chapter  6
DNA Replication
Pages 16

In this chapter and Chapter 7, DNA replication, segregation of DNA, and variations of chromosome partitioning will be addressed. The cell cycle is a good place to start this discussion. All cellular organisms have cell cycles. In Bacteria and Archaea (Figure 6.1 and Figure 6.2), the cycle includes a G1 (gap) phase where each cell has one genome of DNA, followed by an S (synthesis) phase where the chromosomal DNA is replicated, proceeding into a G2 phase, and, nally, through binary ssion to produce two cells, again each having one full set of genes. Other than dying, a cell can exit from the cell cycle in two ways. The rst is by forming a spore. This normally occurs with cells in G1. The cell encases some cytoplasm and its genomic DNA inside a hard shell that will protect the contents for extended period, sometimes for many years. Upon germination of the spore, the cell again enters a normal cell cycle. The second is through differentiation. For example, members of the genus Caulobacter form two types of cells: stalk cells and swarmer cells. Stalk cells are attached to surfaces by a holdfast, and, therefore, these cells cannot move. Swarmer cells are motile and eventually settle onto a surface, grow a holdfast, and stay there, thus differentiating into stalk cells. But, they divide to form two cells. One remains as the stalk cell, and at the other end is a swarmer cell. The stalk cell eventually dies and the swarmer cell swims away to nd a place to attach. Therefore, there is a cell cycle, but one of the cells always differentiates, divides, and then dies.