Depends on which organism your looking at put basically, you have repressor proteins which bind to the DNA directly which inhibit transcription by physically blocking the RNA polymerase. These proteins can be lifted when a signal molecule bind to repressor protein causing a conformational change so that it becomes unbound to the DNA and RNA polymerase can work. This is a negative control. There is a also positive control where the are proteins which bind to promoter regions on the DNA and help increase the rate of transcription. This is what I remember from bio, hope it helps.
In your cells, certain genes are "switched on" and certain genes are "switched off". Genes aren't constantly transcribed because at certain periods of time, the protein product of the gene is not needed. To transcribe the gene when the protein product is not needed would be a waste of the cell's metabolic machinery.
The cell needs to be efficient to use as little energy as possible. Hence, if the protein isn't needed, the gene won't be transcribed.
If you want to look at it from a molecular level, the genes aren't transcribed because they are tightly packed thanks to DNA methylation (the addition of methyl groups to DNA) which causes the DNA to pack very closely together, decreasing the chance of transcription occurring.
The other posters have given adequate descriptions of the mechanisms underlying selective expression of certain genes. Actually, there are a number of genes that ARE being "constantly" transcribed, since their products have a high turnover rate. We refer to such genes as being "housekeeping genes," and their rate of production really doesn't vary much over a fairly wide range of conditions. But most genes are turned on or off depending upon the cell's need.
But your question is really not HOW all that happens, but why? Why not have 'em all turned on, so that every possible condition could be anticipated? Why did the cell evolve complicated mechanisms to turn things on and off? For example, why not simply have degradation mechanisms cranked up at the same rate as production, then have a simple rheostat-like mechanism in the degradation process which could turn down as needed? I believe there are 2 simple reasons for this, and both of them would constitute a significant selective pressure favoring the evolutionary conservation of such mechanisms: 1) the cell has a finite volume, and thus a finite solvating capacity. In addition to all the other smaller solutes in the cell there's only so much protein you can pack into that small volume without them all aggregating into a big, insoluble mess. So, only those genes that are absolutely needed at a given time are made, and the ability of the cell's water to dissolve them all is not overwhelmed. Also, a lot of these proteins are inserted into various membranes of the cell, and there's only so much space in those membranes. You can't keep shoving proteins into membranes willy-nilly without compromising those membranes' integrity. 2) The cell has only so much energy to expend upon expression of genes and their products, since it has to use the majority of its energy to keep ion pumps running. Thus, the cell has to conserve the ATP it has available for only those gene products that it needs at that time.
Some are; most aren't. Each cell contains all the genes needed to make your entire body, from conception to death. Your liver cells don't really need to be making hair, and your brain cells don't need to be making hemoglobin, so they leave those genes turned off.
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Depends on which organism your looking at put basically, you have repressor proteins which bind to the DNA directly which inhibit transcription by physically blocking the RNA polymerase. These proteins can be lifted when a signal molecule bind to repressor protein causing a conformational change so that it becomes unbound to the DNA and RNA polymerase can work. This is a negative control. There is a also positive control where the are proteins which bind to promoter regions on the DNA and help increase the rate of transcription. This is what I remember from bio, hope it helps.
In your cells, certain genes are "switched on" and certain genes are "switched off". Genes aren't constantly transcribed because at certain periods of time, the protein product of the gene is not needed. To transcribe the gene when the protein product is not needed would be a waste of the cell's metabolic machinery.
The cell needs to be efficient to use as little energy as possible. Hence, if the protein isn't needed, the gene won't be transcribed.
If you want to look at it from a molecular level, the genes aren't transcribed because they are tightly packed thanks to DNA methylation (the addition of methyl groups to DNA) which causes the DNA to pack very closely together, decreasing the chance of transcription occurring.
The other posters have given adequate descriptions of the mechanisms underlying selective expression of certain genes. Actually, there are a number of genes that ARE being "constantly" transcribed, since their products have a high turnover rate. We refer to such genes as being "housekeeping genes," and their rate of production really doesn't vary much over a fairly wide range of conditions. But most genes are turned on or off depending upon the cell's need.
But your question is really not HOW all that happens, but why? Why not have 'em all turned on, so that every possible condition could be anticipated? Why did the cell evolve complicated mechanisms to turn things on and off? For example, why not simply have degradation mechanisms cranked up at the same rate as production, then have a simple rheostat-like mechanism in the degradation process which could turn down as needed? I believe there are 2 simple reasons for this, and both of them would constitute a significant selective pressure favoring the evolutionary conservation of such mechanisms: 1) the cell has a finite volume, and thus a finite solvating capacity. In addition to all the other smaller solutes in the cell there's only so much protein you can pack into that small volume without them all aggregating into a big, insoluble mess. So, only those genes that are absolutely needed at a given time are made, and the ability of the cell's water to dissolve them all is not overwhelmed. Also, a lot of these proteins are inserted into various membranes of the cell, and there's only so much space in those membranes. You can't keep shoving proteins into membranes willy-nilly without compromising those membranes' integrity. 2) The cell has only so much energy to expend upon expression of genes and their products, since it has to use the majority of its energy to keep ion pumps running. Thus, the cell has to conserve the ATP it has available for only those gene products that it needs at that time.
Some are; most aren't. Each cell contains all the genes needed to make your entire body, from conception to death. Your liver cells don't really need to be making hair, and your brain cells don't need to be making hemoglobin, so they leave those genes turned off.
Jesus intelligently intervenes. I know because I have a personal relationship with Jesus and he told me.