By the end of this lesson
- Define fertilisation and zygote.
- Relate haploid gametes to a diploid zygote.
- Describe early embryo formation by mitosis.
BIG QUESTION
How does fertilisation begin a new organism without doubling chromosome number every generation?
Meiosis halves chromosome number in gametes; fertilisation restores the diploid state.
Gametes carry one chromosome set
Male and female gametes are haploidDescribes a cell, such as a gamete, with one set of chromosomes (23 in humans).: each contains one set of chromosomes. This prevents chromosome number doubling in every generation.
In animals the gametes are sperm and egg; in flowering plants their nuclei are carried in pollen and ovules.
Fertilisation restores two sets
fertilisationThe fusion of the nuclei of a male gamete and a female gamete to form a diploid zygote. is fusion of male and female gamete nuclei. The resulting zygoteThe single diploid cell formed when the nuclei of a male and a female gamete fuse at fertilisation. is diploid, with one chromosome set from each parent.
Fertilisation is a nuclear event, not simply sperm touching an egg or pollen landing on a stigma.
The zygote becomes an embryo
The zygote divides repeatedly by mitosis to form an embryoAn early stage in the development of an organism, formed by mitosis from the zygote.. Cells later grow and differentiate into specialised tissues.
Mitosis preserves the zygote’s chromosome number and genome in new body cells.
From gametes to embryo
Track chromosome sets and cell processes.
| Stage | Chromosome state | Main event |
|---|---|---|
| Male gamete | Haploid. | Carries one paternal set. |
| Female gamete | Haploid. | Carries one maternal set. |
| Fertilisation | Two haploid nuclei fuse. | Diploid number is restored. |
| Zygote | Diploid. | First cell of the new organism. |
| Embryo | Diploid cells. | Mitosis, growth and differentiation. |
Language that earns precision
- haploid
- Containing one complete set of chromosomes.
- diploid
- Containing two complete sets of chromosomes.
- fertilisation
- Fusion of nuclei of male and female gametes.
- zygote
- Diploid cell formed at fertilisation.
Common misconceptions
Claim: Fertilisation is when gametes meet.
Correction: It specifically requires fusion of their nuclei.
Claim: A zygote is already a multicellular embryo.
Correction: It is the single cell that divides to produce the embryo.
Claim: Gametes have no chromosome pairs because they have no chromosomes.
Correction: They have one chromosome from each homologous pair.
Match the answer to the command word
Write “fusion of nuclei of male and female gametes”.
Haploid + haploid → diploid.
Recall, apply, then justify
Describe fertilisation and early embryo formation.
Hint
Nuclei, zygote, mitosis, embryo.
Suggested marking points
- A male gamete nucleus fuses with a female gamete nucleus.
- A diploid zygote forms.
- The zygote divides repeatedly by mitosis.
- This produces a multicellular embryo.
Explain why gametes must be haploid.
Hint
Follow chromosome number across one generation.
Suggested marking points
- Each gamete contains one chromosome set.
- Fertilisation combines two gametes and restores the diploid number.
- This prevents chromosome number doubling in each generation.
Predict the consequence if diploid gametes fused successfully.
Planning hint
Track chromosome sets into the zygote and next generation.
What a strong design includes
- Each gamete would carry two chromosome sets.
- Fusion would produce a tetraploid zygote with four sets.
- Chromosome number would double relative to the parents.
- Gene dosage and chromosome pairing could be disrupted.
- Development or fertility may be affected.
- If fertile polyploids formed, they could become reproductively isolated.