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Introduction

All living things reproduce. Reproduction is the process of generating offspring. There are two main types of reproduction: sexual and asexual. Some organisms reproduce by only one type of reproduction and others can reproduce by both. This chapter looks at the differences, advantages and disadvantages of sexual and asexual reproduction.

Asexual reproduction

The type of reproduction where cells from only one parent are used, is called asexual. Only genetically-identical organisms are produced by this type of reproduction. Evolutionary asexual reproduction came before sexual reproduction.

Asexual reproduction in bacteria

Asexual reproduction is very common in microorganisms. Bacteria reproduce by binary fission. During binary fission, the cell divides into two daughter cells that are similar in size and shape.

Asexual reproduction in plants

Asexual reproduction in plants is also called vegetative reproduction. It usually involves only the plant's vegetative structures like roots, stems and leaves. For example, raspberries can produce a new generation using their stems; potatoes, using their roots; and geraniums can be grown from any piece of a parent plant. See image 1. 

Sporulation

Some types of mould reproduce through sporulation. They produce reproductive cells - spores - that are stored in special spore cases until they are ready to be released. After they are released they will develop into new, individual organisms. Bread mould reproduces by sporulation.

Asexual reproduction in animals

Some invertebrate animals (without a backbone) reproduce by asexual reproduction. Animals can reproduce asexually in the following ways:

Budding

During budding, a new organism starts growing from the parent's body. At first it looks like a bud. This bud later develops into a mature organism. Sometimes it stays attached to the parent's body and sometimes it breaks off. Hydras reproduce by budding. See image 2.

Gemmules

Gemmules are special structures that are found in sea sponges. A parent sponge releases gemmules that later develop into mature sponges.

Regeneration

In the process of regeneration, detached pieces of the parent's body can develop into a new organism if this body part contains enough genetic information. Some flat worms and starfish can reproduce by regeneration.

Advantages and disadvantages of asexual reproduction

Asexual reproduction works well for organisms that stay in one place. Because they do not move, it is difficult for them to find a mating partner. Stable environments are the best places for organisms that reproduce asexually. Asexual reproduction is also much less time and energy consuming. Asexually-produced generation does not have any genetic variations. That means that these organisms will not have any 'material' for adapting to environmental changes. That is why many asexually-reproducing organisms can reproduce sexually as well.

Sexual reproduction

During sexual reproduction, two gametes from both parents fuse, forming a zygote. A zygote is also referred to as a fertilised egg. All gametes are haploid cells, meaning they have only one set of chromosomes (1n). So, when gametes fuse, they form a diploid organism: 1n+1n=2n.

Sexual reproduction in algae

The simplest form of sexual reproduction in algae is conjugation, in which two similar organisms fuse, exchange genetic material and then break apart. Some multicellular green algae undergo a process called alternation of generations. During this process, generations of different types of organisms are produced: haploid and diploid. Haploid generation reproduces sexually. It is followed by diploid generation that reproduces asexually.

Sexual reproduction in flowering plants

Flowers contain both male and female parts. The female part is called the pistil, which consists of the ovary, ovule, style and stigma at the tip. Inside the ovary are the ovules. Each ovule contains an egg cell. The male structure is called the stamen. It consists of the filament and the pollen-producing anther. A new seed is formed when an egg cell joins with a pollen cell in the process of pollination. Pollination occurs when pollen grains are carried from the anther of the stamen to the stigma of the pistil. See image 3.

Sexual reproduction in animals

Animal male gamete is called spermatozoan or sperm. Sperm is a mobile cell that moves using its 'tail', called flagellum. Female gamete is called an ovum. It does not move and it is much larger than sperm. See image 4.

Types of fertilisation

There are two main types of fertilisation.

Internal fertilisation

During internal fertilisation, eggs are fertilised inside the female's body. Animals, like reptiles and birds, lay eggs after fertilisation. New offspring develop outside the female's body. All eggs are covered by a protective shell. Mammal females, except monotremes, develop a new embryo inside their body. This extra protection increases an organism's chances of survival.

External fertilisation

During external fertilisation, the egg is fertilised outside the female's body. Male and female gametes are released into these species' surroundings where they fuse, forming a zygote. This type of fertilisation usually occurs in water. Amphibians and fish are examples of animals that reproduce in this way.

See animation 1.

Hermaphrodites

Hermaphrodites are animals that have both female and male reproductive organs.
Earthworms and leeches are hermaphrodites, but as they produce eggs and sperm at different times, they need a mate to reproduce. Flatworms are hermaphrodites that can self-fertilise.

Parthenogenesis

In some animal species, eggs can develop without fertilisation in a process called parthenogenesis. Some types of birds and bees can reproduce by parthenogenesis.


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Question 1/5

1. During sexual reproduction two gametes from both parents fuse, forming a

Gemmule

Zygote

Gamete

Ovum

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