Reproduction in Bacteria

Binary Fission

It is the most common mode of asexual reproduction. The cytoplasm and nucleoid of a bacterial cell divide equally into two, following replication of DNA. The cell wall and cytoplasm also split resulting in the formation of two daughter cells.

Under favourable conditions, a bacterial cell divides by fission once in every 20 minutes.

fig. 8.4 - Fission in Bacteria

Sexual Reproduction

In bacterial sexual reproduction there is no meiosis, formation of gametes and zygote. Instead, it involves transfer of a portion of genetic material (DNA) from a donor cell to a recipient cell. This process is called as genetic recombination or parasexuality. It is known to occur in the following three ways:

Transformation

In this process, one kind of bacterium is transformed into another kind. It takes place by a transfer of DNA from a capsulated bacterium into a non-capsulated bacterium. It has been observed in Diplococcus bacteria.

Transduction

In this process, DNA of a bacterial cell (donor) is transferred into another bacterial cell with the help of a bacteriophage. This process is known to occur in several bacterial species such as Salmonella, Escherichia, Micrococcus and Stigella.

Conjugation

It is a process in which the genetic material of a bacterial cell of a particular strain is transferred into that of another bacterial cell of a different strain. Of the two strains of bacteria involved, one acts as donor (or male) and the other as a recipient (or female). The donor cells are known to possess a sex factor or fertility factor (F factor) as a component of its circular DNA (F+ strain). The recipient cell does not have this factor and hence it is described as F- strain. A conjugation between cells of F+ and F- strains always results in the formation of F+ bacterial cells in the progeny.

fig. 8.5 - Genetic Recombinations in Bacteria

Sporulation

Endospore Formation

In certain bacteria like Clostridium and Bacillus, the cells tide over unfavourable conditions by forming endospores. During this process, a portion of the cytoplasm and a copy of the bacterial chromosome undergo dehydration and get surrounded by a three-layered covering. The remaining part of cytoplasm and cell wall degenerate. The resulting structure, called endospore can tolerate extreme environmental conditions and can remain viable for several years. When the environmental conditions are suitable, the endospore absorbs water, swells and the wall splits, releasing the cell inside. It develops a new cell wall and starts functioning as a typical bacterial cell.

Endospore formation is not a method of reproduction. It is only a method meant for tiding over unfavourable conditions and allowing dispersal of the bacterial cells into new habitats.

fig. 8.6 - Endospore of a Bacterium

Other Types of Spores

Formation of sporangiospores, conidia, and cysts.