1.7 - Sexual & Asexual Reproduction

Sexual and asexual reproduction

There are two ways living things reproduce: sexual and asexual.

Sexual reproduction

• Genetic information from two parents, a male and a female, is combined to produce offspring.
• Each parent produces special sex cells called gametes. Reproduction happens when the nucleus of a male gamete fuses with the nucleus of a female gamete, forming a single new cell called a zygote.
• Offspring receive a mixture of genetic information from both parents.
• They are therefore genetically different from each other and from either parent - this mixing leads to variation in the offspring.

Asexual reproduction

• There is only one parent. No gametes are involved and there is no joining of sex cells, so there is no mixing of genetic information.
• The parent produces new cells by mitosis - the type of cell division that makes genetically identical copies.
• Offspring are genetically identical to the parent and to one another. A group of genetically identical offspring produced this way are called clones.
• Examples:
  • Bacteria reproduce asexually by splitting in two
  • Many plants reproduce asexually too, e.g. a strawberry plant sending out runners that grow into new plants
  • Some animals can also reproduce this way

How gametes are made

• Gametes are produced by a type of cell division called meiosis.
• Meiosis halves the chromosome number, so each gamete carries half the genetic information.
• When two gametes join at fertilisation, the full chromosome number is restored.

Gametes - The sex cells

• A gamete is a sex cell - one of the special cells that join together in sexual reproduction.
• Gametes differ from ordinary body cells because they carry only half the genetic information.
• When a male gamete and a female gamete fuse, the full amount of genetic information is restored.
• Sperm and egg cells are examples of specialised cells, each shaped for its particular job in reproduction.

Gametes in animals

• Male gamete - The sperm cell, made in the male reproductive organs, the testes.
• Female gamete - The egg cell (also called the ovum), made in the female reproductive organs, the ovaries.

Gametes in plants

• Plants also reproduce sexually using gametes.
• Male gamete - Carried in a pollen grain.
• Female gamete - The egg cell found inside the flower.

Fertilisation - Forming the zygote and embryo

• Fertilisation is the fusion of the nucleus of a male gamete with the nucleus of a female gamete.
• When a sperm reaches an egg cell, the nucleus of the sperm fuses with the nucleus of the egg cell.
• Each gamete nucleus is haploid, carrying half the normal number of chromosomes, so when the two fuse the new cell has the full number again and is diploid.
• This single fertilised cell is called a zygote.
• After fertilisation, the zygote begins to divide by mitosis, making genetically identical copies of itself again and again.
• The single cell becomes a small ball of cells - the embryo, the very early stage of a new individual.
• Because the zygote received a mixture of chromosomes from both parents, the embryo inherits a combination of features from each of them.
• The random way the gametes combine at fertilisation is part of what produces variation between offspring.

From zygote to implantation in humans

1. Fertilisation takes place in the oviduct - the tube that carries the egg from the ovary.
2. Once the zygote has formed there, it divides by mitosis many times into a ball of cells as it travels along the oviduct to the uterus.
3. On reaching the uterus, the embryo sinks into the thick uterus lining and becomes attached - a step called implantation - where it can continue to develop.