Each human chromosome averages about 1.5 × 108 nucleotide pairs.Eukaryotic chromosomes contain an enormous amount of DNA relative to their condensed length.As a cell prepares for meiosis, its chromatin condenses, forming a characteristic number of short, thick chromosomes that can be distinguished with a light microscope.During interphase of the cell cycle, chromatin fibers are usually highly extended within the nucleus. The resulting chromatin undergoes striking changes in the course of the cell cycle.Eukaryotic DNA is precisely combined with large amounts of protein.While the single circular chromosome of bacteria is coiled and looped in a complex but orderly manner, eukaryotic chromatin is far more complex.Not only is the DNA associated with protein, but also this DNA-protein complex called chromatin is organized into higher structural levels than the DNA-protein complex in prokaryotes.Ĭoncept 19.1 Chromatin structure is based on successive levels of DNA packing.The estimated 25,000 genes in the human genome include an enormous amount of DNA that does not code for RNA or protein.Second, cell specialization limits the expression of many genes to specific cells.First, the typical multicellular eukaryotic genome is much larger than that of a prokaryotic cell.Two features of eukaryotic genomes present a major information-processing challenge.Overview: How Eukaryotic Genomes Work and Evolve
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