And gene regulation is at the bottom of what makes a cell decide to become a red blood cell, or a neuron, or a hepatocyte in the liver, or a muscle cell.
So different gene regulation will give you a different program of genes and different genes expressed. There are several different kinds of gene regulation. Some genes, called housekeeping genes, are expressed in almost every cell. And these require a regulatory network or machinery that keeps them on in almost every cell, so these are the enzymes that help make DNA, and perform glycolysis, and burn sugar, and things like that.
There are other genes that are called tissue-specific genes. These are genes that, say, would only be expressed in a red blood cell or a neuron. Very often, these genes have transcription factors, which are proteins that bind to DNA, near these genes. And those transcription factors actually help the RNA machinery get there and transcribe that gene in those cells, and those tissues, transcription factors, rather, are expressed specifically in those tissues.
There are also factors expressed in those tissues that will be suppressors that can turn a gene off. First, an enzyme nicknamed "Dicer" chops any double-stranded RNA it finds into pieces that are about 22 nucleotides long. This binding blocks translation of viral proteins at least partially, if not completely. The RNAi system could potentially be used to develop treatments for defective genes that cause disease. The treatment would involve making a double-stranded RNA from the diseased gene and introducing it into cells to silence the expression of that gene.
All Rights Reserved. Date last modified: February 2, Created by Wayne W. DNA, Genetics, and Evolution. Contents All Modules. Control of Gene Expression By gene expression we mean the transcription of a gene into mRNA and its subsequent translation into protein. The structural genes contain the code for the proteins products that are to be produced. Regulation of protein production is largely achieved by modulating access of RNA polymerase to the structural gene being transcribed.
The promoter gene doesn't encode anything; it is simply a DNA sequence that is initial binding site for RNA polymerase. 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.
A cancer cell acts different from a normal cell for the same reason: It expresses different genes. 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. Why is this the case? The secret lies in chromatin, or the complex of DNA and histone proteins found within the cellular nucleus.
The histones are among the most evolutionarily conserved proteins known; they are vital for the well-being of eukaryotes and brook little change. When a specific gene is tightly bound with histone, that gene is "off.
This is where the histone code comes into play. 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.
DNA methylation is one mechanism that appears to be coordinated with histone modifications, particularly those that lead to silencing of gene expression. On the other hand, when the tails of histone molecules are acetylated at specific locations, these molecules have less interaction with DNA, thereby leaving it more open. The regulation of the opening of such domains is a hot topic in research. For instance, researchers now know that complexes of proteins called chromatin remodeling complexes use ATP to repackage DNA in more open configurations.
Scientists have also determined that it is possible for cells to maintain the same histone code and DNA methylation patterns through many cell divisions. This persistence without reliance on base pairing is called epigenetics, and there is abundant evidence that epigenetic changes cause many human diseases. For transcription to occur, the area around a prospective transcription zone needs to be unwound. This is a complex process requiring the coordination of histone modifications, transcription factor binding and other chromatin remodeling activities.
Many of these proteins are activators, while others are repressors; in eukaryotes, all such proteins are often called transcription factors TFs. In the test tube, scientists can find a footprint of a TF if that protein binds to its matching motif in a piece of DNA. Some activating TFs even turn on multiple genes at once. All TFs bind at the promoters just upstream of eukaryotic genes, similar to bacterial regulatory proteins.
However, they also bind at regions called enhancers, which can be oriented forward or backwards and located upstream or downstream or even in the introns of a gene, and still activate gene expression. Because many genes are coregulated, studying gene expression across the whole genome via microarrays or massively parallel sequencing allows investigators to see which groups of genes are coregulated during differentiation, cancer, and other states and processes.
Most eukaryotes also make use of small noncoding RNAs to regulate gene expression. For example, the enzyme Dicer finds double-stranded regions of RNA and cuts out short pieces that can serve in a regulatory role.
Argonaute is another enzyme that is important in regulation of small noncoding RNA—dependent systems. Here we offfer an introductory article on these RNAs, but more content is needed; please contact the editors if you are interested in contributing. Imprinting is yet another process involved in eukaryotic gene regulation; this process involves the silencing of one of the two alleles of a gene for a cell's entire life span.
Imprinting affects a minority of genes, but several important growth regulators are included. For some genes, the maternal copy is always silenced, while for different genes, the paternal copy is always silenced. The epigenetic marks placed on these genes during egg or sperm formation are faithfully copied into each subsequent cell, thereby affecting these genes throughout the life of the organism.
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