Checkpoints of the Cell Cycle

1 Checkpoints of the Cell Cycle Objectives:  Describe how the cell knows when it is ready to begin different processes.  Describe the effect of the checkpoints not being in place. Checkpoints in a Cell When a computer runs a program, it follows a set of instructions that tell it what to do. Often, the program will continue to run as it is intended until it needs an input before moving on to the next step in the program. This input will sometimes allow the program to decide what the next step will be. If no input is given, the program may stop or may remain in the first step of the process, never moving beyond it. Cells function in a similar way. Like a computer, the cell uses both internal and external inputs to determine when to run its processes. All cells have DNA, which contains the instructions for what the cell will do. In eukaryotes, the DNA is in the nucleus. In prokaryotes, the DNA is in a circular mass called the nucleoid. To continue the computer analogy, the DNA is like the operating system of the computer. It both processes and provides commands that allow the cell to function. During a cell’s lifetime, it will take in and process materials, create and send out materials, and grow. It continues these life processes until it determines that it is time to reproduce by dividing. The cell reaches a checkpoint, which will tell it that the time has come to divide into two new cells.

2 Many cell cycle checkpoints exist. The three most important for the cell cycle to proceed are the G 1 checkpoint, the G 2 checkpoint, and the spindle checkpoint. These checkpoints tell the cell when it is time to divide and when it is time to move on to certain steps of the process. The G in this case stands for growth. The G 1 phase is the period between one cell division and another. It is the time when the cell does most of its growing and living. This makes the G 1 checkpoint one of the most important checkpoints, because it tells the cell when the time for growing and carrying out normal cellular functions is over and the time to divide has begun. At the G 1 checkpoint, the cell receives input from its environment and its current state to determine if it is ready to divide. When all of the requirements of the checkpoint are met, the cell moves from the G 1 phase into the S phase. In the S phase, the cell synthesizes a copy of its DNA for the new cell. The cell checks four things at the G 1 checkpoint:  It assesses its size to determine whether it is large enough to divide.  It makes sure it has enough nutrients to divide.  It uses molecular signals to communicate with other cells to assess the environmental conditions.  It scans its DNA for damage. If the cell passes all of these checks, the cell moves from the G 1 phase to the S phase. Once the DNA has been replicated, the cell moves from the S phase to the G 2 phase. This is often referred to as the resting phase. The cell rests, creates the new proteins needed for cell division, and grows a little. To determine when the G 2 phase ends, the cell has another checkpoint: the G 2 checkpoint. At the G 2 checkpoint, the cell checks a few more things to determine whether it is time to move on to the next step:  It makes sure that all of the proteins necessary for cell division have been made.  It checks to make sure that the DNA is fully copied.  It checks for any damage or mutations in the new DNA. The last check is extremely important. Damaged or mutated DNA can lead to the formation of cancer cells. Once these checkpoints are cleared, the cell moves on to the next steps in cell division. Before the cell splits into two daughter cells, it has one more checkpoint to clear: the spindle checkpoint. Spindles are small protein fibers that attach to the chromatids and pull them to opposite sides of the cell. At this checkpoint, the cell makes sure that each sister chromatid is attached properly to the spindle, so that one chromatid from each pair goes to each daughter cell. This ensures that each cell receives the correct number of chromosomes. Once it has passed this checkpoint, the cell continues the process of mitosis by pulling the chromatids to opposite sides of the cell, creating new nuclei, and dividing into two new cells.

4 Cancer cells are dangerous because they do not recognize the same checkpoints as healthy cells. A normal cell might need to have certain growth factors satisfied in order to divide, but a cancer cell will simply ignore them, or make their own growth factors and divide anyway. For example, at the G 1 checkpoint, the cell checks to see if the conditions are good for division. If the area is already crowded, the cell will not divide. A cancer cell ignores those conditions and divides anyway. For this reason, tumors often have uncontrolled growth. Other growth factors include replicative immortality. Though normal cells die off after they reproduce a few dozen times, a cancer cell keeps on reproducing. In addition, normal cells undergo a programmed death called apoptosis, often due to DNA damage or overcrowding. Cancer cells keep on dividing. At the G 1 checkpoint, the cell makes sure enough nutrients are available. A cancer cell will not stop dividing if no nutrients are available. As a result, cancerous cells may take the resources that healthy cells need to survive. Not only does the cancer cell multiply, but a healthy cell is lost. Summary Cells are the basic building blocks of life. In single-celled organisms, they carry out all of the necessary functions of life. In multicellular organisms, they perform some of the functions themselves or carry out specialized functions for the rest of the organism. Cell division is one of the most vital functions for the continuation of the organism’s species or life. Without it , single- celled organisms cannot reproduce and multiply and multicellular organisms cannot replace cells lost due to damage or age. A cell continues to carry out its normal functions until it reaches a checkpoint that tells the cell it is time to divide. Once the process of division begins, there are two more checkpoints: one before entering mitosis and one during mitosis. At these checkpoints, the cell determines whether the conditions are right for division and if the DNA is healthy enough to continue. If the DNA is damaged or mutates, a cancer cell could form. Cancer cells ignore many of the checkpoints and continue to divide even when space or nutrients are lacking. Without checkpoints, a cancerous tumor will continue to grow without stopping. Concept Reinforcement 1. What does the cell look for at the G 1 checkpoint? 2. What happens after the cell passes the G 2 checkpoint? 3. What is determined at the spindle checkpoint? 4. How can failure at the G 2 checkpoint lead to cancer?