Reproduction

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In today’s class, we will be talking about reproduction. Enjoy the class!

Reproduction

Reproduction is the ability of living organisms to produce offspring’s i.e. new individuals of their kind or the same species. Reproduction guarantees the continuity of life of all species of organisms. There are two forms of reproduction, They are sexual reproduction and asexual reproduction

Asexual reproduction

In asexual reproduction, an individual produces an offspring by itself i.e. only one parent is present. There is no fusion of nuclei and the cells that give rise to the offspring usually divide by means of mitosis. As a result, asexual reproduction often produces offspring which are identical to the parent. In rare cases, the offspring may not be identical due to mutation.

Asexual reproduction is common among simple organisms and flowering plants. Forms of asexual reproduction include the following:

• Fission: It is commonly found in bacteria and protists. The parent organism simply divides into two or more parts, each of which can exist by itself. e.g. binary fission in bacteria.
• Budding: In budding, the offspring develops as an outgrowth of the parent. The bud may form on an internal or external surface of the parents. Internal buds are formed in some sponges and released when the parent dies. External buds occur in Hydra and Coral polyps. The buds break off from the parent without causing any injury and lead an independent life.
• Spore Formation: Spore is small unicellular bodies which are produced in large numbers. They are small, light and easily dispersed by air. Under favourable conditions, each spore can develop into an independent organism. Spores are commonly produced by Bacteria, Fungi, Protists, Algae, Mosses and Ferns.
• Fragmentation: In this type of asexual reproduction a part of an organism breaks up or fragments from the parent organism and give rise to a new individual. It is a form of regeneration that occurs in simple organisms like algae, coelenterates and sponges.
• Vegetative propagation: It occurs mainly in higher plants where a new plant grows from any portion of an old one other than the seeds. This is the formation of new individuals or plants by a vegetative portion of the plant such as roots, stems and leaves. They are tubers, corns, bulbs, rhizomes, suckers and runners.

Sexual reproduction

Sexual reproduction involves two parents of different sexes (male and female). Each parent produces gamete male and female gametes respectively. There is meiosis during which the number of the chromosomes is halved i.e. from diploid to haploid in the formation of gametes. The egg cell (female gamete is large and non-motile). The sperm (male gamete) is motile and small. During fertilization, the two haploid gametes (male and female) fuse to form a diploid zygote. The zygote undergoes repeated cell division and forms an embryo. The embryo undergoes repeated cell division and differentiation and develops into a young organism similar to the parents.

There are two types of sexual reproduction, they are

• Conjugation – this occurs in paramecium and spirogyra
• Fertilisation – this occurs in higher plants and animals.

Conjugation in spirogyra

In spirogyra, two filaments lie close to each other and outgrowths appear on the walls of the cell lying opposite one another. The cells meet and their walls break and a conjugation tube is formed. One of the gametes passes through the conjugation tube and merges with the gamete in the other cells and their nuclei unite. This results in the formation of a zygospore.

The zygospore can withstand dry conditions and can germinate into a new individual when moisture is available. This process of reproduction is called conjugation. It also occurs in mucor.

Differences between asexual reproduction and sexual reproduction

	Asexual reproduction	Sexual reproduction
1	Only one parent is involved	two parents are involved.
2	No sex cells or gametes are involved	sex cells or gametes are involved
3	No fertilization takes place	fertilization/ fusion of cells takes place
4	A faster rate of producing offspring	a slower rate of producing offspring
5	Offspring are genetically identical	offspring show genetic variation
6   7	The only mitosis occurs during the process   Offspring adapt less to changes in the environment	both meiosis and mitosis occur during the process;   offspring are better adapted to changes in the environment.

 

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