Ans. sexual reproduction does not involve the fusion of gametes. There are many types of asexual reproduction, all producing individuals that are genetically identical to each other and to the parent.
1- Binary Fission
Binary fission means "division into two". It is the simplest and most common method of asexual reproduction.
Examples
It occurs in prokaryotes (bacteria), many unicellular eukaryotes e.g. protozoa and in some invertebrates.
Reproduction
Binary Fission in Prokaryotes
During binary fission in bacteria, the DNA is duplicated and so two copies of ONA are formed. The two copies move towards the opposite poles of cell. The cell membrane invaginates in centre and divides the cytoplasm into two. New cell wall is deposited between two cross membranes. It results in the formation of two daughter bacteria, which grow in size'and divide again.
Binary Fission in Protozoans
During binary fission in unicellular eukaryotes, the nucleus of parent organism divides into two (by mitosis). It is followed by the division of cytoplasm. So two daughter cells of almost equal size are formed. Daughter cells grow in size and then divide again. Figure 14.1
Binary Fission in Invertebrates
Some invertebrates also reproduce asexually through binary fission. During this reproduction, body is cut into two halves (fission) and the missing body parts are regenerated in both halves. This type of asexual reproduction is common in planarian and many echinoderms. Figure 14.2
2- Multiple Fission
Some unicellular organisms (e.g. Amoebae) form hard walls called cysts around them, under unfavorable conditions. When favorable conditions return, the nucleus of parent divides into many daughter nuclei by repeated divisions. This is followed by the divisions of cytoplasm into several parts. Each new part of cytoplasm encloses one nucleus. So a number of daughter cells are formed from a single parent at the same time. This kind of fission is known as multiple fission. Figure 14.3
3- Fragmentation
As certain worms to full size, they spontaneously break up into. 8 or 9 pieces Each piece (fragment) develops into a mature worm, and the process is repeated.If a planarian breaks into many pieces instead of two, it will also be called as fragmentation.
4-Budding
In this type of asexual reproduction, a bud develops as a small outgrowth on parent's body.
Budding in Fungi
In case of yeast (a unicellular fungus) a small bud on one side of cell grows. The
nucleus of cell divides and one of the daughter nuclei is passed into the bud
Parent cell may form more than one bud at a time. Each bud enlarges . and develops the characteristics of parent organism. The bud may separate from parent body. In some cases, the' buds never separate
Reproduction
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and as a result, colonies of individuals are formed. Figure 14.4
Budding in Hydra
Animals such as sponges, Hydra and corals also reproduce by means of budding. In them, a small bud is formed on the side of body, by mitosis. This bud enrages by the formation of more cells. It then detaches from the parent body and grows into new organism. Figure 14.5
Budding in Corals
In corals, the buds do not detach from the parent body. Corals form big colonies, because the bud grow into new organisms by remaining attached to the parent body.
Spore Formation
Spore Formation in Fungi
It is generally seen in most fungi (e.g. Rhizopus) When Rhizopus reaches reproductive age, its body cells form thick walled spore sacs called sporangia (sing Sporangium).Inside each sporangium, a cell divides many times and forms many daughter cells called spores. Each spore is covered with a thick wall called cyst which
can survive unfavorable conditions. When sporangia are mature, they burst and release spores Under favorable conditions. The spores germinate and develop into new Rhizopus. Figure 146
Figure 14.5 Budding in Hydra
Spore Formation in Bacteria
Under unfavorable conditions, some species of bacteria reproduce by forming spores, e.g. Clostridium and Bacillus species. The bacterial spores are also thick-walled. They are formed inside bacterial celts, so are called endospores. Figure 14.7