranded

Telophase

  1. Nucleus id diploid
  2. Two nuclei are formed
  3. Two cells are produced after cytokinesis
  1. It occurs in reproductive cells
  2. Meiosis division is completed in two successive divisions

Prophase

  1. Prophase is very long Subdivided into 5 sub phases
  2. In Zygotene synapses occurs
  3. In Pahcytene homologous chromosome undergo coiling and replication occurs
  4. In diplotene chiasmata formation and crossing over takes place
  5. Variation occurs

Metaphase

  1. Two metaphase plate is formed
  2. Centromere does not divide in Meiosis I but in II

Anaphase

  1. Homologous chromosomes separate and move towards opposite pole
  2. Each chromosome is double stranded n meiosis I

Telophase

  1. Nuclei are haploid
  2. Four nuclei are formed after Telophase II Four cells are produced after cytokinesis II

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