![]() View this video that describes how epigenetic regulation controls gene expression. A gene can be turned on or off depending upon the location and modifications to the histone proteins and DNA. Instead, these changes are temporary (although they often persist through multiple rounds of cell division) and alter the chromosomal structure (open or closed) as needed. Epigenetic means “around genetics.” The changes that occur to the histone proteins and DNA do not alter the nucleotide sequence and are not permanent. This type of gene regulation is called epigenetic regulation. The nucleosomes would pack more tightly together. What impact do you think these changes would have on nucleosome packing? Show Answer In females, one of the two X chromosomes is inactivated during embryonic development because of epigenetic changes to the chromatin. Modifications to the histones and DNA affect nucleosome spacing. Transcription factors can bind, allowing gene expression to occur. When the nucleosomes are spaced far apart (bottom), the DNA is exposed. When nucleosomes are spaced closely together (top), transcription factors cannot bind and gene expression is turned off. Nucleosomes can move to open the chromosome structure to expose a segment of DNA, but do so in a very controlled manner. If DNA encoding a specific gene is to be transcribed into RNA, the nucleosomes surrounding that region of DNA can slide down the DNA to open that specific chromosomal region and allow for the transcriptional machinery (RNA polymerase) to initiate transcription (Figure 2). (credit “micrograph”: modification of work by Chris Woodcock) ![]() When viewed through an electron microscope (b), the nucleosomes look like beads on a string. These nucleosomes control the access of proteins to the underlying DNA. DNA is folded around histone proteins to create (a) nucleosome complexes. These beads (histone proteins) can move along the string (DNA) and change the structure of the molecule. Under the electron microscope, this winding of DNA around histone proteins to form nucleosomes looks like small beads on a string (Figure 1b). Histones package and order DNA into structural units called nucleosome complexes, which can control the access of proteins to the DNA regions (Figure 1a). The first level of organization, or packing, is the winding of DNA strands around histone proteins. It is also organized so that specific segments can be accessed as needed by a specific cell type. The DNA in the nucleus is precisely wound, folded, and compacted into chromosomes so that it will fit into the nucleus. The human genome encodes over 20,000 genes each of the 23 pairs of human chromosomes encodes thousands of genes. ![]() This form of regulation, called epigenetic regulation, occurs even before transcription is initiated. Eukaryotic gene expression begins with control of access to the DNA. Unlike prokaryotic cells, eukaryotic cells can regulate gene expression at many different levels. These different patterns of gene expression cause your various cell types to have different sets of proteins, making each cell type uniquely specialized to do its job.Įukaryotic gene expression is more complex than prokaryotic gene expression because the processes of transcription and translation are physically separated. Thanks to gene regulation, each cell type in your body has a different set of active genes – despite the fact that almost all the cells of your body contain the exact same DNA. Gene regulation is how a cell controls which genes, out of the many genes in its genome, are “turned on” (expressed). Almost all of your cells contain the same set of DNA instructions: so why do they look so different, and do such different jobs? The answer: different gene regulation! Your amazing body contains hundreds of different cell types, from immune cells to skin cells to neurons. What you’ll learn to do: Discuss different components and types of epigenetic gene regulation RNA splicing, the first stage of post-transcriptional control.Post-Transcriptional Control of Gene Expression.Turning Genes Off: Transcriptional Repressors.The Promoter and the Transcription Machinery.Eukaryotic Transcription Gene Regulation.What you’ll learn to do: Discuss different components and types of epigenetic gene regulation.
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