Blog Against Cancer

FUNCTIONAL COMPONENTS OF THE GENE

Every gene consists of several functional components, each involved in a different facet of the process of gene expression (Figure) Broadly speaking, however, there are two main functional units: the “promoter” region and the “coding” region.
The promoter region controls when and in what tissue a gene is expressed. For example, the promoter of the hemoglobin gene is responsible for its expression in erythroid cells and not in brain cells.

How is this tissue-specific expression achieved? In the DNA of the gene’s promoter region, there are specific structural elements, “nucleotide sequences” (see “Structural Considerations” below), that permit the gene to be expressed only in an appropriate cell. These are the elements in the hemoglobin gene that instruct an erythroid cell to transcribe hemoglobin mRNA from that gene. These structures are referred to as “cis”-acting elements because they reside on the same molecule of DNA as the gene. In some cases, other tissue type-specific “cis”-acting elements, called “enhancers,” reside on the same DNA molecule, but at great distances from the coding region of the gene.3,4 In the appropriate cell, the “cis”-acting elements bind protein factors that are physically responsible for transcribing the gene. These proteins are called “trans” - acting factors because they reside in the cell’s nucleus separate from the DNA molecule bearing the gene.

For example, brain cells would not have the right “trans”-acting factors that bind to the hemoglobin promoter, and therefore brain cells would not express hemoglobin. They would, however, have “trans” - acting factors that bind to neuron-specific gene promoters.
The structure of a gene’s protein is specified by the gene‘s “coding” region. The coding region contains the information that directs an erythroid cell to assemble amino acids in the proper order to make the hemoglobin protein. How is this order of amino acids specified? As described in detail below, DNA is a linear polymer consisting of four distinguishable subunits called nucleotides. In the coding region of a gene, the linear sequence of nucleotides “encodes” the amino acid sequence of the protein. This genetic code is in triplet form so that every group of three nucleotides encodes a single amino acid. The 64 triplets that can be formed by four nucleotides exceeds the number of amino acids used to make proteins (20). This makes the code degenerate and allows some amino acids to be encoded by several different triplets.5 The nucleotide sequence of any gene can now be determined . By translating the code, one can derive a predicted amino
acid sequence for the protein encoded by a gene.