Molecular genetics, an important area within Biology Topics, focuses on the structure and function of genes.
Describe Various Modes of Asexual Reproduction in Animals?
So far we have learnt the reproduction in human beings and some other familiar animals which takes place by the process called ‘sexual reproduction’. We will now study the reproduction in very small animals like Amoeba and Hydra. Amoeba and Hydra are the tiny animals which reproduce by the process of asexual reproduction. Asexual reproduction is the production of a new organism from a single parent without the involvement of sex cells (or gametes). The new organism produced by asexual reproduction is exactly identical to the parent. The two most common methods of asexual reproduction in animals are :
- Binary fission, and
We will now describe these two methods of asexual reproduction in animals, one by one. Before we discuss the method called binary fission, we should know the meaning of the terms ‘fission’ and ‘binary fission’ as used in biology. In biology, fission is the process of asexual reproduction in unicellular organisms (or single- celled organisms). In the process of fission, the parent organism splits (or divides) to form two (or more) new organisms. Fission begins with the division of the nucleus followed by the division of cytoplasm to form new organisms.
Fission is of two types : binary fission and multiple fission, depending on whether the parent organism splits to form two new organisms or more than two organisms. In this Class, we will study only the binary fission in detail. Now, the word ‘binary’ means ‘two’ and the word ‘fission’ means ‘splitting’. So, the term ‘binary fission means ‘splitting into two’. Let us now discuss binary fission as the method of asexual reproduction in a microscopic animal called Amoeba.
(i) Binary Fission
Binary fission is an asexual method of reproduction in organisms. In binary fission, the parent organism splits (or divides) to form two new organisms. When this happens, the parent organism ceases to exist and two new organisms come into existence. The unicellular organism (or unicellular animal) called Amoeba reproduces by the method of binary fission. This is described below.
Amoeba reproduces by binary fission by dividing its body into two parts. This happens as follows: When the Amoeba cell has reached its maximum size of growth, then first the nucleus of Amoeba lengthens and divides into two parts. After that the cytoplasm of Amoeba divides into two parts, one part around each nucleus. In this way, one parent Amoeba divides to form two smaller Amoebae (called daughter Amoebae). And we say that one Amoeba produces two Amoebae. The reproduction in Amoeba by binary fission is shown in Figure.
Amoeba takes about an hour to divide into two daughter Amoebae. The two daughter Amoebae produced here grow to their full size by eating food and then divide again to produce four Amoebae, and so on. The daughter Amoebae produced by the process of binary fission are identical to the parent Amoeba.
From the above discussion we conclude that Amoeba reproduces by dividing itself into two. This type of asexual reproduction is called binary fission. Another tiny, unicellular animal called Paramecium also reproduces by the asexual method of binary fission. In multiple fission, the parent organism splits (or divides) to form many new organisms at the same time. We will discuss this is detail in higher classes.
Before we discuss the next asexual method of reproduction called ‘budding’, we should know the meaning of the term ‘bud’. The ‘bud’ here means a ‘small outgrowth’ from the body of an organism (say, an animal). When a bud is formed on the body of an organism, then the nucleus divides into two, and one of the nuclei passes into the bud. Let us discuss the method of ‘budding’ now.
Budding is an asexual method of reproduction. In budding, a small part of the body of the parent organism grows out as a ‘bud’ which then detaches and becomes a new organism. The asexual reproduction by budding is observed in animals like Hydra, sea-anemones, sponges and corals. We will now describe the asexual reproduction in Hydra.
Hydra is a simple multicellular animal [see Figure (a)], Hydra reproduces by the process of budding. This happens as follows: In Hydra, first a small outgrowth called ‘bud’ is formed on the side of its body by the repeated divisions of its cells [see Figure (b)], This bud then grows gradually to form a small Hydra by
developing a mouth and tentacles [see Figure (c)]. And finally the tiny new Hydra detaches itself from the body of parent Hydra and lives as a separate organism [see Figure (d)], In this way, the parent Hydra has produced (or created) a new Hydra. Thus, Hydra reproduces asexually by growing buds from its body. This is called budding. Please note that the bud formed in a Hydra is not a single cell. It is a group of cells. If we collect some pond water and observe it through a hand magnifying glass, we will see a number of Hydrae with small buds attached to their body.
From the above discussion we conclude that in the tiny animal called Hydra, new Hydrae develop from the buds. This method of asexual reproduction in Hydra is called budding. Apart from binary fission and budding, there are some other methods of asexual reproduction by which a single parent produces young ones. We will study these methods in higher classes. Please note that since asexual reproduction involves only a single parent,
so fertilisation is not necessary in asexual reproduction.