Lecture 10- eukaryotic molecular genetics

Lecture 10- eukaryotic molecular genetics - Almost all of...

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Almost all of the cells in your body have the same genes because they were generated from a single cell by mitosis, which of course produces cells that are clones. Yet the cells in different tissues are very different. Why are all your cells not identical the way identical twins are identical? The reason is that different types of cells express different genes. Your blood cells make hemoglobin which makes them red. Your skin cells don't, so your skin is not red. Some skin cells produce melanin, the pigment that gives your skin color. However, very few other tissues do, hence the observation that we are all the same color on the inside. Of course, there are genes that all cells must express that are necessary for basic functions of metabolism, transcription etc. These are often referred to as housekeeping genes. In addition to these are the genes that must be expressed to make a liver cell a liver cell, blood cells blood cells etc. Then there are also genes that are only turned on in response to specific conditions, these can be tissue specific or shared by most or all cells. But how do different cells determine what genes they will express and what genes they won't? There are several points in the transfer of genetic information from DNA to protein where the rate of flow of information and therefore the amount of a given protein might be controlled: at transcription, transport out of the nucleus, mRNA stability, translation, protein function.
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These are all used by some organism somewhere for some gene product. But the main way of determining cell type is regulation of transcription. If you look at different types of cells and the proteins that they express, you will generally find that cells that do not express a protein also do not express the mRNA for that protein. Thus, regulation of the transcription of a particular gene appears to be the main method for controlling what proteins will be present. There are some similarities to the way that prokaryotes regulate transcription as exemplified by the operons we discussed previously but there are also some important differences. What's similar is that what is regulated is the ability of the RNA polymerase to initiate transcription. In both prokaryotes and eukaryotes this is in part determined by the promoter element. The eukaryotic promoter element usually consists of a TATA box about 25 bp upstream (think of transcription as a river flowing, something "upstream" is usually before the transcription start site) and one or a few other types of regulator sequences 50-150 bp upstream (e.g. CAAT box, GC box, Oct element).
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One important difference is that Eukaryotes have three polymerases. Pol I transcribes rRNA, Pol II mRNA and Pol III
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Lecture 10- eukaryotic molecular genetics - Almost all of...

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