READ: The Cell Cycle

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2. Introduction- Cell Division and the Cell Cycle

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Cell Division and the Cell Cycle

What do you think this colorful picture shows? If you guessed that it’s a picture of a cell undergoing cell division, you are right. In fact, the picture is an image of a lung cell stained with fluorescent dyes undergoing mitosis, specifically during early anaphase. You will read about mitosis, a type of cell division, in this chapter.

Cell division is just one of the stages that all cells go through during their life. This includes cells that are harmful, such as cancer cells. Cancer cells divide more often than normal cells, and grow out of control. In fact, this is how cancer cells cause illness. In this chapter, you will read about how cells divide, what other stages cells go through, and what causes cancer cells to divide out of control and harm the body.

Lesson Objectives

• Contrast cell division in prokaryotes and eukaryotes.
• Identify the phases of the eukaryotic cell cycle.
• Explain how the cell cycle is controlled.
• Define cancer, and relate it to the cell cycle.

Vocabulary

Introduction

You consist of a great 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 divide 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. Watch the following video entitled Cell Division and the Cell Cycle.

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3. Cell Division

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.

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. Eukaryotic cells, in contrast, have multiple chromosomes contained within a nucleus and many other organelles. All of these cell parts must be duplicated and then separated when the cell divides.

Cell Division in Prokaryotes

Most prokaryotic cells divide by the process of binary fission. A bacterial cell dividing this way is depicted in Figure below. You can also watch an animation of binary fission:

Binary fission can be broken down into a series of three steps, although it is actually a continuous process. The steps are described below and also illustrated in Figure below. They include DNA replication, chromosome segregation, and cytokinesis.

• Step 1: DNA Replication. Just before the cell divides, its DNA is copied in a process called DNA replication. This 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 (known as poles) 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.

Cell Division in Eukaryotes

Cell division is more complex in eukaryotes than prokaryotes. Prior to dividing, all the DNA in a eukaryotic cell’s multiple chromosomes is replicated. Its organelles are also duplicated. Then, when the cell divides, it 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 of chromosomes. Mitosis is described in greater detail in Lesson 5.2.

• The second major step is cytokinesis. As in prokaryotic cells, during this step the cytoplasm divides and two daughter cells form.

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4. The Cell Cycle

The Cell Cycle

Cell division is just one of several stages that a cell goes through during its lifetime. The cell cycle is a repeating series of events that include growth, DNA synthesis, 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.

Eukaryotic Cell Cycle

The diagram in Figure below represents the cell cycle of a eukaryotic cell. As you can see, the eukaryotic cell cycle has several phases. The mitosis phase (M) actually includes both mitosis and cytokinesis. This is when the nucleus and then the cytoplasm divide. The other three phases (G1, S, and G2) are generally grouped together as interphase. During interphase, the cell grows, performs routine life processes, and prepares to divide. These phases are discussed below.

You can watch a eukaryotic cell going through these phases of the cell cycle at the following link: http://www.cellsalive.com/cell_cycle.htm

Interphase

Interphase of the eukaryotic cell cycle can be subdivided into the following three phases, which are represented in Figure above:

• Growth Phase 1 (G1): during this phase, the cell grows rapidly, while performing routine metabolic processes. It also makes proteins needed for DNA replication and copies some of its organelles in preparation for cell division. A cell typically spends most of its life in this phase.
• Synthesis Phase (S): during this phase, the cell’s DNA is copied in the process of DNA replication.
• Growth Phase 2 (G2): during this phase, the cell makes final preparations to divide. For example, it makes additional proteins and organelles.

Control of the Cell Cycle

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 key checkpoints, which are shown in Figure below. There are three main checkpoints.

1. The G1 checkpoint, just before entry into S phase, makes the key decision of whether the cell should divide.
2. The G2 checkpoint is located at the end of G2 phase. In order for this checkpoint to be passed, the cell has to check a number of factors to ensure the cell is ready for mitosis.
3. The mitotic spindle checkpoint occurs at the point in metaphase where all the chromosomes should have aligned at the mitotic plate.

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 as 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 below). The rapidly dividing cells take up nutrients and space that normal cells need. This can damage tissues and organs and eventually lead to death.

Cancer is discussed in the following video:

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5. Lesson Summary

Lesson 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.

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