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# ⭐ **GAMETOGENESIS — COMPLETE HIGH-YIELD NOTE**

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## **1. Definition**

Gametogenesis = **process of formation of haploid gametes** (sperm & ovum) from diploid germ cells through **meiosis + cellular differentiation**.

Two types:

* **Spermatogenesis** (in males) → Spermatozoa
* **Oogenesis** (in females) → Ovum

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## **2. Chromosomal Basis**

* Starts from **diploid (2n = 46) primordial germ cells (PGCs)**.
* Undergo **mitosis → meiosis I → meiosis II**.
* Final gametes are **haploid (n = 23)**.
* Ensures **genetic variation** via:

* Crossing over
* Independent assortment

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# ⭐ **SPERMATOGENESIS (MALE GAMETOGENESIS)**

Occurs in **seminiferous tubules** of testes. Duration ≈ **64–74 days**.

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## **Stages**

### **1) Spermatocytogenesis (Mitosis)**

* **Spermatogonia (2n)** divide →

* **Type A** (stem cell pool)
* **Type B** (differentiate → primary spermatocyte)

### **2) Meiosis**

* **Primary spermatocyte (2n)** → undergo **Meiosis I** →

* **Secondary spermatocytes (n)**
* **Secondary spermatocytes** undergo **Meiosis II** →

* **Spermatids (n)**

### **3) Spermiogenesis (Maturation)**

Spermatids → spermatozoa
Changes:

* Condensed nucleus
* Acrosome formation
* Tail formation
* Shedding of cytoplasm
* Mitochondria coil in midpiece

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## **Hormonal Control**

* **GnRH (Hypothalamus)** → stimulates FSH & LH
* **FSH** → Sertoli cells → ABP ↑, spermatogenesis ↑
* **LH** → Leydig cells → testosterone production
* **Testosterone** → essential for meiosis I & II
* **Inhibin B** → negative feedback on FSH

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## **Supporting Cells**

* **Sertoli cells**: form blood-testis barrier, nourish sperm, produce inhibin, AMH.
* **Leydig cells**: testosterone synthesis.

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# ⭐ **OOGENESIS (FEMALE GAMETOGENESIS)**

Begins **in fetal life**; completes only after **fertilization**.

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## **Stages**

### **1) Fetal Life**

* Oogonia (2n) → mitosis → **Primary oocytes (2n)**
* Enter **Meiosis I**, arrested at **Prophase I (dictyotene stage)**
* At birth → **1–2 million primary oocytes**
* At puberty → ~400,000 remain

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### **2) Puberty (Each cycle)**

* LH surge → completion of **Meiosis I**
Primary oocyte →
* **Secondary oocyte (n)**
* **First polar body**

Secondary oocyte immediately enters **Meiosis II**, **arrested at Metaphase II**.

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### **3) At Fertilization**

* Completion of **Meiosis II**
* Formation of:

* **Ovum (n)**
* **Second polar body**

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## **Key Differences Between Spermatogenesis vs Oogenesis**

| Feature | Spermatogenesis | Oogenesis |
| ---------------------------- | --------------- | -------------------------------------------- |
| Onset | Puberty | Fetal life |
| Gametes formed per precursor | 4 sperm | 1 ovum + polar bodies |
| Continuous or cyclical | Continuous | Cyclical |
| Arrest points | None | Prophase I (birth), Metaphase II (ovulation) |
| Duration | 74 days | Can span decades |

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# ⭐ **Clinical Correlations**

* **Nondisjunction errors** → trisomies (Down, Edwards, Patau).
* Risk ↑ with **advanced maternal age** (oocyte arrested for years).
* **Klinefelter (47,XXY)**, **Turner (45,XO)** arise due to meiosis errors.
* Sperm abnormalities → oligospermia, teratozoospermia, infertile males.

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# ⭐ **Exam High-Yield Points**

* Ovum is arrested at **Prophase I till puberty**, **Metaphase II till fertilization**.
* Spermiogenesis = transformation **spermatid → spermatozoon**.
* Only **head of sperm** enters ovum; mitochondria remain outside.
* Zona pellucida reaction prevents **polyspermy**.

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# ⭐ **SEO TITLE**

**Gametogenesis: Spermatogenesis & Oogenesis Explained | Complete Notes 2025**

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