Bacteria falls under the monera kingdom of R. H. Whittaker five kingdom classification. All members of monera kingdom are single-celled organisms (unicellular). These are considered plants and not animals.
| Domain | Bacteria |
| Kingdom | Monera |
| Cell type | Prokaryotic |
| Cell wall | Noncellulosic (Polysaccharide + amino acid) |
| Nuclear membrane | Absent |
| Body organization | Cellular |
| Mode of nutrition | Autotrohic (chemosynthetic and photosynthetic) and hetrotrophic (saprophytic / parasitic) |
| Mode of reproduction | Conjugation (Sort of sexual reproduction) |
Prokaryotes
These are the organisms that lack well-defined nuclei but genetic material is embedded in the cytoplasm. An example of a prokaryote is a bacterium.
Bacteria are mostly single-celled plants and usually are spherical or rod-like in shape. They occur almost everywhere, in soil, foodstuffs, water, air as well as in the human body.
They are predominantly heterotrophs. The shape is relatively constant in most bacteria. They vary in size from 1 to 5 microns in length and 0.5 to 1.0 microns in width.
They show relatively little variations in morphological and cytological features. They may be spherical, straight, curved, or rod-like in shape.
In most bacterial species the cell shape is constant. With the light microscope, only cell shape and size are visible. The cell wall varies among different bacterial groups in structure as well as stainability.
The gram stain is used as an aid in the identification of bacteria. With this crystal violet dye and iodine solution, all bacterial cells stain blue.
When this smear is treated with an organic solvent such as ethyl alcohol, the Gram-positive bacteria retain the blue color whereas the others lose it and are termed Gram-negative bacteria.
The nucleus in bacterial cells is lacking, instead, the bacterial cells had one or more nuclear bodies which appear globoid, dumbbell-shaped, or helical.
A strand of DNA embedded in a matrix is founded in the nuclear body which is devoid of nuclear membrane or nucleoli. In E. coli a single chromosome that consists of two chains of DNA wound around each other is present.
The DNA strand is continuous i.e it is ring-shaped and bears hereditary units. Flagella that occur singly or in tufts are found over the surface of the cell. In E. coli, flagella (120-150 Å) are irregularly placed on the surface of the cell.
Sex pili
Male cells have sex pili which are 30-50 Å in diameter. The sex pili help in attaching the male cell to the female cell during sexual reproduction. These are minute hair-like appendages extending from the cell wall of male bacterial cells. During sexual reproduction, the male cells become attached to female cells by sex pili.
Reproduction
Bacteria mostly reproduce by binary fission. The nuclear body first divides which is followed by a wall that develops from the periphery inward and thus divides the protoplasm. Thus the two cells are formed. This type of division may take place every 20 minutes under ideal conditions.
Continuous binary fission in each bacterium will result in a mass equivalent to 2000 tonnes in 20 hours at a normal rate of division.
Fortunately, factors like depletion of nutrient supplies, competition with other organisms, temperature, etc. do not let them grow at this rate.
Sexual reproduction
Tatum and Lederberg observed sexual reproduction in bacteria. Later on, the electron micrographs confirmed this type of reproduction in E. coli. A bridge is formed between two compatible cells. The male cells are distinct from female cells in having the sex pili which are absent in the recipient cells. At this time these male and female cells are haploid.
The genetic material moves through the bridge from the male to the female cell. The “chromosome” of the male cells is transferred to the female cell. As duplicate genes are found in the female cell, it is termed diploid cells although this is not a true diploid condition as in the higher forms. So it does not exhibit the phenomenon of alternation of generation.
In transformation, the DNA of the male initiates inheritable changes in the female cell. Transformation involves only one factor. Transduction is another phenomenon shown by E. coli. The viruses called phages infest cell bodies which may incorporate into the structure of bacterial genetic code.
Transformation and transduction involve a unilateral transfer of genetic material. This is different from sexual reproduction as no Zygote is formed as meiosis occurs.
Conjugation
It is a sexual method of reproduction in which a cytoplasmic bridge is formed between the male and female cells. Through this, a part of the chromosome from the male is introduced into the female cell.
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