Binary Fission - Types of Asexual Reproduction
Binary fission is a procedure in which an organism divides mitotically into two equal individuals which are generally identical to each other. This process is widespread in protozoans. Amoeba, Euglena and Paramecium are the most familiar instances. All the three are freshwater forms. Amoeba is an irregularly shaped unicellular organism. While full grown and when the conditions are favorable it slows down its movements and generates short fine radial pseudopodia. The contractile vacuole stops functioning and may disappear. The nucleus elongates, becomes dumb-bell shaped and goes through mitotic division. The cell constricts in the middle to finally divide into two smaller daughter amoebae each possessing one daughter nucleus. Consequently, the pseudopodia become normal and a new contractile vacuole is formed in each. The whole process of fission in Amoeba may take about 30 minutes at moderately warm temperature. Amoeba thus divides regularly, approximately every 24 hours.
Euglena, a freshwater flagellate, divides longitudinally (longitudinal binary fission). The type of division in it is said to be symmetrogenic that is, producing symmetrical or mirror-image daughter cells. The centrioles first divide into two and the nucleus then starts to divide. Each centriole produces a new basal body and a new flagellum. The contractile vacuole as well divides into two. As mitosis (nuclear division) reaches completion the cytopharynx (gullet) also begins to divide and finally when the organelles have duplicated the anterior end begins dividing as a fork, which deepens posteriorly to ultimately produce two daughter Euglena. Paramecium divides transversely into an anterior half and a posterior half. The fission here is called homothetogenic, meaning looking like the letter "Θ" i.e., "theta" of Greek alphabet. The macronucleus divides amitotically and the micronucleus divides mitotically. A second cytopharynx is generated by the posterior half. Two new contractile vacuoles are formed one going to each half. In the meanwhile a constriction is formed at the middle that deepens to finally separate the two independent daughter paramecia. The entire process of binary fission in Paramecium may take about two hours. As per to one estimate as many as 600 generations can be produced in a single year. It may be seen that all the progeny so produced from a single original parent as a result of binary fission either in Amoeba, Euglena or in Paramecium have the same genetic composition. They are similar in all respects. The term clone is used to refer to such a population. - Plasmotomy is a variant form of fission found in some multinucleate protozoans such as Opalina, a ciliate that lives as a commensal in frog's rectum. In this kind of asexual reproduction the multinucleate protozoan divides into two or more parts without any nuclear division and the existing nuclei are distributed between the new individuals. The new, still multinucleate, daughter opalinas produce more nuclei as they grow.