Until the 1940s, any idea of a sexual reproduction in bacteria was considered ridiculous. In fact, before their prokaryote cell structure was discovered, these organisms were placed in fungi. These methods of asexual reproduction are common in actinomycetes and are same as seen in the fungi. Cysts are formed in Azotobacter and myxobacteria, e.g., Myxococcus, Chondromyces, etc.įragmentation, Sporangiospores, and Conidia: When the cyst wall breaks, the enclosed cell moves out. The whole cell is transformed into a cyst. These are less resistant than endospores. The presence of Fe 3+ and Ca 2+ increases the heat stability of the endospores. (3) The presence of certain cations, particularly Mn 2+ and K +. (2) Relatively high concentrations of oxygen, and (1) Low concentrations of organic and nitrogenous nutrients, The environmental factors that favour endospore formation include: The endospores can survive in boiling water for as long as one and half hours. About 15 per cent of the dry weight of the endospores is due to dipicolinic acid and calcium, which provide extreme resistance to heat. Thus, a new cell is formed within the parent cell, which is then called a sporangium. Additional walls are laid down around this septum, which are called cortex, and outer and inner walls. A septum is laid down around the concentrated genome. Only a single spore is formed in a cell and the whole cell is not consumed in its production. It is an extremely resistant asexual spore found only in genera Bacillus and Clostridium. Many irregular divisions in all planes result in a cluster of cells e.g., Staphylococcus. Three divisions each at right angles result in cubical structure e.g., Sarcina.ģ. Two divisions at right angles result in groups of 4 cells e.g., Tetracoccus.Ģ. When more than two divisions occur in the same plane, a chain of cells results e.g., Streptococcus.ġ. If two divisions occur in the same plane, without separation of daughter cells, then a pair of cells is formed e.g., Diplococcus.Ģ. In the spherical forms (cocci), divisions may occur in any plane, but in bacilli, divisions occur perpendicular to the long axis of the cell.ġ. However, in actinomycetes, cell division occurs without separation and thus the cells are not separated. The cell division is followed by separation of the daughter cells as unicellular bacteria by dissolution of the matrix between the newly synthesized walls. The two events i.e., septum formation and the cross wall synthesis occur simultaneously. Wall material is deposited between the two membranes of the septum. This mechanism of replication was proposed by Cairns in 1963 and is known as the Cairns model.Ī peripheral ring of plasma membrane invaginates and grows centripetally to form a double membrane septum. This type of replication, in which one strand is old and the other is newly synthesized, is called the semiconservative replication. In both the daughter ‘chromosomes’, one strand is derived from the parent and the other is new. It is that point where the two strands become four. The molecule is duplicated at a site that traverses the whole molecule. The bacterial ‘chromosome’ (the circular DNA molecule) divides (replicates) resulting in the production of two circular ‘chromosomes’. Both the events occur simultaneously and are triggered by a mesosome. Fission involves formation of a septum and chromosome division (i.e., replication of genome). In 6 hours a million bacteria are produced from a single parent cell. Division is a very quick process and is completed in 30 minutes. Mostly it is binary fission in which the two daughter cells are identical. The bacterial cell divides into two by fission. The sexual reproduction is represented by transformation, conjugation, transduction and lysogenic conversions. The bacteria reproduce asexually by fission, endospores, cysts, fragmentation, sporangiospores, and conidia. The novel methods of sexual reproduction that occur in bacteria, though different from the classical type involving karyogamy and meiosis, bring about the same end result viz., genetic recombination. The cell division by ‘fission’ is characteristic of bacteria and is shared by none except some yeast (Schizosaccharomyces). Bacteria multiply by all the known methods of reproduction.
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