Gametes and Fertilisation

Section 1 of 7What gametes are

Sex cells made by meiosis

• Sex cells made by meiosis Gametes are the specialised reproductive cells of an organism.
  • They are made by a type of cell division called meiosis.
• Because of meiosis, each gamete carries only half the normal number of chromosomes: 23 in a human gamete, compared with 46 in an ordinary body cell.

Gametes in animals

• Gametes in animals The male gamete is the sperm cell and the female gamete is the egg cell, also called the ovum.

Gametes in flowering plants

• Gametes in flowering plants The male gamete is the nucleus inside a pollen grain.
  • The female gamete is the ovum (egg cell) held inside an ovule in the ovary.

Section 2 of 7Adaptations of sperm and egg

Gamete	Adaptation	How it helps
Sperm cell	Long tail	Lashes from side to side to swim towards the egg
Sperm cell	Many mitochondria	Release the energy needed to keep the tail moving
Sperm cell	Small, streamlined head containing the nucleus	Carries the father's genes with little drag
Egg cell	Large cytoplasm containing food reserves	Supplies energy and nutrients to the early embryo
Egg cell	Haploid nucleus	Provides the mother's half of the genetic information

Section 3 of 7Fertilisation in humans

1Sperm are released

• Sperm are released During intercourse semen is released into the vagina close to the cervix.
  • Sperm swim through the cervix into the uterus and up the oviducts.

2Sperm meets egg

• Sperm meets egg If an egg is present in the oviduct, usually within one to two days of ovulation, a sperm may reach it.

3Nuclei fuse

• Nuclei fuse The nucleus of the sperm joins with the nucleus of the egg.
  • This fusion of a male and a female gamete is fertilisation.

4A zygote forms

• A zygote forms The fertilised egg is called a zygote and has the full 46 chromosomes, 23 from each parent.

Section 4 of 7From zygote to embryo

1

The zygote divides by mitosis, which produces identical copies of the cells and keeps the full chromosome number.

2

The ball of cells that results is called an embryo.

3

As the embryo grows, its cells become specialised (they differentiate) to do particular jobs.

4

These specialised cells form all the tissues and organs of the new individual.

Section 5 of 7Fertilisation in plants

Pollen tube

• Pollen tube After pollination, a pollen tube grows from the pollen grain down towards the ovary.

Delivery of the male nucleus

• Delivery of the male nucleus The male nucleus travels along the tube into the ovary,
  • where it fuses with the nucleus of the ovum to form a zygote that develops into the plant embryo.

Section 6 of 7Key terms

Term	Meaning
Gamete	A haploid sex cell, made by meiosis, that fuses with another gamete at fertilisation.
Fertilisation	The fusion of the nucleus of a male gamete with the nucleus of a female gamete.
Zygote	The single cell formed when two gametes fuse; it has the full chromosome number.
Meiosis	Cell division that halves the chromosome number to make gametes.
Embryo	The ball of cells formed when a zygote divides by mitosis.
Specialised cell	A cell that has changed in structure so that it can carry out a particular job.

Section 7 of 7Exam technique

1

Saying that gametes have 46 chromosomes. Human gametes have 23; the zygote restores the total to 46.

2

Writing that a zygote is made by mitosis. The zygote is made by fertilisation, and mitosis is what happens afterwards to build the embryo.

3

Giving the sperm's adaptation as just having a tail without adding that the mitochondria supply the energy for swimming.

4

Forgetting that fertilisation happens in the oviduct, not in the uterus.