Understanding Cancer

An Introduction to the Biology, Medicine, and Societal Implications of this Disease

J. Richard McIntosh

• Editorial: Garland Science
• Año: 2019
• ISBN:
  9780429576263
• Edición: 1
• Categoría: Medicine
• Categoría: Medical Theory, Practice & Reference

Understanding Cancer is a brand new undergraduate textbook for students without prior training in biology that integrates an introduction to cancer medicine with descriptions of the biological processes that go wrong to cause cancer's onset and progression. It also highlights the human side of cancer with stories of patients and loved ones touched by the disease, dealing with diagnosis, treatment, and the prospect of death as well as the broader societal aspects of cancer and its prevention. Key discoveries that have improved our understanding of cancer are presented in sidebars. In spite of this diversity, the book maintains precision and simplicity in describing what is and is not known about cancer, describing the strengths and limitations of current treatments

1

Cancer and the biology of human cells

CHAPTER 1

WHAT IS A CELL? THE BASICS

WHEN NORMAL CELLS MISBEHAVE

CURRENT TREATMENTS OF CANCER AND PROSPECTS FOR THE FUTURE

Cancer is a result of misbehaving cells. It is a disease that develops when a cell from one’s own body fails to respond to controls that normally regulate cell behavior. When these controls are working properly, they assure that every cell acts in the best interests of the body as a whole. Cancer results from a rogue cell, one in which these controls have failed, so the cell grows and divides when it should not, forming more of its own kind at the expense of other cells. In this way, the rogue cells sap the body’s energy and disrupt normal functions. If these cells are not eliminated, the body as a whole will become ill and may ultimately die. This book will help you understand the nature of cancer in its broadest sense. Since the disease arises from our own cells, an understanding of cancer must begin with an understanding of normal cells; from there we can build an understanding of the malfunctions that lead to cancer. This chapter therefore starts with an introduction to human cells, which is followed by an introduction to cancer as a disease.

LEARNING GOALS

1. List three structures and three functions that are shared by many different types of human cells.

2. Define the terms trait and allele, and give an example of a trait for which your two alleles could be different, given the traits of your parents.

3. Describe how cells that contain the same genetic information can perform different jobs.

4. Describe the changes in cell behavior that occur during cancerous transformation.

5. Identify the two behaviors of a cancer cell that are the most dangerous to a patient.

6. Give examples of how cancer is currently diagnosed and treated, stating one strength and one limitation of each diagnosis and treatment.

7. List the properties of cancerous cells that make this disease so hard to treat.

WHAT IS A CELL? THE BASICS

Cells are systems that can consume materials and energy from their environment and use them for their own growth and reproduction (MOVIE 1.1). In short, cells are the simplest living things. Every structure in your body is composed of cells and of the materials that cells make. However, most human cells are so small they cannot be seen with the naked eye; a cluster containing a million cells is not much bigger than a pinhead. Nonetheless, every cell is very complicated; one estimate by a physicist who cares about complex systems suggests that a single cell is more complex than a supercomputer or an Atlas rocket for launching satellites. Their small size and great complexity make cells hard to understand in detail, but quite a lot has been learned about what they are and how they work. Below is an introduction to cells; each topic relevant to cancer is treated with greater detail in Chapters 3 and 4.

Cells can have many shapes and sizes, depending on the jobs they do.

The egg and sperm that joined to start your life were each cells in their own right, albeit very different in size and shape (FIGURE 1.1A). Sperm, with their small elliptical heads and long tails, are specialized to swim and find an egg. Eggs, which are much bigger, are specialized to contain nutrients that will help the new generation start out well fed. When an egg and a sperm join, in the process called fertilization, they form a single cell that contains two internal compartments called nuclei, one nucleus from the sperm and one from the egg (FIGURE 1.1B). We will discuss the nucleus and its role in cell behavior later in this chapter. Shortly after fertilization, these nuclei fuse, and the result is a cell that is the parent of all the cells in your body. This single cell, which contains material from both egg and sperm, now divides to form two daughter cells, which divide again, making four cells, eight cells, etc. (FIGURE 1.1C,D). These cell divisions gradually build up all the cells needed to make first an embryo, then a fetus, then a child, and then an adult.

FIGURE 1.1 Human egg being fertilized by sperm to make the cell that then divides to form all the cells of an embryo. (A) A picture showing many sperm (blue) trying to gain entry into an egg and become the one that fertilizes it. (B) A light micrograph of a slice through a fertilized egg showing the coat that surrounds the egg to protect it, the egg cell itself, and two interior spheres, which are the nuclei contributed by the egg and the sperm cells (arrows). The two nuclei will fuse to form one, whereupon this cell is called a zygote. It will divide many times to form all the cells of the body. (C) The results of the first division: this structure is now a two-cell embryo. (D) Three cell divisions have occurred since the egg became a zygote; now there are eight cells. (A and B, courtesy of Jonathan VanBlerkom, University of Colorado Boulder. C and D, courtesy of the Copenhagen Fertility Center.)

As those many cells form, they specialize, taking on different structures so they can accomplish different jobs for your body. Some become blood cells, some become nerve cells, some turn into muscle, and some help to make covering layers, like the skin that surrounds your body and the tissues that line the inside of your mouth, throat, stomach, and intestine. The task of understanding cells is in part learning what is common to them all and in part learning how and why cells become different.

All cells are built from materials with familiar names.

The most prevalent substance in a cell is water, but dissolved in that liquid are a number of materials you know about from the names of different kinds of food: proteins, fats, and sugars (whose general name is carbohydrates). Proteins and fats are important for cells because they are the materials from which much cellular structure is built; they also contribute to many internal cellular functions. Carbohydrates are important because cells use them to store energy as well as to accomplish some other jobs. In addition, cells contain minerals, like sodium, potassium, calcium, iron, and magnesium, as well as many small molecules, such as vitamins, sugars, and amino acids. Although most of these molecules are dissolved in water, some are attached to one another to build structures that are the cell’s framework.

Finally, and most significant for this book, cells contain DNA, a long molecule made from two strings of smaller molecules hooked together end-to-end, like pop-it beads. DNA is important because it is the material cells use to store the information they need to be able to reproduce and specialize for making body tissues, such as muscle, skin, and bone. You will learn more about DNA in later chapters, because it is especially important for our understanding of cancer.

All our body’s cells share some essential features.

All cells have a well-organized structure, though they can be remarkably different in appearance. The structure of each cell helps it to perform its many functions. One structure common to all cells is a very thin membrane, made of proteins and fat, that serves the cell as a boundary (FIGURE 1.2). This structure, called the plasma membrane, separates the inside of the cell from the rest of the world. Although it is thin, it is impermeable to most things, so the many molecules that a cell needs to survive don’t leak out. However, anything that enters the cell, like molecules of food, must cross this boundary. Cells also make additional membranes that divide their interior into compartments that can accomplish specific functions. For example, membrane-bounded compartments called mitochondria (Figure 1.2B) convert the nourishment a cell consumes into a molecule that stores energy in a form that cells can use efficiently.

Many of a cell’s components are positioned so they can contribute effectively to the well-being of the cell as a whole. For example, most of the cell’s DNA is in another membrane-bounded compartment, the nucleus, where it is concentrated for active use (Figure 1.2). The nucleus of every human cell contains 46 long pieces of DNA, each of which is called a chromosome.

FIGURE 1.2 The internal structures of a human cell. (A) Image taken with a light microscope showing a human skin cell growing in a laboratory on a piece of glass. Optical tricks have been used to make cellular structures stand out. In this image you can see an overview of the cell and a few of the structures inside it, as labeled. (B) A diagram of a similar cell highlights multiple organelles. The plasma membrane surrounds the cell, defining its boundary. The nucleus contains the cell’s DNA. The cytoplasm is the region inside the plasma membrane and outside the nucleus; it holds many organelles, such as mitochondria, the organelles responsible for producing the form of energy that cells use most efficiently. Two systems of cytoplasmic fibers are shown; these help to give the cell its shape. (A, courtesy of Casey Cunningham.)

One of the most important aspects of cells is their ability to store the instructions necessary to make a new cell. Much of this information is written into the structure of the cell’s DNA. Since all cells need this kind of information, one of the processes required for cell growth and division is the duplication of its DNA. This process forms two identical copies of the original DNA, so there is a copy of the information for each of the daughter cells that will form when the parent cell divides.

SIDE QUESTION 1.1

If the plasma membrane is the boundary that holds in all the contents of the cell, why not just make it completely impermeable, so nothing can leak out?

Many other essential cellular processes occur in the space between the nucleus and the plasma membrane, a region called the cytoplasm. Here the cell processes food to obtain both the energy and the materials needed to make more DNA, proteins, carbohydrates, and fats. The cytoplasm is also the place where many cellular functions are carried out. For example, it is here that cells make multiple copies of a few specific proteins that assemble into fibers that give a cell its shape and allow it to move (Figure 1.2). The cytoplasm is also the place where cells generate and interpret signals that they send out to and receive from other cells in the body.

Our bodies are formed by a combination of cell reproduction and cell differentiation.

The fertilized egg from which you started life gave rise to every cell in your body. When you consider that there are trillions of cells in an adult human, this is quite amazing. Cell growth and division are the processes by which emb...