Asexual Reproduction
Definition:-Production of offspring by a single parent without the formation and fusion of gametes is called Asexual reproduction. The youngones received all the genes from its single parent.Asexual reproduction is also known as agamogenesis or agamogeny, due to mitotic cell division it is also termed as somatogenic reproduction.
Clone ;- Asexual Reproduction produces identical offspring .so, they are commonly referred as clones.
Occurrence:- Asexual Reproduction occurs most commonly in unicellular organisms such as, Monerans and Protista, and in plants and some animals also.It doesn't occur in highest non vertebrates and all vertebrates.
Types of Asexual Reproduction
Asexual Reproduction occurs in different ways in different organisms. Some common types are discussed below:- Binary Fission. Multiple Fission, Budding, Spore formation
1.BINARY FISSION:-
It is the division of parent into two small , nearly equal sized daughter individuals.
EXAMPLE 1 :- Binary fission in bacteria-
It is the most common method of asexual reproduction in actively growing bacteria. The cytoplasm and nucleoid divide into two without mitosis. During the division of nucleoid, the DNA replicates while attached to mesosomes. A new mesosome develops which get attached to the daughter chromosome. At the same time the cell wall and plasma membrane starts growing transversely inwards from the middle of the dividing cell between the two mesosomes The dividing septum originates centripetally and separates the parent cell into two nearly equal daughter cells, each having a nuclear body. These cells may separate from each other or may remain attached for some time to form chain.
Under favourable conditions, the process of binary fission completes in about 20-30 minutes. Thus, a very large number of bacterial cells are produced in just 24 hours.
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EXAMPLE 2 Binary fission in Protozoan Protists:
In protozoans, The replicated chromosomes are separated by intranuclear(closed) mitosis and the nucleus divides by furrowing. Then the cytoplasm gradually constricts between the two separating nuclei, ultimately forming two equal sized daughter individuals, each having nucleus. the offspring grow to the size of the parent before dividing. e.g it occurs in Amoeba, Paramecium, Euglena and Trypanosoma.
It is explained as the three types of binary fission.
Binary fission is of 3 types with regards ti the plane of division:
- Irregular
- Longitudinal
- Transverse
i) Irregular Binary Fission:-
It occurs in amoeba . Plane of division of cytoplasm is variable , but always in right angle to the elongated dividing nucleus.
ii) Longitudinal Binary fission:-
It occurs in flagellates such as Euglena. The cytoplasm splits lengthwise from before backward , forming two similar daughter individuals.
iii) Transverse Binary Fission:-
It occurs in ciliates such as paramecium. The cytoplasm divides transversely between two sets of nuclei, forming two dissimilar daughter individuals.\
Example 3 Binary Fission in Planarians
Planarians divide by transverse binary fission. The worms fixes the posterior part of the body firmly to the substratum and moves the anterior part forward. Thus, the anterior part exerts a pull over the posterior part, which holds on . Due to the pull, middle part is stretched, constricts and finally breaks. The anterior part regenerates the posterior part , and the posterior part regrows the anterior part. Thus, two complete worms are formed.
