6.6 Growth and Reproduction in Bacteria
Growth in bacteria means an increase in the total population rather than an increase in the size of the organism. Their growth is very fast and depends on suitable temperature, availability of nutrients, pH, and ionic concentration; If conditions are favorable then most bacteria divide after every 20 minutes, e.g., E.coli. The interval between two I species. successive divisions is known as generation time. It is different in e
6.6.1 The Growth Phases of Bacteria
There are following four phases of growth in bacteria. for coming division i.e., Lag phase (no growth): Bacteria prepare themselves to adapt to their new environment and growth has not yet achieved its maximum rate.
Log phase (rapid growth period): Fast growth occurs at this phase. In human, the disease symptoms develop during the log phase because the bacterial production attains such a high level which damages the tissues.
Stationary phase (equal birth and death rate): After the log phase, the growth slows down because of a shortage of nutrients. Thus the rate of reproduction and death of bacteria becomes equal.
Death phase (decline phase): In this phase death rate increases and the reproduction rate decreases. It is due to the exhaustion of nutrients and the accumulation of toxic wastes.
6.6.2 Reproduction in Bacteria
Bacteria reproduce both asexually and sexually. Asexual Reproduction (Binary fission): All bacteria reproduce asexually using binary fission. There is a single chromosome, having a circular DNA molecule. First DNA is replicated and attached to the plasma membrane. After duplication, the two chromosomes move towards their respective sides.
The plasma membrane pushes inward at the center of the cell. The cell wall grows inwards to separate both daughter cells from each other thus two daughter bacteria are formed. In most bacteria, it takes 20 minutes, if conditions are favorable.
- Cell elongates DNA is replicated
- The cell wall and plasma membrane begin to constrict
- Cross-wall falls completely separating the two DNA
- Cell separate
Sexual Reproduction in Bacteria:
Bacteria lack traditional sexual reproduction (gametogenesis). However, bacteria exhibit genetic recombination that is cells do not fuse, and only a piece of DNA or plasmid of the donor cell is inserted in the recipient cell. This process occurs by conjugation, transduction, and transformation.
Conjugation:
It is the process by which one bacterium transfers genetic material to another bacterium through a tube- formed by pili called a conjugating tube or bridge. The bacterium that gives the DNA is called the donor and the bacterium that receives DNA is called the recipient. This process was. first studied experimentally by Lederberg and Tatum in 1946 in E. coli.
Later studies made with the help of an electron microscope confirmed the close contact and the formation of a conjugatory bridge between the bacterial cells. The process of conjugation may be explained in the following st S: The donor cell produces the pilus, which is a structure and begins contact with a recipient cell. projects out of the cell and recipient cell.
She pilus enables direct contact between the SpThe donor plasmid consists of a double-stranded DNA molecule forming a circular structure, it is attached at both ends, and an enzyme picks one of the two DNA strands of the donor plasmid and this strand is transferred to the recipient cell.
Sp-4: In the last step the donor cell and the recipient cell both containing single-stranded DNA, replicate this DNA and thus end up forming a double-stranded DNA.
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