10.1Gene Types in Terms of Their Expression


House-keeping Genes Function in All Organisms

House-keeping genes are essential for the sustenance and multiplication of cells and include genes for enzymes associated with energy production; intermediary metabolism of sugars, lipids, amino acids, etc.; and biosynthesis of nucleic acids and proteins. The functions performed by house-keeping genes indicate that they can also be considered as vital genes. Indeed, genes necessary for survival differ between autotrophs (organisms that can produce organic compounds from inorganic molecules, such as plants) and heterotrophs (organisms that ingest organic compounds as food, such as animals).

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Luxury Genes Function in Multicellular Organisms

In addition, multicellular organisms such as humans possess various differentiated cells. Each cell has specific gene expression patterns for each differentiated function. Humans comprise approximately 200 different types of cells, such as skin cells, liver cells, and nerve cells. Each cell varies in both structure and function because different sets of genes are expressed in each of them. For example, the serum albumin gene is expressed only in hepatocytes, while the insulin gene is expressed only in pancreatic beta cells. These genes are not expressed in other cells. Furthermore, gene functions that are not required in unicellular organisms become essential for multiple cells to form an organism. These functions include cell adhesion at a micro level and the organization of connective tissue*1 at a macro level. Genes associated with intercellular signaling pathways involved in the coordinated regulation of an individual organism are also necessary. Thus, many of the genes essential for the survival of multicellular organisms are often called luxury genes and are expressed only in certain cell types or at certain stages of development.

*1 All cells other than germ cells in multicellular organisms are called somatic cells. Most of the cells that compose an organism are therefore somatic cells.

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Somatic Cells of a Particular Individual Possess the Same Genes

Each differentiated cell expresses specific genes, but all somatic cells*1 that comprise an individual are considered to carry the same set of genes, namely two sets of genes, one derived from the mother and one from the father. Although this is difficult to prove, it has long been known that a cloned plant can be produced from a single somatic cell, and cloned animals from several mammal species have recently been successfully produced from single somatic cells. This indicates that the set of genes housed in a single somatic cell is capable of forming all cells that compose an organism (including somatic*2 and germ cells). Although this has not been confirmed in humans, it is assumed that humans are not an exception. Thus, although all somatic cells possess the same genes, the fact that there are differentiated cells in which different genes are expressed indicates the presence of a mechanism of gene expression regulation.

*2 The β-galactosidase gene was formally referred to as the lacZ gene, but here we have referred to it as β-gal for simplicity.

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Genes with Regulated and Unregulated Expression

Genes involved in cellular energy metabolism and protein synthesis must function continuously to allow cell survival. This constant gene expression is called constitutive expression. On the other hand, situational expression of genes is called regulatory expression. House-keeping genes are generally constitutively expressed; however, even genes for enzymes involved in energy metabolism often change their expression levels according to changes in cell environments. Expression of genes involved in proliferation is suppressed during growth arrest.
At least two expression regulation mechanisms exist for genes that are clearly involved in differentiation functions. The first mechanism is tissue (or cell) specific and is exemplified by the following system: among the various cells in the bodies of chickens, only the oviduct epithelial cells express the ovalbumin gene, while other cells do not. The second one is contextual regulation of expression. In the oviduct epithelium, the ovalbumin gene is not always expressed; rather, there is a mechanism in which estrogen (the female hormone) simultaneously stimulates both expression of the ovalbumin gene and ovulation. These two regulatory mechanisms are observed during differentiation of not only the oviduct cells but all cells in the body.


Only Lymphocytes Possess Different Genes

Although all somatic cells of a single human possess the same genes, lymphocytes are an exception in that they have different genes. A human can produce hundreds of millions of types of antibodies, which are proteins. This implies that a human must also have as many genes to produce these antibodies; however, all somatic cells only have the precursors for antibody genes, which cannot produce antibodies. During differentiation of lymphocytes, recombination occurs in antibody genes, thereby equipping each lymphocyte with only one antibody gene. As a result, although an individual may possess genes for producing hundreds of millions of antibody types, each lymphocyte possesses only one type of antibody gene. If a somatic cell clone is generated from a lymphocyte, then that clone will be able to synthesize only one antibody type (or may even be unable to synthesize any antibody).

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