Suparna D.

Interphase (Before cell division starts)

Interphase is the longest phase of the cell cycle, where the cell grows and prepares for division. It has three stages:

1️⃣ G₁ phase (Gap 1)

Cell grows in size.

Proteins and organelles are synthesized.

Cell performs its normal functions.

The cell decides whether to continue to divide or enter G₀ (resting phase).

2️⃣ S phase (Synthesis)

DNA replication occurs (each chromosome duplicates to form two sister chromatids).

Centrosome also duplicates.

3️⃣ G₂ phase (Gap 2)

Cell continues to grow.

More proteins and enzymes needed for division are produced.

Cell checks and repairs any DNA errors.

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🔬 Mitosis (Equational division)

Occurs in somatic (body) cells for growth and repair; daughter cells are genetically identical to parent.

1️⃣ Prophase

Chromatin condenses into visible chromosomes (each with two sister chromatids).

Nuclear envelope starts to break down.Nucleolous, golgi apparatus, endoplasmic reticulum disappears.

Centrosomes move to opposite poles and spindle fibers form.

2️⃣ Metaphase

Chromosomes line up along the equatorial (metaphase) plate.

Spindle fibers attach to centromeres (via kinetochores).

3️⃣ Anaphase

Sister chromatids separate and move to opposite poles, pulled by spindle fibers.

4️⃣ Telophase

Chromatids reach poles and decondense into chromatin.

Nuclear envelope reforms around each set.

Nucleolus,golgi apparatus, Endoplasmic reticulum reappears.

Cytokinesis: Cytoplasm divides, forming two identical daughter cells.

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🌾 Meiosis (Reductional division)

Occurs in germ cells (to produce gametes), results in four haploid cells.
It has two successive divisions: Meiosis I and Meiosis II.

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Interphase before meiosis

Same as mitotic interphase: G₁, S (DNA replication), and G₂ phases.

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🌀 Meiosis I (Reductional division)

Reduces chromosome number to half (diploid → haploid).

1️⃣ Prophase I (Longest, most complex)

It is divided into five stages:

Leptotene: Chromosomes start condensing.
Zygotene: Synapsis (homolog pairing), bivalents form.
Pachytene: Crossing over happens.
Diplotene :Chiasmata visible, homologs start separating.
Diakinesis: Chromosomes fully condensed, nuclear membrane breaks down.

2️⃣ Metaphase I

Homologous chromosome pairs align at equatorial plate.

Spindle fibers attach to centromeres.

3️⃣ Anaphase I

Homologous chromosomes separate (sister chromatids remain together) and move to opposite poles.

4️⃣ Telophase I

Chromosomes reach poles.

Nuclear membrane may reform (varies by species).

Cytokinesis occurs, forming two haploid cells.

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🌀 Meiosis II (Equational division)

Similar to mitosis, separates sister chromatids.

1️⃣ Prophase II

Chromosomes condense again (if decondensed).

New spindle forms.

Nuclear envelope breaks down (if reformed).

2️⃣ Metaphase II

Chromosomes align at equatorial plate.

3️⃣ Anaphase II

Sister chromatids finally separate and move to opposite poles.

4️⃣ Telophase II

Chromatids (now chromosomes) reach poles.

Nuclear envelope reforms.

Cytokinesis occurs, resulting in four haploid daughter cells.