Describe the various methods of reproduction in Bacteria.

Q. Describe the various methods of reproduction in Bacteria. 
Or, Describe the methods of multiplication in bacteria. 
Ans. Asexual Reproduction
1. By Cell Division :
Binary Fission: Bacteria is generally divided by binary fission producing two daughter cells of equal size. The bacterial division is visibly initiated by the ingrowth of cytoplasmic membrane, often in association with a mesosome. This process is accompanied by ingrowth of wall and eventually leads to the formation of complete septum which is thicker than the ordinary cell wall. Cell separation begins by cleavage of the septum progressing from its periphery towards its centre.
Nuclear division: The bacterial chromosome (the circular DNA molecule) divides to form 2 circular chromosome. The bacterial cell does not require a mitotic spindle because it does not have a set of chromosomes to be segregated into the two daughter cells. Nevertheless, the daughters of the single bacterial chromosome must be regularly segregated.
Jacob and Ryter have provided evidence that this process does not depend simply on migration toward the two poles of the cell before septum formation. Rather, serial sections showed that the nuclei in a bacillus are regularly attached to the cytoplasmic membrane often via mesosome. It was therefore suggested that the mesosomes divide along with the attached nucleus. Then in cell division the two progeny mesosomes, through their attachment to the plasma membranes, migrate in opposite directions from the site of septum formation carrying the two nuclei with them. In this way the cytoplasmic membrane serves as a primitive, mitotic apparatus, with mesosome substituting from the mitotic spindle. Moreover the mesosomes are also associated with spetum formation, migration of a nucleus and mesosome to the middle of each daughter cell might relate to the initiation of septum formation at the site at the next cell division.
2. By Budding: In some species of bacteria (e.g. Hyphomicrobium) a bud is given out, which on separation develops into a new bacteria.
3. By Spore Formation : Certain gram positive rods (bacillus. Clostridium) have developed a powerful, ecialized mechanism for survival through hard times, the formation of spores. These cells, like the seeds of higher plants or the cysts of Protozoa, are in a state of cryptobiosis with no metabolic activity and they exhibit a marked increase in resistance to the lethal effect of heat, drying, freezing, chemicals and radiation. Thus the survival of a vegetative culture on drying is statistical, the survivor being a small fraction of the population and their number decreases rapidly on storage. Bacteria spores in contrast, survive drying quantitatively and die very slowly on storage.
Resistance to heat has been the most conspicuous property of spores and spore forming species are readily isolated from natural materials by pasteurizing the sample before inoculation.
Bacteria spores are endospores formed by a process in which a vegetative cell gives rise within itself another cell of novel structure and composition. Sporulation thus represents differentiation in an unicellular organism and it is attracting increasing attention as a model for cell differentiation in general.
Spores are usually dehydrated cells, whose refractility and physical density are close to those of fully dried proteins. They also do not take ordinary stains and they are resistant to disinfectants.
The electron microscopic study of nature of spore reveals a very thick envelope which occupies nearly 1/3 the radius and 1/2 the volume of the spore. Several layers can be distinguished.
(i) Surrounding the core is the innermost layer, the thin spore wall or membrane. On germination it gives rise to the membrane and wall of the new vegetative cell; but in the spore it is much thinner and is usually difficult to resolve into a membrane and a wall component.
(ii) Next is the thickest layer, the cortex, when unstained is less dense than the other layers.
(iii) Outside the cortex are two coats, which frequently separate to produce an artificial gap. The inner coat appears to be thin, laminated boundary of the cortex. The outer coat is the dense structure of the unstained spore and presumably are further loosely surrounded in a delicate exosporium of unknown function.
In coccus forms during unfavourable conditions protoplasm dries up and bacterial cell is termed as orthospores.
Sexual Reproduction: 
1. By Conjugation: The fusion of the chromatin matter of two related species or types of bacteria is known as conjugation and resembles a reduced sexual process. The transmission of chromatin particles from the donor cell to the recipient cell occurs in a defined direction and with a known frequency. It was first discovered by Leaderberg and Tatum in Escherichia coli in 1946.
At present conjugation is considered to be a one side transport of genetic material from one cell to another. The presence of specific fertility factor (designated as F) in one conjugating cell is a necessary condition for conjugation.
There are two mating types of cells in E.coli; one is donor (male) and the other recipient (female). When a suspension of donor cells is mixed with recipient cells, a conjugation bridge develops between the two conjugants at the point of contact. The donor then injects its chromosome into the recipient and dies.
Conjugation can occur among cells of different species. Conjugation between Escherichia coli and dysentery bacteria leads to the most varied changes in bacteria known as typical strains.
The process of conjugation can be controlled and definite characters can be made to pass from one bacterial cell to the other.
2. Other Methods: In bacteria, the sexual reproduction (karyogamy followed by meiosis) is not the only method of genetic recombination but they employ several novel methods. There exists mechanisms of genetic transfer in bacteria other than conjugation.
(a) Transformation : Discovered by Griffith (1928), the DNA is absorbed from the external medium. In this process, DNA of one strain of bacterium is freed from the cells and directly enters into another cell. DNA moves from cell to cell in solution. In general, transformation is intercellular transfer of a fragment of DNA from the donor genome which fragment through a process of recombination replaces a particular gene of the recipient genome.
(b) Transduction : Reported by Lederberg and Zinder’s (1952) by his experiments on Salmonella. The bacterial viruses or phages are responsible for the transfer of genetic material. Thus, it is a phage-mediated genetic transfer.
(c) Bacterial Sexduction : Episomes responsible for the transfer of genetic material.
(d) Lysogeny: It involves association of genetic material of a virus with that of the bacterium. It also provides a permanent genetic modification of the bacterial genome.
When the DNA enters the recipient cell by any of these mechanisms, it can synapse with the homologous region of the recipient genome and undergo recombination to give new genome types.

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