Reproduction in Organisms

The process through which organisms – including plants and animals – produce other organisms or “offspring” of the same species is known as reproduction. This assures that, despite the fact that individual creatures will always perish, the species, and so life itself, will continue to exist on Earth. From a single-celled amoeba to complex human beings, all species reproduce to make new progeny. Moreover, the process of reproduction supports the process of evolution and maintains the diversity of life on earth. Reproduction in organisms is of two types – asexual and sexual reproduction.

Read the entire article to get all the necessary information regarding reproduction in organisms such as the types of reproduction, how do living organisms reproduce, advantages and disadvantages of different types of reproduction etc. Continue reading to know more.

What is Reproduction in Organisms?

Reproduction in Organisms Class 12 Notes: Reproduction is one of the salient features of all organisms. Reproduction is a mode of multiplication and conservation or perpetuation of the pre-existing individuals giving rise to new young ones as the older individuals are prone to deteriorate or face death. It has been a model of conservation of species over course of evolution. The newborn is known as offspring. Genetic material is often transferred from generation to generation, sometimes undergoing changes in composition due to the crossing over of genetic material and cell division. The cell division observed here is meiosis.

Reproduction in Organisms: Why is it Necessary?

Reproduction is essential for the continuity of a species and life itself on the planet. The process of reproduction helps to maintain a precise balance amongst different biotic components in the ecosystem. Life would cease to exist in the process of reproduction does not happen. Reproduction also paves the way for evolution as it leads to variations through the intermingling of species as seen in sexual reproduction.

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Types of Reproduction

Reproduction is chiefly of two types:

Asexual Reproduction

In the asexual mode of reproduction in organisms, a new offspring is produced from a single parent only. There is no change in chromosome number and genes. The offsprings produced in the process are identical copies of their parent because in this process a single parent divides itself to reproduce its offspring. It is also known as agamogamy or agamogenesis.

Asexual reproduction takes place in 5 important ways. The different types of asexual reproduction are as follows:

• Binary fission
• Multiple fission
• Budding
• Fragmentation
• Vegetative propagation

1) Binary fission: Binary fission is the most basic of the asexual reproduction by which the organisms split into two equal-sized daughter cells by mitosis. Here the nucleus undergoes division after which cytoplasm gets divided and the daughter cells are genetically identical to the parent cell. It is found in amoeba (simple binary fission), paramecium (transverse binary fission), and Euglena (longitudinal binary fission).

2) Multiple fission: Multiple fission is the type of fission in which the nucleus followed by cytoplasm undergo multiple division, to form daughter cells. The type of cell division here is amitosis. It is observed in plasmodium and Entamoeba histolytica.

3) Budding: Budding is a process wherein small buds arise from the parent cell, by the process of mitosis. It is found in the lower class of organisms like yeast, hydra, sponges.

4) Fragmentation: Fragmentation is a type of asexual reproduction, where the individual organism gets split into multiple fragments each further developing into new individuals. It is observed in flatworms and microstomia.

5) Vegetative propagation: It is a type of asexual reproduction observed in plants, in which a new plant can arise from a part of the parent plant or out of a specialized region. Here the plants reproduce from stem or tubers (potato, mint), leaves (bryophyllum), root (sweet potato, tapioca), or reproductive parts (onion, agave). Vegetative propagation is a natural process but can also be induced artificially.

Artificial Vegetative Propagation

It is widely used by horticulturists to obtain variety in the yield as compared to the native plant and further clone it since the genetic material remains unchanged. This method is widely used in plants where sexual reproduction or seeds are not available and the process is easier and cheaper compared to the traditional propagation of plants.

The different types include Cutting, such as in stem (rose), leaves (bryophyllum), roots (apple, lemon, tamarind), and placed in a suitable medium to grow into a new plant.

Grafting: Here the desired parts of two different plants are fused to form a new plant. The root is called stock and the part of the plant grafted to; is called Scion. The scion and stock should be compatible with each other. The types of grafting can be done in plants such as mango (wedge grafting), citrus plant (crown grafting), apple (tongue grafting), almonds (budding).

Layering: It is the method of inducing roots development at stem regions. Some of the types of layering include – simple layering (rhododendron), tip layering (raspberries, blackberries), stool layering (apple), compound layering (grapes), air layering (oleander).

Advantages & Disadvantages of Asexual Reproduction

Advantages:

1. Reproduction can be achieved by a single parent.
2. The cultivation of seedless plants is easy.
3. There is no loss of genetic material and hence chances of survival is stronger.
4. There are multiple methods by which reproduction can be achieved.
5. Asexually reproducing plants mature more quickly, hence, it is possible to achieve multiple yields in a shorter time.

Disadvantages:

1. Any harmful mutant genetic material is carried forward.
2. Useful genetic variation in the next generations rarely occur.
3. Leads to negligible resistance to pest invasions.
4. Asexual reproduction makes no genetic contribution in the course of evolution.
5. Asexually reproducing organisms have a shorter lifespan and are limited to certain habitat.

Sexual Reproduction

In the sexual mode of reproduction in organisms, a new offspring is produced by the participation of two parents of the opposite sex, such as the male and female. This type of reproduction is seen in all multicellular organisms including birds, reptiles, dogs, cats, cattle, elephants, etc. The exchange of genetic material takes place in the chromosomes of the specialized sex cells called the gamete. This process of fusion of male and female gametes, in which the offspring receives half of the genetic material from each of the parents, is called Fertilization. The male gamete is sperm and the female gamete is the egg. The complete process of sexual reproduction consists of the following set of events:

Pre-fertilization Stage:

In this phase, events such as gametogenesis and gamete transfer take place.

Gametogenesis:

Gametogenesis is a process in which the formation of gametes: male and female gametes, takes place. The gametes are haploid cells that develop from their parent cells that undergo cell division. Gametogenesis is further classified into oogenesis and spermatogenesis.

a) Oogenesis: It is a process of formation of the egg from the oocyte. Oogonium, a germ cell(diploid) undergoes mitosis to increase its number to a few million cells. The primary oocyte is diploid in nature, undergoes its first meiotic division at adolescence, gets arrested, and later develops into the follicle, giving rise to the secondary oocyte and a polar body. The secondary oocyte further undergoes meiotic division and gets arrested and completes the division at the time of fertilization. The polar body formed during oogenesis is degenerated and used up by the body.

b) Spermatogenesis: In this process, the spermatogonium, a stem cell is deposited at the time of birth and forms sperms in adolescence. The spermatogonium, the diploid cell undergoes mitosis to increase in number and further continue to undergo meiosis to form haploid sperms.

Gamete transfer:

The gametes must be brought in contact with each other to achieve fertilization. The gametes are transferred to the region of fertilization, as a result of which, the haploid gametes fuse to form a fertilized diploid egg. In most organisms, sperm is motile, and the egg stays within the organism. All the sperms do not reach the egg, hence they are produced in millions of numbers, so fertilization can be achieved.

In plants, pollination is involved in the transfer of pollen present on anther, containing the male gamete into the ovary of the plant containing the egg. It is easier in self-pollinating plants, as the anther and stigma are placed close to each other. In cross-pollinating plants, the pollen on anther of one plant is transferred to the stigma of the other plant of the same species, which is usually achieved by bees or by the wind.

Fertilization Stage

It is the process of fusion of male and female gamete, resulting in the formation of fertilized egg or zygote, a pre-cursor to embryo which usually forms inside the female organism. Sometimes, the female gamete undergoes the transformation into an embryo and then the offspring without fertilization, it is called parthenocarpy, bearing a seedless variety of fruits or vegetables in plants. In animals, it is called parthenogenesis, observed in certain insects and lizards.

Post-fertilization Stage

This stage is marked by the formation of a zygote.

Zygote formation: Zygote is the fertilized egg, which is diploid in nature in all sexually reproducing organisms. It is either formed on a medium such as water, in case of external fertilization and inside the female organism, in case of internal fertilization. The zygote is a precursor to an embryo.

Embryogenesis: It is the development of the embryo from the zygote. The zygote undergoes several mitotic divisions to form specialized cells, which eventually transform into organs and organisms. Cell division and cell differentiation are the key events in this stage. Cell division increases the number of cells in the embryo and these cells become further specialized based on the structural and functional organization of the organism. Cell differentiation helps in the complete development of the organism. In lower organisms, a thick wall is formed around the cell wall to prevent desiccation or damage to the cell in the period of rest, the post which they germinate by dissolving the wall.

In the case of plants, the zygote is formed inside the ovule of a flower. The ovule develops into seeds while the zygote develops into embryos and the ovary forms the fruit. The outside protective layer called the pericarp develops on the surface of the fruit. Seed dispersal is one of the ways of plant propagation.

Advantages & Disadvantages of Sexual Reproduction

Advantages:

1. Genetic variation can be achieved within the species level.
2. Resistance to diseases is possible due to immunity in the organisms.
3. Adaptability to evolution is a prime feature.

Disadvantages:

1. Certain genetic features may be lost due to genetic variation
2. Slow process, hence less offspring can be produced compared to asexual reproduction.
3. The rate of success is not 100%, since one of the organisms involved in fertilization may be sterile or unproductive.

Reproduction in organisms has evolved over time and it has made way for several kinds of research and discoveries leading to prominent solutions and a better way of living.

FAQs

Here are some of the frequently asked questions on reproduction in organisms:

Q1: Explain why meiosis and gametogenesis are always interlinked.
Ans: Meiosis is the process that occurs during gametogenesis. Gametogenesis refers to the process of how gametes are formed. Meiosis is necessary for the formation of gametes. Hence, both the processes are said to be interlinked.

Q2: What is vegetative propagation? Give two suitable examples.
Ans: Vegetative propagation is a common form of asexual reproduction observed in plants. In this, a fragment of a plant is used to grow another plant. Many plants reproduce naturally as well as artificially by vegetative propagation and the offspring produced are genetically identical.

Q3: Define external fertilization. Mention its disadvantages.
Ans: External fertilization is a mode of reproduction characterized by the fertilization of male and female gametes outside the body of the organisms. External fertilization is observed in amphibians such as frogs and toads. However, there are a few drawbacks of external fertilization:
i) The chances of survival of the gametes are very less.
ii) Not all gametes are fertilized.
iii) The gametes might desiccate.
iv) The predators usually eat the eggs.

Q4: How does the progeny formed from asexual reproduction differ from those formed by sexual reproduction in organisms?
Ans: The progeny formed by asexual reproduction involves a single parent and are genetically identical to the parent whereas the progeny formed by sexual reproduction are formed when male and female gametes fuse together and are genetically unique.

Q5: Why is reproduction in organisms essential?
Ans: Reproduction is essential for the continuity of species on earth. If the organisms do not reproduce, life will cease to exist.

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