GROWTH AND REPRODUCTION OF BACTERIA
The increase in number of bacterial cells is called as bacterial growth.
This increase in number occurs by asexual reproduction.
In bacteria asexual reproduction occurs by means of binary fission.
The division of a prokaryotic cell into two identical daughter cells is called as binary fission. Following processes occur in binary fission:
- Parent cell enlarges,
- Its chromosome duplicates,
- Plasma membrane pinches at the center of the cell.
- When nuclear material has been equally distributed, the cell wall inward to separate cell into two.
Repeating the Process:
This sequence is repeated at intervals by each new daughter cell. As a result the population of cells increases: Once the division is complete, bacteria grow and develop their specific features.
The time interval until the completion of next division is known as generation time. In the fastest growing bacteria such divisions may occur as often as every 20 minutes.
Bacterial Growth Curve:
Four different phases are known in bacterial growth curve.
It is phase of no growth. Bacteria prepare themselves for division.
It is phase of rapid growth. Bacteria divide at exponential rate.
Bacterial death rate is equal to the bacterial rate of reproduction and multiplication.
Death / Decline Phase:
Bacteria start dying. Death rate is more than reproduction rate.
- In 1946 it was discovered that bacteria can take part in a primitive form of sexual reproduction. The usual process of sexual reproduction and mitosis is absent in bacteria.
- Gametes are not involved, but the exchange of genetic material does take place and is called genetic recombination. It is generally by conjugation.
- Conjugation involves transfer of DNA between cells in direct contact. One cell acts as the donor (‘male’) and the other as the recipient (‘female’).
- The ability to serve as a donor is determined by genes in a special type of plasmid called the sex factor, or F factor (F for fertility). This forms the protein of a special type of pilus, the F pilus or sex pilus. This enables cells to come into contact. The pilus is hollow and the DNA passes through the pilus from the donor (F+) to the recipient (F–).
Advantages of Sexual Reproduction (Conjugation):
Due to conjugation new genetic combinations are produced. As a result these bacteria can survive in many different conditions.