08.PMCB.lecture11 - 9/16/2008 1 lant Molecular and Cellular...

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Unformatted text preview: 9/16/2008 1 lant Molecular and Cellular Biology Lecture 9: Nuclear Genome Organization: Chromosome Structure, Chromatin, DNA Packaging, Mitosis Gary Peter 9/16/2008 PMCB Lecture 11: G. Peter 2 Learning Objectives 1. List and explain how DNA is packaged in the nucleus 2. Explain how euchromatin and heterochromatin differ 3. Explain proposed mechanisms that maintain heterochromatin in its state 4. Explain the structure, functions of enzymes that mediate chromatin remodeling and their proposed mechanisms 9/16/2008 PMCB Lecture 11: G. Peter 3 DNA in the Nucleus is Organized into Chromosomes Chromosomes z One very long linear dsDNA molecule/chromosome with z Single copy, Repetitive, and Highly repetitive sequences z Centromere sequences z Two teleomere sequences z Multiple origins of replication z Proteins that fold and pack the long DNA strand into more compact chromatin z Histones z Nonhistone chromosomal proteins 9/16/2008 PMCB Lecture 11: G. Peter 4 Example of Human Genome z A human cell contains 2 m of DNA stretched end to end that must fit into a nucleus that is ~6 uM in diameter z A maize cell contains 2 m of DNA stretched end to end that must fit in a nucleus that is <10 uM in diameter z Compaction is ~1000 fold for interphase chromosomes and 10,000 fold between dsDNA and mitotic chromosomes 9/16/2008 PMCB Lecture 11: G. Peter 5 Packing of DNA into the Nucleus: Multiple Levels of Compaction Interphase Mitotic 3-fold 27 fold 700 fold ~1000 fold 9/16/2008 PMCB Lecture 11: G. Peter 6 Evidence for Nucleosomes as the Basic Unit of Chromosome Structure z Histone mass = the mass of DNA in chromatin z Gentle lysis of nuclei and TEM analysis shows that chromatin is a 30nm wide thread z Decondensation of chromatin reveals beads on string 9/16/2008 PMCB Lecture 11: G. Peter 7 ucleosome Isolation & Organization z Unfolded chromatin is digested with micrococcal nuclease z Limited digestion leaves histone H1 + nucleosomal core with an average of 200bp of DNA z More extensive digestion releases H1 and yields core particles with 146bp of DNA protected from nuclease digestion z The 54bp on average is a linker DNA (Linker varies from 5-80bp) z Nuclesome cores dissociated with high salt removes the 146 bp DNA from the octameric histone core 9/16/2008 PMCB Lecture 11: G. Peter 8 Nucleosome Core Structure: X-ray Crystal Structure z Core is a histone octamer with 2 subunits of H2A, H2B, H3 and H4 with DNA wrapped around 1.65 turns in a left-handed coil z Histones are basic proteins rich in lysine and arginine that make salt bridges with the backbone phosphates z Extensive hydrogen bonds (146) between histones and DNA with...
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08.PMCB.lecture11 - 9/16/2008 1 lant Molecular and Cellular...

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