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EukaryGeneRegulation211S - Eukaryotic Gene Expression and...

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Eukaryotic Gene Expression and Regulation - 1 We have recently discussed some examples of how viruses regulate gene expression and the operon of prokaryotic gene expression. The eukaryotic genome is more complex than the prokaryotic genome. Not only do eukaryotes have more DNA and more genes than prokaryotes, in multicellular organisms different genes are activated or repressed in different tissues from the time of early development, so we have differential or selective gene expression depending on the cell and tissue type. There are also more variations in the DNA of eukaryotes. As we shall study in our section on genomics, only about 2 – 5% (or less) of genes code for proteins. Another portion codes for RNA, but most of our DNA is not transcribed. Being able to sequence genomes of many species, including the human genome, has been of tremendous benefit to the growing understanding of eukaryotic DNA and its gene expression. We will look at genomics and the characteristics of eukaryotic DNA in our next section. To begin our discussion of gene regulation in eukaryotes, let's briefly review the structure of the eukaryotic DNA molecule. We have learned that a chromosome is composed of DNA that is 2 nm in diameter wrapped around a set of histone proteins to form a bead-like set of nucleosomes that are about 10nm in diameter. We know, too, that during mitosis, chromosomes condense. (There are also intermediate levels of DNA organization that are important to our understanding of gene function and regulation.) Nucleosomes coil to form a chromatin fiber of about 30nm in diameter. The chromatin fibers continue to fold to form 300nm looped domains along the chromosome during prophase. The loops are anchored to a non-histone protein scaffold.
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Eukaryotic Gene Expression and Regulation - 2 Ultimately, at metaphase, chromosomes achieve their densest packing of about 700nm per chromatid and are readily visible with a microscope. However, packing of chromosomes is not restricted to condensation for mitosis. During Interphase, many regions of a chromosome remain as 10nm and 30nm chromatin fibers, anchored to the nuclear matrix and some regions, including telomeres and centromeres, remain tightly condensed and clumped. Highly condensed chromatin regions are called heterochromatin. The heterochromatin is too compacted to be transcribed. The less compacted regions of chromatin are known as euchromatin. Only the more stretched out or dispersed euchromatin can be transcribed. Cells also have chemical means of blocking DNA access for gene expression, as we shall discuss. Another function of the nuclear matrix is to organize the chromosomes during Interphase. Each chromosome has a fixed, non-overlapping territory within the nuclear matrix.
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