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What are the phases of mitosis?

Class 11 BiologyNCERT chapter: Cell Cycle and Cell Divisionइसे हिंदी में पढ़ें

Exam-ready answer

Mitosis, the equational division, has four stages of karyokinesis: prophase, metaphase, anaphase and telophase. In prophase chromosomes condense; in metaphase they line up at the metaphase plate; in anaphase sister chromatids separate and move to opposite poles; in telophase nuclear envelopes re-form. Cytokinesis then divides the cytoplasm into two daughter cells.

Explanation

In prophase, the chromosomal material condenses into compact mitotic chromosomes, each made of two chromatids attached at the centromere. The centrosome, which duplicated during the S phase of interphase, moves towards opposite poles, and spindle fibres begin to form. By the end of prophase the Golgi complex, endoplasmic reticulum, nucleolus and nuclear envelope are no longer seen.

Metaphase starts when the nuclear envelope has completely disintegrated. Chromosomes are now most condensed, so their morphology is studied best at this stage. Spindle fibres attach to the kinetochores of the chromosomes, which line up at the equator to form the metaphase plate. In anaphase, each centromere splits and the sister chromatids, now called daughter chromosomes, are pulled to opposite poles.

In telophase, the chromosomes reach the poles, decondense and lose their individuality. A nuclear envelope forms around each cluster, and the nucleolus, Golgi complex and ER reappear. Cytokinesis follows: in an animal cell a furrow in the plasma membrane deepens and splits the cell in two, while a plant cell forms a cell plate from the centre outward. Each daughter cell gets the same chromosome number as the parent.

Points to remember

  1. Mitosis is called equational division because the daughter cells get the same chromosome number as the parent cell.
  2. Karyokinesis has four stages: prophase, metaphase, anaphase and telophase.
  3. Metaphase plate: the plane at the equator where chromosomes align; chromosome morphology is studied best at metaphase.
  4. Anaphase: centromeres split and sister chromatids (daughter chromosomes) move to opposite poles.
  5. Cytokinesis: furrow formation in animal cells; cell plate formation from centre to periphery in plant cells.

Exam tip

Draw labelled diagrams of all four stages in order and write one key event under each; mention the metaphase plate and the splitting of centromeres in anaphase.

Questions students also ask

In which phase are chromosomes studied best?
Chromosomes are studied best at metaphase. At this stage they are most condensed, and each chromosome, with its two sister chromatids held at the centromere, lies clearly at the metaphase plate, so its shape and size are easy to see under a microscope.
How does cytokinesis differ in plant and animal cells?
In animal cells a furrow appears in the plasma membrane and deepens until it meets in the centre, dividing the cytoplasm. Plant cells have a rigid wall, so a cell plate forms in the centre and grows outward to meet the side walls; it later becomes the middle lamella.
What is the significance of mitosis?
Mitosis usually produces diploid daughter cells with identical genetic material. It allows growth of multicellular organisms, restores the nucleo-cytoplasmic ratio and helps in cell repair, such as the constant replacement of cells of the upper layer of the epidermis, the lining of the gut and blood cells.

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