Chapter 1: Cell Differentiation Lesson 6

Chapter 1 • Lesson 6 Cell Differentiation Key terms gene • cell differentiation • zygote • stem cell • gene expression • gene regulation Getting the Idea New cells in eukaryotic organisms form through mitosis. In most cases, each cell produced through mitosis is an exact duplicate of its parent cell. However, most organisms are not uniform creatures made up of a single type of cell. Instead, they are composed of different types of cells that are specialized to perform different functions. Specialized Cells of Multicellular Organisms Recall from Lesson 1 that the organelles in eukaryotic cells have shapes and structures that are related to their functions. The same is true of the cells that make up multicellular organisms. Most multicellular organisms are made up of many different kinds of cells, each of which is specialized for certain functions. Animals have many types of specialized cells. The diagram shows three common examples. As you read about each type of cell, notice how the cell's shape is related to its function. The first cell shown is a neuron, or nerve cell. Nerve cells send information from one part of the body to another. The long, thin structure of a nerve cell is well suited to this function. Red blood cells are specialized for transporting oxygen. They contain hemoglobin, a protein that binds to oxygen molecules. The small size and flat disk shape of red blood cells allow them to move easily through the thin blood vessels that transport them throughout the body. The sperm cell is a gamete, or sex cell. Gametes are specialized cells used only for sexual reproduction. A sperm cell is the male gamete. It carries the genetic information of the male parent. Its function is to travel to and penetrate an egg cell (the female gamete), fertilizing it. A sperm cell's strong tail helps it travel quickly. The cell's distinctive head contains enzymes that allow the sperm to enter an egg by breaking through its plasma membrane. You will learn more about gametes and fertilization in Lesson 23. Muscle cells are another type of specialized cell common to animals. Muscle cells are specialized to enable movement. Human muscle cells contain fibers that are arranged in a tight pattern. The fibers contain a protein called myosin that causes the fibers to shorten, or contract. Specialized cells are not unique to animals. Plants and fungi also have specialized cells. Two kinds of specialized cells present in many plants are the cells that make up xylem and phloem. The main function of xylem is to transport water up through a plant. Phloem transports food made in a plant's leaves to the rest of the plant. The cells that make up xylem and phloem are long and thin, like pipes. Cell Differentiation Recall that genetic information and instructions for cell activities are carried in DNA. In multicellular organisms, the DNA is bundled into structures called chromosomes, which are located in the cell nucleus. Chromosomes, in turn, are made up of smaller segments called genes. Genes are sections of DNA that code for specific traits. You have just read that the cells in multicellular organisms are specialized to carry out different functions. However, these cells all start with the same genes, and nearly all cells in a multicellular organism have the same set of genes. How do these cells become specialized? The answer is cell differentiation. Cell differentiation is a process that produces specialized cells with different structures and functions. Most multicellular organisms start as fertilized eggs, or zygotes. The zygote's DNA serves as the instructions for the development of the entire organism. As the organism grows by repeated cell division, all the new cells receive the same complete set of chromosomes and DNA. With a few exceptions, such as gametes and red blood cells, all of an organism's cells have the same DNA. Although almost every cell in an organism has the same genetic information, each cell uses only the portion of that information that it needs. During cell differentiation, cells begin to "read" different parts of the DNA and ignore other parts. To understand this, think about how you use a dictionary. If you want to look up differentiation, you turn to the D section of the dictionary and find the word you want. You do not read about egg in the E section or about sperm in the S section. You look up only the information you need. In a similar way, cells use only the information they need to carry out their functions. Most cell differentiation occurs early in the development of an organism. Each cell starts with the potential to specialize and become any type of cell. However, once a cell differentiates, the process cannot be reversed. For example, a nerve cell cannot change into a muscle cell, nor can a xylem cell of a plant change into a phloem cell. When a differentiated cell divides by mitosis, it produces daughter cells of the same type as the parent cell from which they formed. Stem Cells The unspecialized cells that reproduce themselves and have the capacity to differentiate into one of many types of specialized cells are called stem cells. You will read more about how stem cells differentiate in the next section. For now, you need to know that there are two types of stem cells: embryonic stem cells and adult stem cells. Embryonic stem cells are the stem cells found in embryos, or developing offspring. These stem cells divide and differentiate to produce most of the body cells an organism will have. Mature organisms have another type of stem cell, called adult stem cells, which aid in growth and repair. In humans, for example, the adult stem cells in bone marrow differentiate to produce blood and bone cells. In recent years, researchers have learned how to cause both embryonic and adult stem cells to differentiate into different types of body cells under the right laboratory conditions. You will learn more about this type of research in Lesson 26. Gene Expression and Regulation The main function of genes is to control the production of proteins. Recall that proteins are large molecules made up of amino acids. An organism's traits depend on the kind and number of proteins it makes, based on the information in its genes. The process by which the information in genes is used to make proteins is called gene expression. Not every gene in an organism is expressed, or makes proteins. In fact, only a fraction of the genes in a cell are expressed at any time. For example, the gene that codes for hemoglobin—the protein that enables red blood cells to transport oxygen—is expressed in red blood cells but not in the cells of the pancreas. Similarly, the genes that code for insulin production in the cells of the pancreas are not expressed in red blood cells. A complex mechanism known as gene regulation determines whether a gene is turned on or off in a cell. If the gene is turned off, the protein it codes for is not produced. Gene regulation controls the production of specialized cells. In organisms that reproduce sexually, gene expression begins soon after the fertilized egg (zygote) begins to divide to form the cells that will make up the new organism. In humans and many other animals, the dividing cells quickly form a ball of cells that is composed of three layers, called germ layers. Although the cells of each germ layer are undifferentiated and initially appear the same, they will develop into different structures. Which genes are expressed is determined by the position of a cell within the growing mass of cells. For example, in humans, cells of the outer layer, or ectoderm, will form skin, nerves, and sense organs. Cells in the middle layer, or mesoderm, will form bones, muscles, and connective tissue. Cells of the inner layer, or endoderm, will form the digestive system and the lungs. Cell differentiation begins with the formation of these three layers. As a multicellular organism develops, different genes are expressed in the cells of different parts of the body. For example, the genes expressed by a skin cell are different from those expressed by a blood cell or a nerve cell. Similarly, some genes are expressed only at certain times, to control processes that take place in the organism. For example, the genes that control the production of certain hormones, such as estrogen, are not expressed until puberty. The genes for bone growth are turned off in adults, so bones do not continue to grow. Which genes are activated in a cell depends on the cell's history and environment. Environmental conditions that affect gene expression include the presence or absence of chemicals and what other cells are nearby. For example, some cells release chemical signals, in the form of hormones, that influence the development of other cells.