Humans have about 20,000 protein-coding genes. Scientists have found that about 3800 of those genes are continually expressed in all cells (Eisenberg and Levanon 2013). These so-called housekeeping genes encode proteins, such as RNA polymerase, that all cells need to make at a steady rate in order to survive. But the remaining 14,200 genes are required in some tissues at certain times—though not in others. The cells that produce our hair do not produce hemoglobin; likewise, our blood cells do not express the keratin proteins found in our hair.
Our cells express different combinations of genes thanks to a complex interaction of molecules (See Table 1). The first type of regulation to be carefully documented targets the transcription of DNA into RNA. As we saw earlier, RNA polymerase and other molecules must attach to a site upstream of the protein-coding region of a gene to commence transcribing it. That upstream section, known as the gene control region, may also contain small segments of DNA where molecules can bind. If a protein called a repressor attaches to a site in that upstream region, for example, it will block the advance of the transcribing molecules, and the gene will not be expressed. Other proteins, called transcription factors, bind to sites called enhancers in the gene control region, where they activate gene expression
Table 1: The process of regulating genes in eukaryotes is extremely flexible, and a variety of mechanisms are available at every stage. Mutations affecting any of these regulatory steps can lead to genetic variation for the expression of organismal phenotypes.
Coiling or packing of DNA.
Can render a gene more or less accessible to RNA polymerase and to cis-regulatory factors necessary for transcription.
Methylation of DNA. Binding specificities of RNA polymerase, repressors, activators, transcription factors, hormones/signals.
Any given gene control region may contain a number of regulatory regions where different molecules can bind. A cell can thus exercise exquisite control over the precise conditions under which it will express a given gene. Likewise, a cell can also coordinate the expression of many genes at once. Hundreds of genes often carry identical regulatory regions where the same transcription factor can bind. In many cases, those genes encode other transcription factors of their own that switch even more genes on and off. Thus a single protein can trigger the expression of a cascade of genes.
Scientists first discovered gene regulation in the late 1950s. Researchers in France identified a repressor protein that turned off a gene in the bacteria Escherichia coli, preventing it from feeding on lactose. And for many years afterward, scientists found other proteins controlling the expression of genes. More recently, however, scientists have come to appreciate that RNA molecules can also regulate gene expression.
For example, microRNAs silence genes by binding to mRNA molecules that would otherwise be translated into proteins. They act like switches, turning genes on and off in response to changes in the environment. Some RNA molecules help to coordinate the development of embryos. For a human embryo to develop different tissues and organs, for example, certain genes must make proteins in certain cells while other genes must be blocked.