Phases of Mitosis — Revision Notes

⚡ 30-Second Revision

Interphase: — G1 (growth), S (DNA replication, $2C \to 4C$), G2 (growth, preparation).
Prophase: — Chromatin condenses, chromosomes visible (2 chromatids each). Nuclear envelope & nucleolus disappear. Spindle forms.
Metaphase: — Chromosomes align at metaphase plate. Spindle fibers attach to kinetochores.
Anaphase: — Sister chromatids separate, move to opposite poles (now individual chromosomes). Chromosome number temporarily doubles ($2n \to 4n$).
Telophase: — Chromosomes decondense. Nuclear envelope & nucleolus reform. Spindle disappears.
Cytokinesis: — Cytoplasm divides. Animal: cleavage furrow. Plant: cell plate.
Outcome: — Two genetically identical daughter cells, each $2n$ chromosomes, $2C$ DNA.

2-Minute Revision

Mitosis is the process of nuclear division (karyokinesis) followed by cytoplasmic division (cytokinesis), resulting in two genetically identical daughter cells. It's preceded by Interphase, where DNA replicates, making each chromosome consist of two sister chromatids.

Prophase initiates mitosis: chromatin condenses into visible chromosomes, the nuclear envelope and nucleolus disappear, and the mitotic spindle begins to form from centrosomes moving to poles.

Metaphase is characterized by the precise alignment of all condensed chromosomes at the cell's equatorial plane, known as the metaphase plate. Spindle fibers are fully formed and attached to the kinetochores of each sister chromatid.

Anaphase is the crucial separation stage: sister chromatids detach from each other and are pulled towards opposite poles of the cell by shortening spindle fibers. Each separated chromatid is now considered an individual chromosome, temporarily doubling the chromosome count.

Telophase reverses prophase events: chromosomes decondense, new nuclear envelopes form around the two sets of chromosomes at the poles, and the nucleolus reappears. The spindle apparatus disassembles.

Finally, Cytokinesis divides the cytoplasm. In animal cells, a cleavage furrow pinches the cell in two. In plant cells, a cell plate forms in the middle, growing outwards to create a new cell wall. This completes the formation of two identical daughter cells.

5-Minute Revision

Mitosis is the process of somatic cell division, ensuring the production of two genetically identical daughter cells from a single parent cell. This process is vital for growth, tissue repair, and asexual reproduction. It is part of the M-phase of the cell cycle, following Interphase (G1, S, G2 phases) where DNA replication occurs, resulting in chromosomes composed of two sister chromatids.

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Prophase: — This is the longest phase of mitosis. The diffuse chromatin fibers condense extensively, becoming visible as distinct, compact chromosomes under a light microscope. Each chromosome is clearly seen to consist of two sister chromatids joined at the centromere. Concurrently, the nuclear envelope and nucleolus begin to disappear. In animal cells, the centrosomes migrate to opposite poles, initiating the formation of the mitotic spindle apparatus.

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Metaphase: — This phase is defined by the precise arrangement of all chromosomes at the metaphase plate (equatorial plate), an imaginary plane equidistant from the two spindle poles. The spindle fibers (kinetochore microtubules) are fully formed and firmly attached to the kinetochores located at the centromere of each sister chromatid. This alignment is critical for ensuring equal distribution of genetic material.

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Anaphase: — The shortest and most dynamic phase. The cohesin proteins holding sister chromatids together are cleaved, allowing the sister chromatids to separate abruptly. Each separated chromatid is now considered an individual chromosome. These newly formed chromosomes are then pulled towards opposite poles of the cell by the shortening kinetochore microtubules, while polar microtubules elongate, pushing the poles further apart. The centromere leads the movement, giving chromosomes a V or J shape.

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Telophase: — This phase essentially reverses the events of prophase. Once the chromosomes reach their respective poles, they begin to decondense, returning to their extended chromatin state. New nuclear envelopes form around each set of chromosomes at the poles, and the nucleolus reappears within each nascent nucleus. The mitotic spindle disassembles.

Cytokinesis: Overlapping with anaphase and telophase, cytokinesis is the division of the cytoplasm. In animal cells, a contractile ring of actin and myosin filaments forms a cleavage furrow that pinches the cell into two.

In plant cells, Golgi-derived vesicles fuse at the equatorial plane to form a cell plate, which grows outwards to become a new cell wall, dividing the cell. The final outcome is two daughter cells, each diploid (2n) and genetically identical to the parent cell in its G1 state.

Prelims Revision Notes

Mitosis is somatic cell division, producing two genetically identical diploid (2n) daughter cells. It's part of the M-phase, preceded by Interphase (G1, S, G2).

Interphase:

G1: — Cell growth, normal metabolic activity. Chromosomes are unduplicated (1 chromatid each). DNA content = 2C.
S: — DNA synthesis/replication. Each chromosome duplicates, forming two sister chromatids. Chromosome number remains 2n, but DNA content becomes 4C.
G2: — Cell growth, organelle duplication, preparation for mitosis.

Karyokinesis (Nuclear Division):

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Prophase:

* Chromatin condenses into visible chromosomes (each with 2 sister chromatids). * Nuclear envelope disintegrates. * Nucleolus disappears. * Centrosomes move to poles, mitotic spindle forms.

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Metaphase:

* Chromosomes align at the metaphase plate (equatorial plate). * Spindle fibers (kinetochore microtubules) attach to kinetochores on centromeres. * Metaphase checkpoint ensures proper attachment.

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Anaphase:

* Sister chromatids separate (cohesin cleavage by separase). * Separated chromatids are now individual chromosomes. * Chromosomes move to opposite poles (centromere leading). * Temporarily, chromosome number doubles (e.g., from 2n to 4n in a diploid cell), DNA content remains 4C.

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Telophase:

* Chromosomes decondense at poles. * New nuclear envelopes form around each set of chromosomes. * Nucleolus reappears. * Spindle disassembles.

Cytokinesis (Cytoplasmic Division):

Animal Cells: — Cleavage furrow forms (contractile ring of actin/myosin) and pinches cell into two.
Plant Cells: — Cell plate forms (fusion of Golgi vesicles) in the middle, growing outwards to form new cell wall.

Outcome: Two daughter cells, each 2n chromosomes and 2C DNA, genetically identical to the parent G1 cell.

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Interphase: Growth, DNA replication
Prophase: Chromosomes condense, Nuclear envelope disappears
Metaphase: Chromosomes align at Metaphase plate
Anaphase: Sister chromatids move Apart
Telophase: Two nuclei form, Chromosomes decondense
Cytokinesis: Cell divides (Cytoplasm)