Gametes and fertilisation in mammals and plants

What you’ll learn

• Relate mammalian gamete structures to their functions.
• Explain the acrosome reaction, cortical reaction and restoration of diploidy.
• Trace pollen-tube growth and double fertilisation in flowering plants.

Understand the idea

A sperm cell carries a haploid nucleus, an acrosome containing enzymes, a midpiece with mitochondria and a flagellum for movement. An egg cell is large, with cytoplasm containing resources for early development, a haploid nucleus and surrounding layers including the zona pellucida. Their different structures reflect their roles: one delivers paternal genetic material, while the other contributes most of the zygote’s cytoplasm as well as maternal genetic material.

In mammalian fertilisation, sperm interaction with the egg’s coverings triggers the acrosome reaction, helping penetration. Fusion of the gamete membranes triggers release of cortical-granule contents. These modify the zona pellucida and help prevent additional sperm entering. The haploid genetic complements then unite to form the diploid zygote nucleus. Preventing polyspermy matters because extra sperm nuclei would disrupt the normal chromosome complement.

In flowering plants, a compatible pollen grain germinates on the stigma and a pollen tube grows through the style towards an ovule, entering through the micropyle. Two male nuclei are delivered to the embryo sac. One fuses with the egg nucleus to form the diploid zygote; the other fuses with the two polar nuclei to produce the typically triploid primary endosperm nucleus. This double fertilisation establishes the embryo and a tissue that supports its nutrition. Pollination is the transfer of pollen to the stigma and is distinct from nuclear fusion.

Key formulae

$$n+n=2n\quad\text{zygote}$$

$$n+n+n=3n\quad\text{typical endosperm nucleus}$$

Worked example

A flowering plant has 18 chromosomes in each diploid body cell. Find the expected chromosome counts in a male gamete nucleus, the zygote and a typical endosperm nucleus.

1. The haploid number is 18/2 = 9, so a male gamete nucleus contains 9 chromosomes.
2. Fusion of male and egg nuclei gives 9 + 9 = 18 in the zygote.
3. Fusion with two polar nuclei gives 9 + 9 + 9 = 27 in the endosperm nucleus.

Answer: 9, 18 and 27 chromosomes respectively.