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chapter 19 outline 2

chapter 19 outline 2 - charged DNA • Histones are similar...

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How Eukaryotic Genomes Work and Evolve Human genome is much more complicated than the genomes of prokaryotes Chromatin, which is a DNA-protein complex, is organized into a higher structural level in eukaryotes than prokaryotes Chromatin Structure is Based on Successive Levels of DNA Packing Eukaryotic DNA is combined with a large amount of protein and chromatin undergoes many changes during the cell cycle o Interphase- chromatin is a diffuse mass in the nucleus o As the cell prepares for mitosis- chromatin coils and folds up (condenses), eventually forming short, thick chromosomes that are distinguishable from each other Eukaryotic chromosomes contain an enormous amount of DNA relative to their condensed length. All the DNA fits into the nucleus through a multilevel system of DNA packing 1) Nucleosomes or “Beads on a String” Seen during interphase Histone (proteins) mass in chromatin is equal to DNA mass Histones are positively charged amino acids (lysine and arginine) and they bind tightly to negatively
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Unformatted text preview: charged DNA • Histones are similar in all eukaryotes and even prokaryotes Chromatin appears as beads on a string (10 nm in diameter) • Each bead is a nucleosome (basic unit of DNA packing) o Nucleosomes consist of DNA wound around a protein core composed of two molecules each of four types of histone (H2A, H2B, H3, H4). The amino end of each histone protein extends outward from the nucleosome. The fifth histone (H1) attaches to the DNA near the nucleosome when a 10 nm chromatin undergoes the next level of packing. • The string between the beads is linker DNA DNA and histones remain intact throughout the cell cycle • Histones leave DNA only transiently during DNA replication and almost always stay with DNA during transcription. o DNA is transcribed while being wrapped around histones because changes in shapes and positions of nucleosomes allow RNA synthesizing polymerases to move along DNA...
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