Topoisomerase Iv

The number of recombination sites determines how chromosomes are unwound. TopoIV can readily untangle circles with even numbers of crossovers, whereas Resolvase XerCD is additionally needed to separate circles with odd numbers of crossovers.

1. Bacterial cell with circular chromosome

2. Newly replicated DNA

Chromosome

Cell wall

FIGURE 5.25 Elongation of the Cell Separates Chromosomes

Segregation of chromosomes is caused by elongation of the cell. Subsequently, a partition, or septum, is formed that completes cell division.

3. Daughter chromosomes attached to membrane

4. Septum forms between chromosomes

Septum

5. Chromosomes in daughter cells

Cell Division in Bacteria Occurs after Replication of Chromosomes

Bacterial cells grow longer and replicate their DNA simultaneously. Then they divide.

Bacteria divide by binary fission, or splitting. Bacteria have only one chromosome, which lies within the cytoplasm. Bacterial cell division is thus relatively simple and may be divided for convenience into four stages, although some of the processes overlap slightly:

1. Replication of the chromosome

2. Partition of the daughter chromosomes

3. Cell elongation

4. Separation of the two daughter cells by formation of a cross-wall.

Replication proceeds in both directions at once around the circular bacterial chromosome. Eventually, the two replication forks meet and merge, yielding two new circular chromosomes. These are attached to the cell membrane, probably at their origins, as suggested above. As the cell elongates, the chromosomes are pulled apart (Fig. 5.25). The final step of cell division is the formation of a cross-wall, or septum.

How Long Does It Take for Bacteria to Replicate?

The time required for an E. coli cell to divide, the generation time, ranges from 20 minutes to several hours, depending on the conditions. Despite this, duplication of the binary fission Simple form of cell division, by splitting down the middle, found among bacteria generation time The time from the start of one cell division to the start of the next septum Cross-wall that separates two new bacterial cells after division

The Concept of the Replicon 125

Pause Chromosome duplication - 40 min Cell division - 20 min

80 min

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