Chapter 19 - Chapter 19 Eukaryotic Genomes: Organization,...

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Unformatted text preview: Chapter 19 Eukaryotic Genomes: Organization, Regulation, and Evolution Overview: How Eukaryotic Genomes Work and Evolve In eukaryotes, DNA-protein complex, called chromatin Ordered into higher structural levels than DNA-protein complex in prokaryotes Figure 19.1 Both prokaryotes & eukaryotes Must alter patterns of gene expression in response to changes in environmental conditions Concept 19.1: Chromatin structure is based on successive levels of DNA packing Eukaryotic DNA Combined with large amount of protein Eukaryotic chromosomes Contain enormous amount of DNA relative to condensed length Nucleosomes, or Beads on a String Proteins called histones 1 st level of DNA packing in chromatin Bind tightly to DNA In electron micrographs Unfolded chromatin has appearance of beads on a string Each bead is a nucleosome Basic unit of DNA packing Figure 19.2 a 2 nm 10 nm DNA double helix Histone tails His- tones Linker DNA (string) Nucleosome (bad) Histone H1 (a) Nucleosomes (10-nm fiber) Nucleosome 30 nm (b) 30-nm fiber Higher Levels of DNA Packing Next level of packing Forms 30-nm chromatin fiber Figure 19.2 b The 30-nm fiber Forms looped domains , making 300-nm fiber Figure 19.2 c Protein scaffold 300 nm (c) Looped domains (300-nm fiber) Loops Scaffold In mitotic chromosome Looped domains coil forming characteristic metaphase chromosome Figure 19.2 d 700 nm 1,400 nm (d) Metaphase chromosome In interphase cells Most chromatin is in highly extended form called euchromatin Concept 19.2: Gene expression can be regulated at any stage, but the key step is transcription All organisms Must regulate which genes are expressed at any given time During development of a multicellular organism Cells undergo specialization in form & function called cell differentiation Differential Gene Expression Each cell Expresses only a fraction of its genes In each type of differentiated cell A unique subset of genes is expressed Regulation of Chromatin Structure Genes within highly packed heterochromatin Are usually not expressed Histone Modification Chemical modification of histone tails Can affect configuration of chromatin & thus gene expression Figure 19.4a (a) Histone tails protrude outward from a nucleosome Chromatin changes Transcription RNA processing mRNA degradation Translation Protein processing and degradation DNA double helix Amino acids available for chemical modification Histone tails Histone acetylation Seems to loosen chromatin structure & thereby enhance transcription Figure 19.4 b (b) Acetylation of histone tails promotes loose chromatin structure that permits transcription Unacetylated histones Acetylated histones DNA Methylation Addition of methyl groups to certain bases in DNA Is associated with reduced transcription in some species Epigenetic Inheritance Traits transmitted by mechanisms not directly involving nucleotide sequence Regulation of Transcription...
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This note was uploaded on 01/18/2011 for the course BIO 103 taught by Professor Sanders during the Spring '10 term at Northwestern IA.

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Chapter 19 - Chapter 19 Eukaryotic Genomes: Organization,...

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