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Unformatted text preview: CHAPTER 19 THE ORGANIZATION AND CONTROL OF EUKARYOTIC GENOMES Introduction • Gene expression in eukaryotes has two main differences from the same process in prokaryotes. • First, the typical multicellular eukaryotic genome is much larger than that of a bacterium. • Second, cell specialization limits the expression of many genes to specific cells. • The estimated 35,000 genes in the human genome includes an enormous amount of DNA that does not program the synthesis of RNA or protein. • This DNA is elaborately organized. • Not only is the DNA associated with protein to form chromatin, but the chromatin is organized into higher organizational levels. • Level of packing is one way that gene expression is regulated. • Densely packed areas are inactivated. • Loosely packed areas are being actively transcribed. A. Eukaryotic Chromatin Structure 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. • During interphase of the cell cycle, chromatin fibers are usually highly extended within the nucleus. • Eukaryotic chromosomes contain an enormous amount of DNA relative to their condensed length. • Each human chromosome averages about 2 x 108 nucleotide pairs. • If extended, each DNA molecule would be about 6 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. • Their positively charged amino acids bind tightly to negatively charged DNA. • The five types of histones are very similar from one eukaryote to another and are even present in bacteria. • Unfolded chromatin has the appearance of beads on a string, a nucleosome , in which DNA winds around a core of histone proteins. • The beaded string seems to remain essentially intact throughout the cell cycle. • 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. • As chromosomes enter mitosis the beaded string undergoes higher-order packing. • The beaded string coils to form the 30- nm chromatin fiber . • This fiber forms looped domains attached to a scaffold of nonhistone proteins. • In a mitotic chromosome, the looped domains coil and fold to produce the characteristic metaphase chromosome....
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This note was uploaded on 08/28/2010 for the course SCIENCE 101 taught by Professor Wong during the Spring '10 term at Rutgers.
- Spring '10