Fertilisation joins two haploid gametes into one diploid zygote, and the zygote then divides and implants. Learn the events as a sequence in which each step causes the next.
This lesson follows comparing gametogenesis in the reproduction, development and growth chapter.
What is the sequence?
- Fertilisation: a sperm nucleus fuses with the ovum nucleus in the oviduct, forming a zygote.
- Cell division: the zygote divides by mitosis into two cells, then four, then more, while moving toward the uterus.
- Ball of cells: the dividing cells form an embryo that reaches the uterus.
- Implantation: the embryo attaches to the thick, blood-rich lining of the uterus.
- Development: the embryo keeps dividing and forming tissues, and the placenta forms to exchange substances.
Worked example: explain each step
Question: describe what happens between fertilisation and implantation, and explain why the zygote divides by mitosis. [4 marks]
A full answer:
- Fertilisation occurs in the oviduct, where the sperm nucleus fuses with the nucleus of the ovum to form a diploid zygote.
- The zygote divides by mitosis repeatedly as it moves down the oviduct, forming a ball of cells.
- The ball of cells reaches the uterus and implants in the lining, which is thick and rich in blood vessels.
- Mitosis is used because it produces cells that are genetically identical to the zygote, with the same chromosome number.
Notice that the answer gives the place for each step. Place and process are both needed for full marks.
The mistake that costs marks
The common slip is to swap fertilisation and implantation, for example writing “fertilisation is when the embryo attaches to the uterus”. The two words describe different events in different places.
| Wrong | Right | |
|---|---|---|
| Fertilisation | Embryo attaches to the uterus | Sperm and ovum nuclei fuse, in the oviduct |
| Implantation | Sperm meets ovum | Embryo attaches to the uterus lining |
| Zygote division | Meiosis | Mitosis |
A useful memory line: fertilisation forms the first cell, and implantation gives it a place to grow.
Check yourself
Why must implantation occur in a thick, blood-rich uterus lining?
Answer
The embryo needs oxygen and nutrients to keep dividing and growing, and it needs a way to remove waste.
The thick, blood-rich lining supplies these substances and supports the placenta. The lining is prepared each cycle by hormones, which links to the next lesson on hormonal-cycle graphs.
What to study next
The lining is prepared by hormones, so continue with interpreting hormonal-cycle graphs educationally. The biological data interpretation trainer gives more practice with diagrams.
Record any stages you swapped in the mistake log and paper-error review. For a teacher to check your sequence answers, see online one-to-one Biology tuition.