Cell Division and Reproduction What do you think this picture shows? If you guessed that it s a picture of two cells, you are right. In fact, the picture shows human cancer cells, and they are nearing the end of the process of cell division. In this process, a single cell divides into two cells. Cell division is just one of the stages that all cells go through during their life. This is true of normal cells as well as cancer cells, although cancer cells divide more often and grow out of control. In fact, this is how cancer cells cause illness. Before Reading: RECALL: What is the main difference between prokaryotic and eukaryotic cells? PREDICT: Which of these two cell types do you think will grow faster? Why? Introduction You consist of many cells, but like all other organisms, you started life as a single cell. How did you develop from a single cell into an organism with trillions of cells? The answer is cell division. After cells grow to their maximum size, they separate into two new cells. These new cells are small at first, but they grow quickly and eventually divide and produce more new cells. This process keeps repeating in a continuous cycle.
Cell Division Cell division is the process in which one cell, called the parent cell, divides to form two new cells, referred to as daughter cells. How this happens depends on whether the cell is prokaryotic or eukaryotic. Remember that a prokaryotic cell does not have a nucleus. Cell division is simpler in prokaryotes than eukaryotes because prokaryotic cells themselves are simpler. Prokaryotic cells have a single circular chromosome, no nucleus, and few other organelles. Compare this to eukaryotic cells, which have multiple chromosomes (pieces of DNA) contained within a nucleus and many other organelles. A eukaryote makes copies of all of these cell parts and then separates them when the cell divides. Reading Check 1: 1. Which type of cells go through the process of cell division? a) prokaryotic cells b) eukaryotic cells c) prokaryotic and eukaryotic cells d) neither 2. Complete this sentence. Cell division is a more complex process in eukaryotes because eukaryotic cells have a (which includes multiple ) as well as many other ; the eukaryotic cell must make of all these parts, and then them when the cell divides. 3. If you were conducting a scientific experiment in a laboratory, which type of cell would be best to study? Why? Cell Division in Prokaryotes Most prokaryotic cells divide by the process of binary fission. The term binary means consisting of, indicating, or involving two. Binary fission is the process in which one prokaryote splits into 2 cells. Figure 3.1: Binary Fission in a Bacterial Cell Cell division is relatively simple in prokaryotic cells. The cell on the left is dividing by binary fission. Notice how its plasma membrane is starting to grow into the center of the cell. It will eventually pinch apart the parent cell to form two identical daughter cells.
Binary Fission Although binary fission is actually a continuous process, it can be understood as a series of three steps. The steps are described below and also illustrated below in Figure 3.2. The steps include DNA replication, chromosome segregation, and cytokinesis. Step 1 DNA Replication: Just before the cell divides, its DNA is replicated in a process called DNA replication. This duplication results in two identical chromosomes instead of just one. This step is necessary so that when the cell divides, each daughter cell will have its own chromosome. Step 2 Chromosome Segregation: The two chromosomes segregate, or separate, and move to opposite ends of the cell. Step 3 Cytokinesis: A new plasma membrane starts growing into the center of the cell, and the cytoplasm splits apart, forming two daughter cells. This process is called cytokinesis. The two daughter cells that result are genetically identical to each other and to the parent cell. Figure 3.2: Binary Fission Reading Check 2: 1. In the process of binary fission, what is the result of DNA replication? a) one identical chromosome b) two identical chromosomes c) daughter cells 2. Why does each daughter cell need its own chromosome? 3. During the chromosome segregation phase, each chromosome moves to ends of the cell in preparation for. 4. In cytokinesis, what two cell organelles are involved? (circle both) a) cytoplasm b) nucleus c) mitochondria d) cell membrane 5. The two daughter cells are genetically identical to and to.
Cell Division in Eukaryotes Cell division is more complex in eukaryotes than prokaryotes. Before dividing, the DNA in a eukaryotic cell s multiple chromosomes is replicated. Its organelles are also duplicated. Then, when the cell divides, the process occurs in two major steps: The first step is mitosis, a multi-phase process in which the nucleus of the cell divides. During mitosis, the nuclear membrane breaks down and later reforms. The chromosomes are also sorted and separated to ensure that each daughter cell receives a complete set. Mitosis is described in greater detail in the next section. The second major step is cytokinesis. As in prokaryotic cells, during this step the cytoplasm divides and two daughter cells form. Reading Check 3: 1. What processes take place in eukaryotic cells before cell division? (circle all correct answers) a) mitosis b) DNA replication c) organelle duplication d) cytokinesis 2. The two major steps of eukaryotic cell division are and. The Cell Cycle Cell division is just one of several stages that a cell goes through during its lifetime. A cycle is a number of related events that happen again and again in the same order. The cell cycle is a repeating series of events that include growth, DNA replication, organelle duplication, and cell division. The cell cycle in prokaryotes is quite simple: the cell grows, its DNA replicates, and the cell divides. In eukaryotes, the cell cycle is more complicated. As discussed previously, the eukaryotic cell cycle is a more complex process because the cell itself is more complex. Eukaryotic Cell Cycle The eukaryotic cell cycle has several phases. The first phase is called interphase. During interphase, the cell grows, performs routine life processes, and prepares to divide by replicating the DNA and duplicating organelles. Interphase takes a much longer time than cell division. A phase is one of the stages of a process. Cell division consists of 2 phases and includes both mitosis, when the nucleus divides, and then cytokinesis, when the cytoplasm divides.
Reading Check 4: 1. How many phases are there in the eukaryotic cell cycle? a) 2 b) 3 c) 4 d) 5 2. What are the phases? 3. What 4 things happen during interphase? 4. Define mitosis and cytokinesis. Figure 3.3: Eukaryotic Cell Cycle
Control of the Cell Cycle Imagine driving somewhere with no rules, no signs, and no traffic signals. You wouldn't know when to stop or go. It would be very difficult to safely get where you need to go! Cells need rules as well. If the cell cycle occurred without regulation, cells might go from one phase to the next before they were ready. What controls the cell cycle? How does the cell know when to grow, synthesize DNA, and divide? The cell cycle is controlled mainly by regulatory proteins. These proteins control the cycle by signaling the cell to either start or delay the next phase of the cycle. They ensure that the cell completes the previous phase before moving on. Regulatory proteins control the cell cycle at three key checkpoints Cancer and the Cell Cycle Cancer is a disease that occurs when the cell cycle is no longer regulated. This may happen because a cell s DNA becomes damaged. Damage can occur due to exposure to hazards such radiation or toxic chemicals. Cancerous cells generally divide much faster than normal cells. They may form a mass of abnormal cells called a tumor (see Figure 3.5). The rapidly dividing cells take up nutrients and space that normal cells need. This can damage tissues and organs and can eventually lead to death. Figure 3.5: These cells are cancer cells, growing out of control and forming a tumor. Summary Cell division is part of the life cycle of virtually all cells. It is a more complicated process in eukaryotic than prokaryotic cells because eukaryotic cells have multiple chromosomes and a nucleus. The cell cycle is a repeating series of events that cells go through. It includes growth, DNA synthesis, and cell division. In eukaryotic cells, there are two growth phases, and cell division includes mitosis. The cell cycle is controlled by regulatory proteins at three key checkpoints in the cycle. The proteins signal the cell to either start or delay the next phase of the cycle. Cancer is a disease that occurs when the cell cycle is no longer regulated. Cancer cells grow rapidly and may form a mass of abnormal cells called a tumor.