Question

Define oogenesis. Describe the process of oogenesis with a diagram.

Hint:

Track the two pause points in meiosis: one lasts until puberty, the other lasts until fertilisation.

Answer 1

Step 1: Define oogenesis.
Oogenesis is the process by which a diploid oogonium (egg mother cell) in the ovary develops into a mature haploid female gamete, the ovum (egg), through a specific sequence of growth and meiotic division.

Step 2: Formation of primary oocytes, before birth.
In the foetal ovary, oogonia multiply by mitosis, then some enter meiosis I but stop partway through, arresting at the prophase I stage. Each arrested cell, now called a primary oocyte, gets surrounded by a layer of granulosa cells, forming a primary follicle. Millions of these primary follicles are already present at birth.

Step 3: Growth and completion of meiosis I, at puberty.
From puberty onward, in each menstrual cycle a batch of primary follicles starts growing further under hormonal control, and usually one dominant follicle matures fully into a Graafian follicle. Just before ovulation, the primary oocyte inside it completes meiosis I, producing two unequal-sized cells, a large secondary oocyte and a tiny first polar body.

Step 4: Ovulation and completion of meiosis II.
The secondary oocyte is released at ovulation and starts meiosis II, but again stops partway, this time at metaphase II. It only completes this second division, forming the mature ovum and a second polar body, if a sperm actually fertilises it.

Step 5: Diagram description.
A simple flow diagram would show: oogonium (mitosis) -> primary oocyte (arrested in meiosis I, inside a primary follicle) -> [puberty, growth] -> secondary oocyte + first polar body (meiosis I complete) -> [after fertilisation] -> ovum + second polar body (meiosis II complete).

Final Answer:
Oogenesis converts a diploid oogonium into a haploid ovum through two meiotic divisions that pause at prophase I (until puberty) and at metaphase II (until fertilisation), each division producing one large cell and one small polar body.