Question

Give the schematic representation of oogenesis in human female. Indicate chromosome number also.

Hint:

Oogonium (46) to primary oocyte (46) to secondary oocyte (23) with first polar body, then ovum (23) with second polar body.

Answer 1

Step 1: Understanding the process:
Oogenesis is the formation of the mature ovum (egg) in the ovary of a human female.
It starts before birth, pauses twice, and ends only if a sperm enters the egg.

Step 2: Before birth: oogonia to primary oocytes:
In the fetal ovary, oogonia (diploid, \( 2n = 46 \)) divide by mitosis to make many cells.
Each cell enters meiosis I and stops at prophase I. It is now a primary oocyte (\( 2n = 46 \)) inside a primary follicle.
No new primary oocytes form after birth. The oocyte stays arrested until puberty.

Step 3: At puberty: meiosis I finishes:
Each month a few follicles grow into secondary follicles. Then one grows into a Graafian follicle.
The primary oocyte completes meiosis I just before ovulation. It divides unequally.
This gives one large secondary oocyte (\( n = 23 \)) and one tiny first polar body (\( n = 23 \)).

Step 4: Ovulation and meiosis II:
The secondary oocyte stops at metaphase II and is released at ovulation.
Meiosis II is completed only if a sperm enters. It gives one ovum (\( n = 23 \)) and a second polar body (\( n = 23 \)).
The polar bodies degenerate. Only the ovum can be fertilised.

Step 5: Schematic representation:
Oogonium (\( 2n = 46 \))
\( \downarrow \) mitosis (fetal life)
Primary oocyte (\( 2n = 46 \)), arrested at prophase I
\( \downarrow \) meiosis I (at puberty, before ovulation)
Secondary oocyte (\( n = 23 \)) + first polar body (\( n = 23 \))
\( \downarrow \) meiosis II (only after sperm entry)
Ovum (\( n = 23 \)) + second polar body (\( n = 23 \))

Final Answer:
Oogenesis runs: oogonium (46) to primary oocyte (46) to secondary oocyte (23) plus polar body (23) to ovum (23) plus polar body (23). A neat flow chart with these chromosome numbers is expected.