In this article we will discuss about the cell division and cell cycle in prokaryotes.
Bacterial cell division is also termed as fission. The bacterial chromosome is circular and attached to the plasma membrane at a specific point. Its replication starts from an initiation point. Both the copies of the chromosome are attached to the plasma membrane, close to each other.
Then new membrane material is added between the attachment sites of the two chromosomes. This causes the cell elongation and gradual separation of both the chromosomes from each other. When the separation is sufficient, the plasma membrane invaginates across the mid-point of the cell and forms a double layered transverse septum.
Cell wall material is also deposited into the space formed by folding of plasma membrane and thus a cross wall is formed. Thickness of the cross wall is greater than the normal cell wall. Then the cleavage occurs and two daughter cells, each possessing a single chromosome are produced, which are similar to their mother cell (Fig. 9.1).
The chromosome of the bacterium Escherichia coli contains 1100 pm DNA in length and it takes 40 minutes for replication, at a rate of about 30 pm per minute. It has been calculated that DNA double helix must unwind at about 10,000 turns per minute. Typically, the bacterial chromosome contains a single replicon. Replication of bacterial chromosome is initiated at a single point on DNA molecule and it is bidirectional.
In bacterial cell cycle, there have been proposed the existence of two time constants:
(i) Constant “C” which refers to the duration of time required for replication of DNA,
(ii) Constant “D” which represents the minimum amount of time necessary between the ends of DNA replication and splitting into two cells.
In case, the generation time is one hour, it takes 40 minutes to complete the DNA replication, i.e., the value of “C” is 40 minutes. Thus the period of constant “D” is 20 minutes. If the generation time equals the time period of DNA replication i.e., 40 minutes, a new round of DNA replication starts just after the first found is finished.
Thus the time phases of “C” and “D” overlap each other. If the generation time is below 40 minutes, the DNA replication begins before the completion of preceding round of replication.