Saturday, 30 January 2021

Reproduction in Bacteria by Binary Fission (steps)


When the bacteria are inoculated in to a nutrient medium and incubated, they grow and Increase in number. Bacteria reproduce mainly by transverse binary fission. However, some bacteria show unusual methods of reproduction.

Reproduction in Bacteria by Binary Fission :

This is the most common mode of reproduction in bacteria. where a single cell divides in to two daughter cells of equal size. A cell just needs to grow to its double the size and then splits in to two daughter cells after development of a transverse septum. The cell division is preceded by chromosome replication, so that each daughter cell gets a copy of genome.

Events leading to new cell formation :

The new cell formation Initiates by the septum formation in the centre of the cell, when the cell reaches to a maximum size and cell mass. This is followed by formation of two daughter cells. The steps leading to binary fission are as under:

1). Septum formation, leading to cell division starts only after the completion of chromosomal replication.
2). Inward growth of cytoplasmic membrane starts, at the middle of the cell. Mesosomes play important role in this event.
3). Now the new cell wall material deposits along the Invigilating cell membrane. This results into septum formation and formation of two daughter cells.
4). In case of cocci, the septum formation starts at the equatorial ridge in the cell wall. In this case, the new cell wall material is approximately one half of the total.
5). In case of gram-positive rods, the septum formation occurs in the same manner. Only 15% of the new wall is dertved from septum. The remainder is formed along the cylindrical part of the cell. This is because the cell grows by elongation.
  In the cell, the new cell wall material is being added as the inner layers. During growth, outer old layers get spread thinly and ultimately destroyed by degradative enzymes.

Septum formation in the gram-negative becteria.
a) Cell wall layers of bacteria. b) Formation of fold on outer membrane at a site where septum will be formed. c) Downward synthesise of cell membrane and peptidoglycan layer, resulting in septum formation. d) Completion of septum formation. e) Invagination of outer membrane, followed by cell division. 

6). In gram-negative bacteria, a fold appears in the outer membrane, at which the septum formation starts. Cell membrane and peptidoglycan layer grow inward and ultimately divides the cell. The outer membrane Invaginates only at the linal stage of cell division.
7). It is proposed that central mesosome plays important role in cell division. Genome of cell remains attached to mesosome. During binary fission, the mesosome divides and the genome replicates. Hence, each daughter cell gets complete genome.


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