This code includes modifications of the histones' positively charged amino acids to create some domains in which DNA is more open and others in which it is very tightly bound up. When a specific gene is tightly bound with histone, that gene is "off." But how, then, do eukaryotic genes manage to escape this silencing? This is where the histone code comes into play. The histones are among the most evolutionarily conserved proteins known they are vital for the well-being of eukaryotes and brook little change. Why is this the case? The secret lies in chromatin, or the complex of DNA and histone proteins found within the cellular nucleus. (Using microarray analysis, scientists can use such differences to assist in diagnosis and selection of appropriate cancer treatment.) Interestingly, in eukaryotes, the default state of gene expression is "off" rather than "on," as in prokaryotes. A cancer cell acts different from a normal cell for the same reason: It expresses different genes. For instance, an undifferentiated fertilized egg looks and acts quite different from a skin cell, a neuron, or a muscle cell because of differences in the genes each cell expresses. For eukaryotes, cell-cell differences are determined by expression of different sets of genes. Here, the articles on prokaryotic regulation delve into each of these topics, leading to primary literature in many cases. Here, the sigma factor of RNA polymerase changes several times to produce heat- and desiccation-resistant spores. (In eukaryotes, there is no exact equivalent of attenuation, because transcription occurs in the nucleus and translation occurs in the cytoplasm, making this sort of coordinated effect impossible.) Yet another layer of prokaryotic regulation affects the structure of RNA polymerase, which turns on large groups of genes. Furthermore, some repressors have a fine-tuning system known as attenuation, which uses mRNA structure to stop both transcription and translation depending on the concentration of an operon's end-product enzymes. For instance, some repressors bind near the start of mRNA production for an entire operon, or cluster of coregulated genes. In prokaryotes, most regulatory proteins are specific to one gene, although there are a few proteins that act more widely. Some regulatory proteins must have a ligand attached to them to be able to bind, whereas others are unable to bind when attached to a ligand. The repressor or activator protein binds near its regulatory target: the gene. Here, the cells rely on protein–small molecule binding, in which a ligand or small molecule signals the state of the cell and whether gene expression is needed. For prokaryotes, most regulatory proteins are negative and therefore turn genes off. In contrast, regulated genes are needed only occasionally - but how do these genes get turned "on" and "off"? What specific molecules control when they are expressed? It turns out that the regulation of such genes differs between prokaryotes and eukaryotes. These genes also control protein synthesis and much of an organism's central metabolism. Such genes are among the most important elements of a cell's genome, and they control the ability of DNA to replicate, express itself, and repair itself. Some genes are constitutive, or always "on," regardless of environmental conditions. Genes can't control an organism on their own rather, they must interact with and respond to the organism's environment. Next, we turn to the regulation of genes. Along the way, the article set also examines the nature of the genetic code, how the elements of code were predicted, and how the actual codons were determined. Thus, this collection or articles begins by showing how a quiet, well-guarded string of DNA is expressed to make RNA, and how the messenger RNA is translated from nucleic acid coding to protein coding to form a protein. How does a gene, which consists of a string of DNA hidden in a cell's nucleus, know when it should express itself? How does this gene cause the production of a string of amino acids called a protein? How do different types of cells know which types of proteins they must manufacture? The answers to such questions lie in the study of gene expression.
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