BCHM323 Gene Structure

Genes are the instruction manuals for the composition and functioning of all living things. They are made up of DNA. One strand of DNA may contain several genes. A typical example of a gene is the gene for the synthesis of the β-globin chain of the hemoglobin molecule. There are two general types of genes in the human genome: non-coding RNA genes and protein-coding genes. Gene products can either serve a structural or catalytic function in the cell.


Protein-coding genes represent the majority of the total genes expressed, and are in two stages: transcription and translation (see Gene expression ). The protein coding genes are incredibly diverse in size and organization and hence have no characteristic structure. There are, however, several conserved features as shown below Figure1.








Figure 1: Structure of a Eukaryotic gene


The boundaries of a protein-encoding gene are defined as the points at which transcription begins and ends. The core of the gene is the coding region, which contains the nucleotide sequence that is eventually translated into the sequence of amino acids in the protein.


Most eukaryotic genes can be compartmentalized into several compartments with different functions that enable the precise expression of the gene see Figure 1. The promoter region of the gene plays a unique function of signaling the beginning of the gene and is uniquely recognized and bound by sigma factor and RNA polymerase to initiate transcription in prokaryotes (usually more complicated in eukaryotes). The transcribed region of the gene is divided into exons and introns. The exons are the sections that are found in the mature transcript (messenger RNA), while the introns are removed from the primary transcript by a process called splicing (see Gene expression ). Gene enhancers are regions of the DNA that can recognize and bind proteins to enhance the transcription of a particular gene or group of genes. Enhancers can be located close to or far from the gene they influence.


The smallest protein-coding gene in the human genome is only 500 nucleotides long and has no introns. It encodes a histone protein (see Epigenetics ). The largest human gene encodes the protein dystrophin, which is missing or non-functional in the disease muscular dystrophy. This gene is 2.5 million nucleotides in length and it takes over 16 hours to produce a single transcript. However, more than 99 per cent of the gene made up of its 79 introns.


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