Oogenesis is the process that makes the female gamete (ovum) inside the ovary. It begins in the foetal ovary before birth: oogonia are made and grow into primary oocytes, which then STOP in the middle of meiosis-I. They stay stopped for years. Meiosis-I finishes just before ovulation (making a secondary oocyte + first polar body), and meiosis-II finishes only when a sperm enters the egg. Memory hook: "One egg, two long pauses" — the female cell waits at prophase-I from birth, and again at metaphase-II until the sperm arrives.
Oogenesis flow: the oogonium becomes a primary oocyte before birth and pauses in prophase-I; meiosis-I finishes just before ovulation to give a secondary oocyte plus the first polar body; meiosis-II finishes only when a sperm enters, giving one ovum plus the second polar body. One oogonium yields just one functional egg.
Your doubts, answered
Does oogenesis begin at birth or at puberty?
Oogenesis is INITIATED during the embryonic (foetal) stage, not at puberty. Inside each foetal ovary, a couple of million oogonia (gamete mother cells) are formed. No new oogonia are added after birth. This is a very common NEET trap: spermatogenesis begins at puberty, but oogenesis STARTS long before birth. What happens at puberty in the female is only that the paused cycle restarts each month, not the start of oogenesis itself.
What is the difference between a primary oocyte and a secondary oocyte?
A primary oocyte is diploid (2n) and is arrested (paused) in prophase of meiosis-I. It stays this way from birth until it is picked for a cycle. When meiosis-I completes, it forms ONE secondary oocyte (haploid, n, with most of the cytoplasm) plus a small first polar body. So: primary oocyte = before meiosis-I finishes; secondary oocyte = after meiosis-I finishes but before meiosis-II finishes.
When is meiosis-II completed in the female?
Meiosis-II of the secondary oocyte is completed ONLY at fertilisation — when the sperm enters and fuses with the egg. Sperm entry triggers the secondary oocyte to finish meiosis-II, forming the second polar body and the mature haploid ovum. This is why the cell RELEASED at ovulation is technically a secondary oocyte, not a fully finished ovum. NEET repeatedly tests this exact timing.
How many functional eggs come from one oogonium?
Only ONE functional ovum comes from one oogonium. The divisions are unequal: meiosis-I gives one secondary oocyte + one (first) polar body, and meiosis-II gives one ovum + one (second) polar body. The polar bodies are tiny cells with almost no cytoplasm and they degenerate. This is different from spermatogenesis, where one cell gives four functional sperms.
Why are the divisions in oogenesis unequal?
The divisions are unequal so that ONE cell keeps almost all the cytoplasm and stored food. A future embryo needs a large store of nutrients before implantation, so the egg keeps the cytoplasm while the polar bodies get almost none. This is why oogenesis makes only one big functional egg instead of four equal cells.
⚠️ The NEET trap ✗ The cell released from the ovary at ovulation is a fully formed haploid ovum, and meiosis is already complete. ✓ The cell released at ovulation is a SECONDARY OOCYTE. Its meiosis-II (second meiotic division) is completed only after a sperm enters it — the second polar body is extruded after sperm entry, and only then is the true ovum formed. 🧠 Egg leaves the ovary UNFINISHED. It says 'I will finish only if a sperm comes' — meiosis-II waits for the sperm.
Real NEET questions
2022
At which stage of life is the oogenesis process initiated?
A · Puberty
B · Embryonic development stage ✓
C · Birth
D · Adult
Solution: Oogenesis is initiated during the embryonic (foetal) development stage, when a couple of million oogonia (gamete mother cells) form inside each foetal ovary. No new oogonia are added after birth. This contrasts with spermatogenesis, which begins at puberty. Correct answer: Embryonic development stage.
2020
Meiotic division of the secondary oocyte is completed:
A · After zygote formation
B · At the time of fusion of a sperm with an ovum ✓
C · Prior to ovulation
D · At the time of copulation
Solution: The secondary oocyte stays arrested in meiosis-II after ovulation. Its meiosis is completed only when the sperm enters and contacts the egg. Sperm entry triggers completion of meiosis-II, forming the second polar body and the haploid ovum, after which the nuclei fuse. So meiosis of the secondary oocyte is completed at the time of fusion of sperm with ovum.
2019
Extrusion of the second polar body from the egg nucleus occurs:
A · After entry of sperm but before fertilization ✓
B · After fertilization
C · Before entry of sperm into ovum
D · Simultaneously with first cleavage
Solution: Sperm entry through the zona pellucida and plasma membrane induces the secondary oocyte to complete meiosis-II. This produces the second polar body and the haploid ovum. The second polar body is therefore extruded AFTER sperm entry but BEFORE the fusion of the two nuclei (fertilisation) is complete.
Solved Human Reproduction NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Is the egg released at ovulation an ovum or a secondary oocyte?
It is a secondary oocyte. It has finished meiosis-I but is still paused in meiosis-II. It becomes a true ovum only after a sperm enters it and meiosis-II completes.
How many polar bodies are formed in oogenesis?
Two. The first polar body forms when meiosis-I finishes (before ovulation) and the second polar body forms when meiosis-II finishes (at fertilisation). Both are tiny and degenerate.
How is oogenesis different from spermatogenesis in output?
One oogonium gives only ONE functional ovum plus two tiny polar bodies. One spermatogonium gives FOUR functional sperms. Oogenesis divisions are unequal; spermatogenesis divisions are equal.
When does the primary oocyte form?
Primary oocytes form before birth, during foetal life. The oogonia start meiosis-I, become primary oocytes, and then pause in prophase-I until much later, one at a time in each cycle.
Why must a NEET student remember the two pauses in oogenesis?
Because NTA repeatedly tests exact timing: meiosis-I completes just before ovulation, and meiosis-II completes only at fertilisation. Marking these two pause points correctly gives easy marks in Human Reproduction.
