Anatomy chapters
Cell division and gametogenesis
How cells divide by mitosis and meiosis, and how sperm and eggs are made, with the timings and chromosome numbers examiners ask about.
- Mitosis makes two identical diploid cells; meiosis makes four haploid cells for reproduction.
- Sperm are made continuously from puberty; all primary oocytes are formed before birth.
- The oocyte pauses twice: in prophase I (until puberty) and in metaphase II (until fertilisation).
On this page
Overview
Body cells divide by mitosis to grow and repair. Germ cells divide by meiosis to make gametes with half the number of chromosomes.
Gametogenesis is the making of gametes: spermatogenesis in the testis and oogenesis in the ovary.
When a sperm (23 chromosomes) fertilises an oocyte (23 chromosomes), the zygote gets the full 46 again.
Mitosis
One diploid cell divides into two identical diploid cells.
The stages are prophase, metaphase, anaphase and telophase. DNA is copied beforehand, in the S phase of interphase.
- Prophase: chromosomes condense.
- Metaphase: chromosomes line up at the equator.
- Anaphase: sister chromatids are pulled apart.
- Telophase: two new nuclei form, then the cytoplasm divides.
Meiosis
Meiosis has two divisions after one round of DNA copying. One diploid cell gives four haploid cells.
Meiosis I separates the paired homologous chromosomes. It halves the chromosome number (reduction division).
Meiosis II separates the sister chromatids, like mitosis.
Crossing over (exchange of segments between paired homologous chromosomes) happens in prophase of meiosis I. It mixes genes and makes every gamete unique.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Where | Body (somatic) cells | Germ cells |
| Divisions | One | Two |
| Daughter cells | 2, diploid | 4, haploid |
| Pairing of homologues | No | Yes |
| Crossing over | No | Yes |
| Daughter cells identical | Yes | No |
Nondisjunction
Sometimes chromosomes fail to separate in meiosis. One gamete gets an extra chromosome and another gets one too few.
After fertilisation this gives a trisomy (47 chromosomes) or a monosomy (45).
| Example | Karyotype |
|---|---|
| Down syndrome | Trisomy 21 |
| Edwards syndrome | Trisomy 18 |
| Patau syndrome | Trisomy 13 |
| Klinefelter syndrome | 47,XXY |
| Turner syndrome | 45,X |
Most cases of Down syndrome come from nondisjunction in the mother's meiosis, and the risk rises with maternal age.
Spermatogenesis
Spermatogenesis takes place in the seminiferous tubules of the testis, from puberty onwards.
- Spermatogonia (2n) divide by mitosis to keep the supply going.
- Some become primary spermatocytes (2n), which enter meiosis I.
- Meiosis I gives two secondary spermatocytes (n).
- Meiosis II gives four spermatids (n).
Sertoli cells support and nourish the developing cells. Leydig cells, between the tubules, make testosterone.
Spermiogenesis
Spermiogenesis is the change of a round spermatid into a sperm. There is no cell division.
- The Golgi apparatus forms the acrosome, a cap of enzymes over the head.
- The nucleus condenses.
- A tail (flagellum) grows; mitochondria wrap around the middle piece.
- Most of the cytoplasm is shed.
Spermatogenesis is the whole process from spermatogonium to sperm. Spermiogenesis is only the last, shape-changing step.
The whole process takes about 64 days in classical teaching. Sperm then mature further in the epididymis, and gain full ability to fertilise (capacitation) in the female tract.
Oogenesis
Oogenesis starts before birth.
- Oogonia multiply by mitosis in the fetal ovary.
- By birth they have all become primary oocytes, which have started meiosis I and stopped.
- The primary oocyte, with its layer of cells, forms a primordial follicle.
The primary oocyte is arrested in the diplotene stage of prophase I from fetal life until puberty, when each month a few resume meiosis.
At ovulation
Just before ovulation, the primary oocyte finishes meiosis I. It gives a secondary oocyte (with most of the cytoplasm) and a small first polar body.
The secondary oocyte starts meiosis II and stops again at metaphase II. It is ovulated in this state.
Meiosis II is only completed if fertilisation occurs. Then the oocyte forms the mature ovum and a second polar body.
The cell released at ovulation is a secondary oocyte, not a mature ovum.
Spermatogenesis vs oogenesis
| Feature | Spermatogenesis | Oogenesis |
|---|---|---|
| Starts | At puberty | Before birth |
| Continues | Throughout adult life | Stops at menopause |
| Products of one primary cell | 4 sperm | 1 oocyte + polar bodies |
| Cytoplasm | Divided equally | Unequal; the oocyte keeps most |
| Arrest points | None | Prophase I and metaphase II |
Clinical relevance
Maternal age and trisomy
How it happens: primary oocytes wait in prophase I for decades. The longer the wait, the more likely chromosomes fail to separate.
What you see: rising risk of trisomies, especially Down syndrome, with maternal age.
Male infertility
How it happens: problems with spermatogenesis, such as undescended testes (too warm), infection (mumps orchitis) or genetic causes like Klinefelter syndrome.
What you see: low or absent sperm count.
The favourite questions are the stage of arrest of the primary oocyte, the stage at which the secondary oocyte waits, the difference between spermatogenesis and spermiogenesis, and the link between nondisjunction and trisomies.
Quick revision
- Mitosis: two identical diploid cells. Meiosis: four haploid cells.
- Crossing over: prophase I.
- Primary oocytes: all formed before birth; arrested in diplotene of prophase I.
- Secondary oocyte: arrested in metaphase II; finishes only if fertilised.
- Spermiogenesis: spermatid changes into sperm with no division.
- Nondisjunction: trisomy 21, 18, 13; risk rises with maternal age.
Written by The House Job from standard embryology references (Langman's Medical Embryology).
FAQ
When does the primary oocyte finish its first meiotic division?
Just before ovulation, after being arrested in prophase I since fetal life.
In which phase is the ovulated oocyte arrested?
Metaphase of meiosis II. It completes meiosis II only if it is fertilised.
What is spermiogenesis?
The final stage of spermatogenesis, in which a spermatid changes shape into a sperm without dividing.
Where is crossing over seen?
In prophase of the first meiotic division.