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chap19studyguide - Chapter 19 Eukaryotic Genomes...

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Chapter 19 Eukaryotic Genomes: Organization, Regulation, and Evolution Study Guide Overview: How Eukaryotic Genomes Work and Evolve Two features of eukaryotic genomes present a major information-processing challenge. ° First, the typical multicellular eukaryotic genome is much larger than that of a prokaryotic cell. ° Second, cell specialization limits the expression of many genes to specific cells. The estimated 25,000 genes in the human genome include an enormous amount of DNA that does not code for RNA or protein. This DNA is elaborately organized. ° 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. Concept 19.1 Chromatin structure is based on successive levels of DNA packing While the single circular chromosome of bacteria is coiled and looped in a complex but orderly manner, eukaryotic chromatin is far more complex. Eukaryotic DNA is precisely combined with large amounts of protein. ° The resulting chromatin undergoes striking changes in the course of the cell cycle. Eukaryotic chromosomes contain an enormous amount of DNA relative to their condensed length. ° Each human chromosome averages about 1.5 × 10 8 nucleotide pairs. ° If extended, each DNA molecule would be about 4 cm long, thousands of times longer than the cell diameter. ° This chromosome and 45 other human chromosomes fit into the nucleus. ° This occurs through an elaborate, multilevel system of DNA packing. Histone proteins are responsible for the first level of DNA packaging. ° The mass of histone in chromatin is approximately equal to the mass of DNA. ° Their positively charged amino acids bind tightly to negatively charged DNA. Unfolded chromatin has the appearance of beads on a string. ° In this configuration, a chromatin fiber is 10 nm in diameter (the 10-nm fiber).
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Each bead of chromatin is a nucleosome, the basic unit of DNA packing. Histones leave the DNA only transiently during DNA replication. They stay with the DNA during transcription. ° By changing shape and position, nucleosomes allow RNA-synthesizing polymerases to move along the DNA. Chromosomes have highly condensed areas, heterochromatin, and less compacted areas, euchromatin. Heterochromatin DNA is largely inaccessible to transcription enzymes. ° Looser packing of euchromatin makes its DNA accessible to enzymes and available for transcription. Concept 19.2 Gene expression can be regulated at any stage, but the key step is transcription Like unicellular organisms, the tens of thousands of genes in the cells of multicellular eukaryotes are continually turned on and off in response to signals from their internal and external environments. Gene expression must be controlled on a long-term basis during
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chap19studyguide - Chapter 19 Eukaryotic Genomes...

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