4.1Gene Types in Terms of Expression
The Function of Housekeeping Genes in All Organisms
Housekeeping genes are essential for the sustenance and multiplication of cells, and include genes associated with energy production, intermediary metabolism of sugars, lipids, amino acids, etc., and biosynthesis of nucleic acids and proteins. The functions performed by housekeeping genes mean they can also be thought of as vital genes. Needless to say, those 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).
Additional Gene Function in Multicellular Organisms
Multicellular organisms such as humans have many types of differentiated cells, and each cell has specific gene expression patterns for each of diverse cell functions. Humans are said to consist of some 200 types of cell. Skin cells, liver cells and nerve cells, among others, have different shapes and functions because different sets of genes are expressed in each of them. As an example, the serum albumin gene is expressed only in hepatocytes, while the insulin gene is expressed only in pancreatic β-cells. These genes are not expressed in other cells. For multiple cells to form an organism, gene functions that are not required in unicellular organisms become essential. Such functions include cell adhesion on a micro level and the organization of connective tissue*1 on a macro level. Genes associated with intercellular signaling pathway- i.e., those involved in the biological regulation of an organism - are also necessary. Thus, many of the genes essential for the survival of multicellular organisms are expressed only in certain cell types or at certain stages of development.
Connective tissue: A type of tissue with a rich extracellular matrix (see Chapter 11). Connects cells and tissues and, as a frame, maintains the shape and strength of an animal’s body.
The Identical Nature of All Somatic Cells from a Particular Individual
While each differentiated cell expresses a unique set of genes, all somatic cells that make up an individual are believed to share the same two sets of genes, one inherited from each parent. Although this is difficult to prove, in the plant kingdom it has long been known that a clone plant body can be produced from a single somatic cell, and cloned animals from several mammal species have recently been successfully created from single somatic cells. This indicates that a gene set housed in a single somatic cell is capable of creating all cells that make up an organism (including somatic cells*2 and germ cells). While this has not been confirmed in humans, it is assumed that we are not an exception. Differentiated cells in which different sets of genes are expressed do exist, despite the assumption that all somatic cells have the same gene sets, because mechanisms are in action to regulate gene expression.
Somatic cells: All cells other than germ cells in multicellular organisms are called somatic cells. Most of the cells that make up an organism are therefore somatic cells.
Genes Subject to and Free of Expression
Genes involved in energy metabolism and protein synthesis in cells must act continuously to maintain the life of those cells. This constant gene expression is called constitutive expression. On the other hand, situational expression of genes is called regulated expression. Very roughly speaking, housekeeping genes include those that express constitutively; however, even genes for enzymes involved in energy metabolism often change their expression levels according to cell environments, and the expression of genes involved in proliferation is suppressed during growth arrest.
It is clear that at least two expression regulation mechanisms exist for genes that are involved in differentiation functions. One is the mechanism in which, among the various cells in the body, only β-cells in the pancreas express insulin genes while other cells do not. The other is the mechanism of β-cells, in which the expression level of insulin genes is increased when blood sugar rises and is reduced when it falls. These two regulation mechanisms are found in the differentiation functions not only of β-cells but of all cells.
Lymphocytes - the Only Cells with Different Genes
All human somatic cells except lymphocytes have the same genes. Humans can produce hundreds of millions of antibody types (proteins). However, although we have an enormous number of genes, all our somatic cells have only precursors of antibody genes, which cannot produce antibody proteins. During the differentiation process of lymphocytes, recombination occurs in antibody genes, thereby equipping each lymphocyte with just one antibody gene. As a result, a single human has genes to produce hundreds of millions of antibody types, whereas each lymphocyte has a gene for only one antibody type. If a clone is made from a lymphocyte, the clone will be able to synthesize only one antibody type (or may not be able to synthesize antibodies at all).