7 Reproduction

Reproduction: making offspring

• Reproduction according to embedded genetic instructions is a characteristic of living organisms.

• Two types of reproduction can occur: asexual and sexual.

• Offspring produced through asexual reproduction are clones of the single parent and of each other.

• Offspring produced through sexual reproduction show variation.

Reproduction without sex

• Asexual reproduction produces clones of a single parent or cell.

• In prokaryotes (bacteria), asexual reproduction occurs through binary fission.

• Asexual reproduction in eukaryotes involves the process of mitosis.

• Asexual reproduction in some eukaryotes, such as protists and simple animals, involves splitting and budding.

Reproduction without sex

• Asexual reproduction in plants involves structures such as runners, rhizomes and bulbs.

• Plant clones can be produced through tissue culture.

• Artificial cloning of mammals has been achieved through separating embryo cells and through somatic cell cloning.

Sexual reproduction

• Male gametes involved in sexual reproduction are sperm (and pollen) and female gametes are eggs.

• Plant life cycles have an alternation of generations, with a diploid spore producing stage alternating with a gamete-producing stage.

Sexual reproduction

• In some animals, egg and sperm production occur in separate female and male organisms.

• An hermaphrodite animal can produce both male and female gametes.

• Flowering plants show a variety of conditions regarding egg and pollen formation.

Getting gametes together

• Sexual reproduction involves fusion of an egg and a sperm at fertilisation.

• In animals, fertilisation may be internal or external.

• Some species that use external fertilisation have developed strategies to increase the chance of fertilisation occurring.

Getting gametes together

• With internal fertilisation, there is a greater chance of gametes meeting and hence the chance of fertilisation is increased.

• Cross-pollination in plants is more common than self-pollination.

• Various natural agents assist the transfer of pollen between different plant species.

• Natural breeding can be altered by various technologies.

Meiosis: making gametes

Meiosis: making gametes

• Meiosis is involved in gamete production.• Starting from a diploid cell (2n), meiosis results in

four haploid (n) cells.• Meiosis involves a reshuffling of genetic material.• Fertilisation restores the diploid number of

chromosomes.• In flowering plants, fertilisation involves the

fusion of an egg produced in the ovule with a sperm nucleus formed in the pollen grain.

Meiosis: making gametes

• Double fertilisation occurs only in flowering plants.

• In flowering plants, the developing embryo is enclosed within a seed formed from the ovule.

• In flowering plants, one or more seeds are enclosed within fruit formed from the ovary.

Human reproduction

• Human eggs and sperm are produced in ovaries and testes respectively.

• From one starting cell undergoing meiosis, only one egg is produced by a human female but four sperm result.

• A human egg completes meiosis only after fertilisation.