Gene Expression

Some genes are expressed, or turned on to produce their specific proteins, in some parts of the plant and not in others. Another set of genes code for proteins that regulate the expression of genes. These regulatory genes and their protein products form complex interacting systems that ultimately turn flower color genes on in flowers and turn chlorophyll genes on in leaves, and neither set of genes in roots.

An example of regulatory proteins that control the expression of other genes is the family of so-called "Helix-Loop-Helix" proteins. This group of proteins got its name from a characteristic three-dimensional structure that occurs in each of their molecules -- two helical segments connected by a loop. These Helix-Loop-Helix proteins bind to specific genes at specific points along the DNA chains comprising the genes. The binding of a regulatory protein to the DNA of a gene can either activate the gene (promoting its expression) or block it (thereby preventing its expression). That is to say, there are "ON" switches and there are "OFF" switches.

Another example of regulatory genes is found in the "Homeo Box" family of genes. Homeo Box genes are found throughout the eukaryotic world, from fruit flies to mice, men, and flowers. These code for proteins which bind to the DNA segments ahead of the starting points of the genes that are involved in the development of the structures of an organism's body, controlling formation of similar parts such as segments, limbs, and flower petals. A group of Homeo Box genes of particular interest are the MADS proteins. In flowering plants, the MADS proteins control the structural development of the blooms. Many double flowers, where the stamens have become petalloids, are probably the result of modified MADS genes. (MADS is an acronym from the names of the first four genes of this group which were identified, from mice, Arabidopsis, Drosophila, and humans.)

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Last revised on 4 January 2004
© Copyright 1995, 2001, 2004 James E. Shields
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