Chapter 19 - Chapter 19 - Eukaryotic Genomes Chapter 19...

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Chapter 19 - Eukaryotic Genomes Chapter 19 Eukaryotic Genomes Lecture Outline Overview: How Eukaryotic Genomes Work and Evolve Two features of eukaryotic genomes present a major information-processing challenge. o First, the typical multicellular eukaryotic genome is much larger than that of a prokaryotic cell. o 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. o 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. o The resulting chromatin undergoes striking changes in the course of the cell cycle. During interphase of the cell cycle, chromatin fibers are usually highly extended within the nucleus. 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. Eukaryotic chromosomes contain an enormous amount of DNA relative to their condensed length. o Each human chromosome averages about 1.5 × 108 nucleotide pairs. o If extended, each DNA molecule would be about 4 cm long, thousands of times longer than the cell diameter. o This chromosome and 45 other human chromosomes fit into the nucleus. o This occurs through an elaborate, multilevel system of DNA packing. Histone proteins are responsible for the first level of DNA packaging. o The mass of histone in chromatin is approximately equal to the mass of DNA. o Their positively charged amino acids bind tightly to negatively charged DNA. o The five types of histones are very similar from one eukaryote to another, and similar proteins are found in prokaryotes. o The conservation of histone genes during evolution reflects their pivotal role in organizing DNA within cells. Unfolded chromatin has the appearance of beads on a string. o In this configuration, a chromatin fiber is 10 nm in diameter (the 10-nm fiber). Each bead of chromatin is a nucleosome, the basic unit of DNA packing.
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o The “string” between the beads is called linker DNA. A nucleosome consists of DNA wound around a protein core composed of two molecules each of four types of histone: H2A, H2B, H3, and H4. o
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This note was uploaded on 07/22/2009 for the course BIOL 01 taught by Professor Sheeter during the Spring '09 term at St. Edwards.

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Chapter 19 - Chapter 19 - Eukaryotic Genomes Chapter 19...

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