The closed complex is converted into an open complex of 1.3 turns of the double helix in a series of steps by first “melting” a short region of DNA around the -10 region, giving an unstable intermediate open complex within the sequence bound by the enzyme.  Two bases in the non-template strand of the –10 element (A11 and T7) flip out from their base-stacking interactions and instead insert into pockets within the s protein where they make more favorable interactions. By stabilizing the single-stranded form of the –10 element, these interactions drive melting of the promoter region.

TLS Online TPP Program

#Id: 5024

#Unit 3. Fundamental Processes

DnaA protein, An initiator protein is synthesized continuously throughout the cell cycle; and its accumulation of a critical amount would trigger initiation.

TLS Online TPP Program

#Id: 5025

#Unit 3. Fundamental Processes

Bacteria have two links between replication and cell growth:

• The frequency of initiation of cycles of replication is adjusted to fit the rate at which the cell is growing. 

• The completion of a replication cycle is connected with division of the cell.

TLS Online TPP Program

#Id: 5026

#Unit 3. Fundamental Processes

The rate of bacterial growth is assessed by the doubling time, the period required for the number of cells to double.

The shorter the doubling time, the faster the bacteria are growing.

Escherichia coli growth rates can range from doubling times as fast as 18 minutes to slower than 180 minutes.

TLS Online TPP Program

#Id: 5027

#Unit 3. Fundamental Processes

The bacterial chromosome is a single replicon; thus, the frequency of replication cycles is controlled by the number of Initiation events at the single origin.

TLS Online TPP Program

#Id: 5028

#Unit 3. Fundamental Processes

The replication cycle can be defined in terms of two constants: C and D

C is the fixed time of 40 minutes required to replicate the entire E.coli chromosome. Its duration corresponds to a rate of replication fork movement of 50,000 bp/minute.

D is the fixed time of 20 minutes that elapses between the completion of a round of replication and the cell division with which it is connected. This period may represent the time required to assemblethe components needed for division.

TLS Online TPP Program

#Id: 5029

#Unit 3. Fundamental Processes

A cycle of chromosome replication must be initiated at a fixed time of C + D =60 minutes before cell division.