7 reproduction. reproduction: making offspring reproduction according to embedded genetic...
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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.