Gene Expression In Eukaryotes

Word Roots and Origins intron and exon

The "int" in the word intron comes from the "int" in the word intervening. The "ex" in the word exon comes from the "ex" in the word expressed.

Eukaryotes are vastly different from prokaryotes. Their genomes are much larger than those of prokaryotes. In addition, the DNA of eukaryotic cells is located in several individual chromosomes instead of in the single circular chromosome that occurs in prokary-otes. Finally, most eukaryotes are multicellular organisms made of specialized cells. Although each cell type contains a complete set of the organism's genes, only some of these genes are expressed at a given time. Different cell types produce different proteins. Not surprisingly, the control of gene expression in eukaryotes is far more complex than it is in prokaryotes. Although operons are common in prokaryotes, they have not been found often in eukaryotes.

Structure of a Eukaryotic Gene

Much of the control of gene expression in eukaryotes occurs at the level of the individual chromosome. In eukaryotes, gene expression is partly related to the coiling and uncoiling of DNA within each chromosome. Recall that eukaryotic DNA is organized as fibers of chromatin wrapped around small specialized proteins called histones. Prior to mitosis or meiosis, the DNA and histones coil tightly, forming the structures we recognize as chromosomes. After mitosis or meiosis, certain regions of the DNA coils relax, thus making transcription possible. This uncoiled form, known as euchromatin (yoo-KROH-muh-tin), is the site of active transcription of DNA into RNA. However, some portions of the chromatin in specific cells remain permanently coiled, so their genes can never be transcribed. Thus, the degree to which DNA is uncoiled indicates the degree of gene expression.

As in prokaryotes, the promoter is the binding site of RNA polymerase in eukaryotes. In the eukaryotic gene, there are two kinds of segments beyond the promoter: introns and exons. Introns (IN-trahnz) are sections of a structural gene that are transcribed but are not translated. Exons (EK-sahnz) are the sections of a structural gene that, when expressed, are transcribed and translated.

The benefits of the intron-exon pattern of gene organization are not yet fully understood. For many years, scientists were uncertain of the role of introns in the cell. However, recent research suggests that the noncoding RNA transcribed from introns performs important functions even though it is not translated. Some of these functions include regulating RNA that is translated, interacting with this coding RNA to influence gene expression, and acting as "switches" that allow protein production only when "turned on" by the presence of certain chemical targets. Scientists continue to explore the role of introns and noncoding RNA. For example, some researchers are investigating medicines that work by affecting the actions of introns and noncoding RNA.

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