Gene Expression In Development

All multicellular, sexually reproducing organisms begin life as a fertilized egg, or zygote. Although every cell in the developing zygote contains all of the organism's genes, only a small number of the genes are expressed. Certain genes are turned on and off as various proteins are needed at different times during the organism's life. For example, as eukaryotes grow, cells become specialized to perform different tasks. Muscle cells specialize in movement, and liver cells specialize in making enzymes that break down fat. The development of cells that have specialized functions is known as cell differentiation (DlF-uhr-EN-shee-AY-shuhn). As organisms grow and develop, organs and tissues develop to produce a characteristic form. This development of form in an organism is called morphogenesis (MOR-foh-JEN-uh-sis).

Homeotic Genes

Homeotic (HOH-mee-AH-tik) genes are regulatory genes that determine where certain anatomical structures, such as appendages, will develop in an organism during morphogenesis. Homeotic genes seem to be master genes of development that determine the overall body organization of multicellular organisms.

When a homeotic gene is transcribed and translated, regulatory proteins are formed. It is thought that these proteins regulate development by switching groups of developmental genes on or off. Such control of gene expression increases or decreases the rates of cell division in various areas of the developing organism. The resultant variation in growth rates in specific areas of the organism produces specific patterns of structural development.

objectives

• Summarize the role of gene expression in an organism's development.

• Describe the influence of homeotic genes in eukaryotic development.

• State the role of the homeobox in eukaryotic development.

• Summarize the effects of mutations in causing cancer.

• Compare the characteristics of cancer cells with those of normal cells.

vocabulary cell differentiation homeotic gene homeobox proto-oncogene oncogene tumor cancer tumor-suppressor gene metastasis carcinogen carcinoma sarcoma lymphoma leukemia

Word Roots and Origins homeotic from the Greek homoioun, meaning "to make like"

figure 11-5

(a) Homeotic genes are expressed normally in this fruit fly. (b) This fruit fly has legs growing out of its head. This abnormality is caused by a homeotic mutation.

Antennapedia Fruit Fly

figure 11-6

The lighted spots on this grid of a DNA chip indicate to scientists which genes are being expressed in the cells being studied.

The lighted spots on this grid of a DNA chip indicate to scientists which genes are being expressed in the cells being studied.

Homeobox Sequences

One of the best-known examples of homeotic genes is found in fruit flies of the genus Drosophila, shown in Figure 11-5a. Each homeotic gene of this fruit fly shares a common DNA sequence of 180 nucleotide pairs. This specific DNA sequence within a homeotic gene is called a homeobox, and the homeobox codes for proteins that regulate patterns of development. As the fruit fly embryo becomes an elongated larva, specific homeoboxes control the morphogenesis of specific regions in the larva. Each of these home-oboxes will also control a specific part of the adult fruit fly. As Figure 11-5b shows, a mutation in a homeotic gene can lead to abnormalities. The same or very similar homeobox sequences have been found in homeotic genes of many eukaryotic organisms. It is thought that all organisms may have similar homeoboxes that code for their anatomy.

Tracking Changes in Gene Expression

The control of gene expression is important not only in the development of an organism but throughout the organism's life. Only a fraction of an organism's genes are expressed in any one cell. And cells constantly switch genes on and off. In the 1990s, researchers developed a tool for tracking gene expression called a DNA chip.

DNA chips contain a microscopic grid with thousands of known DNA fragments that are "tagged" with a fluorescent compound. A sample of mRNA from the organism being studied is spread over the grid. When spots on the grid light up, as shown in Figure 11-6, mRNA segments from the sample have linked with complementary sequences of DNA on the chip. Scientists can use this information to determine at once which genes are being expressed.

DNA chips have many practical applications but will likely have a significant impact in medicine. In fact, the technology is already being used to better understand gene expression in cancer.

GENE EXPRESSION, CELL DIVISION, AND CANCER

The division of cells is regulated by many genes, including genes called proto-oncogenes (PROHT-oh-AHNG-kuh-JEENZ), which regulate cell growth, cell division, and the ability of cells to adhere to one another. These genes code for regulatory proteins that ensure that the events of cell division occur in the proper sequence and at the correct rate.

A mutation in a proto-oncogene can change the gene into an oncogene, a gene that can cause uncontrolled cell proliferation. The mutation may lead to the overexpression of proteins that initiate cell division or to the expression of such proteins at inappropriate times during the cell cycle. These conditions can lead to uncontrolled cell division.

Tumor Development

A tumor is an abnormal proliferation of cells that results from uncontrolled, abnormal cell division. The cells that make up a benign (bi-NIEN) tumor remain within a mass. Benign tumors generally pose no threat to life unless they are allowed to grow until they compress vital organs. Examples of benign tumors are the fibroid cysts that can occur in a woman's breasts or uterus. Most benign tumors can be removed by surgery if necessary.

In a malignant (muh-LIG-nuhnt) tumor, the uncontrolled dividing cells may invade and destroy healthy tissues elsewhere in the body. This uncontrolled growth of cells that can invade other parts of the body is called cancer.

Some genes act as "brakes" to suppress tumor formation. Tumor-suppressor genes code for proteins that prevent cell division from occurring too often. In cancer, these tumor-suppressor genes are damaged, and a decrease in the activity of tumor-suppressing proteins can increase the rate of cell division. Cells have three types of tumor-suppressing genes, all of which must be damaged before cancer can occur. Figure 11-7 illustrates how mutations in proto-oncogenes and tumor-suppressor genes may lead to cancer.

www.scilinks.org Topic: Cancer Gene (Oncogenes) Keyword: HM60210

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