Unit Three. The Continuity of Life
The cell you see above is a dividing cell of the Oregon newt Taricha granulosa, a kind of salamander. The micrograph (that is, a photo taken through a microscope) captures the cell in the metaphase stage of mitosis, when all the blue-stained chromosomes are lined up on the metaphase plate. Soon the red-stained spindle fibers will draw duplicates of the homologous chromosomes to opposite poles of the cell. When cell division is complete, two daughter cells will result, each containing the same amount of DNA as the parent cell. Different types of cells divide at different rates. Some human cells divide frequently, particularly those subjected to a lot of wear and tear. The epithelial cells of your skin divide so often that your skin replaces itself every two weeks. The lining of your stomach is replaced every few days! Nerve cells, on the other hand, can live for 100 years without dividing. Cells use a battery of genes to regulate when and how frequently they divide. If some of these genes become disabled, a cell may begin to divide ceaselessly, a condition we call cancer. Exposure to DNA-damaging chemicals such as those in cigarette smoke greatly increases the chance of this sort of event occurring in the tissues exposed to the smoke, which is why smokers will more likely get lung cancer than colon cancer.
8.1. Prokaryotes Have a Simple Cell Cycle
All species reproduce, passing their hereditary information on to their offspring. In this chapter, we begin our consideration of heredity with a look at how cells reproduce. Cell division in prokaryotes takes place in two stages, which together make up a simple cell cycle. First the DNA is copied, and then the cell splits in half by a process called binary fission. The cell in figure 8.1a is undergoing binary fission.
In prokaryotes, the hereditary information—that is, the genes that specify the prokaryote—is encoded in a single circle of DNA, called a prokaryotic chromosome. Before the cell itself divides, the DNA circle makes a copy of itself, a process called replication. Starting at one point, the origin of replication (the point where the two strands of DNA are connected at the top of figure 8.1b), the double helix of DNA begins to unzip, exposing the two strands. The enlargement on the right of figure 8.1b shows how the DNA replicates. The purple strand is from the original DNA and the red strand is the newly formed DNA. The new double helix is formed from each naked strand by placing on each exposed nucleotide its complementary nucleotide (that is, A with T, G with C, as discussed in chapter 3). DNA replication is discussed in more detail in chapter 11. When the unzipping has gone all the way around the circle, the cell possesses two copies of its hereditary information.
When the DNA has been copied, the cell grows, resulting in elongation. The newly replicated DNA molecules are partitioned toward each end of the cell. This partitioning process involves DNA sequences near the origin of replication, and results in these sequences being attached to the membrane. When the cell reaches an appropriate size, the prokaryotic cell begins to split into two equal halves. New plasma membrane and cell wall are added at a point between where the two DNA copies are partitioned, indicated by the green divider in figure 8.1b. As the growing plasma membrane pushes inward, the cell is constricted in two, eventually forming two daughter cells. Each contains one prokaryotic chromosome and is a complete living cell in its own right.
Figure 8.1. Cell division in prokaryotes.
(a) Prokaryotes divide by a process of binary fission. Here, a cell has divided in two and is about to be pinched apart by the growing plasma membrane. (b) Before the cell splits, the circular DNA molecule of a prokaryote initiates replication at a single site, called the origin of replication, moving out in both directions. When the two moving replication points meet on the far side of the molecule, its replication is complete. The cell then undergoes binary fission, where the cell divides into two daughter cells.
Key Learning Outcome 8.1. Prokaryotes divide by binary fission after the DNA has replicated.