Each nucleosome bead includes dna plus eight histones

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Each nucleosome bead includes DNA plus eight histones. Stretches of DNA, called linkers, join consecutive nucleosomes.
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3 Eukaryotic chromosomes undergo multiple levels of folding and coiling, called DNA packing. At the next level of packing, the beaded string is wrapped into a tight helical fiber. This fiber coils further into a thick supercoil. Looping and folding can further compact the DNA. DNA packing tends to prevent gene expression by preventing RNA polymerase and other transcription proteins from contacting the DNA. Higher levels of packing can therefore inactivate genes for the long term. Highly compacted chromatin, found in varying regions of interphase chromosomes, is generally not expressed at all. Chemical modification of DNA bases or histone proteins can result in epigenetic inheritance. Certain enzymes can add a methyl group to DNA bases, without changing the sequence of the bases. Individual jeans are usually more methylated in cells in which the genes are not expressed. Once methylated, genes usually stay that way through successive cell divisions in an individual. Removal of the extra methyl groups can turn on some of these genes. Inheritance of traits transmitted by mechanisms not directly involving the nucleotide sequence is called epigenetic inheritance. These modifications can be reversed by processes not yet fully understood. X chromosome inactivation In female mammals, one of the two X chromosomes is chemically modified and highly compacted. Either the maternal or paternal chromosome is randomly inactivated. Inactivation occurs early in embryonic development, and all cellular descendants have the same inactivated chromosome. An inactivated X chromosome is called a Barr body. Tortoiseshell fur coloration is due to inactivation of X chromosomes in heterozygous female cats.
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4 11.3 Prokaryotes and eukaryotes employ regulatory proteins (activators and repressors) that bind to specific segments of DNA and either promote or block the binding of RNA polymerase, turning the transcription of genes on and off. In eukaryotes, activator proteins seem to be more important than repressors. Thus, in multicellular eukaryotes, the default state for most genes seems to be off. A typical plant or animal cell needs to turn on and transcribe only a small percentage of its genes. Eukaryotic RNA polymerase requires the assistance of proteins called transcription factors. Transcription factors include activator proteins, which bind to DNA sequences called enhancers and initiate gene transcription, and other transcription factor proteins that interact with the bound activators, which then collectively bind as a complex at the gene’s promoter.
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  • Fall '16
  • Stacy Ort
  • DNA

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