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Chromosome Structure Changes as Eukaryotic Ceils Divide 143 Sister Chromatid Cohesion and Chromosome-

Condensation Are Mediated by SMC Proteins 144 Mitosis Maintains the Parental Chromosome Number 146 The Gap Phases of the Cell Cycle Allow Time to Prepare for the Next Cell Cycle Stage while also Checking that the Previous Stage Is Finished Correctly 146 Me[osis Reduces the Parental Chromosome Number f 48 Different Levels of Chromosome Structure

Can Be Observed by Microscopy 150 THE NUCLEOSOME 151 Nueleosomes Are the Building Blocks of Chromosomes 151 Box 7-1 Mierocctcal Nuclease arid the DNA Associated mth the "Nucleosome 152

Histories Are Small, Positively-Charged Proteins 153

The Atomic Structure of the Nucleosome 154

Many DNA Sequence-^I ndependent Contacts Mediate the Interaction between the Core 1 listones and DNA 156 The H is co ne N-Term mal Tails Stabtliie DNA

Wrapping around the Octamer 159 HIGHER-ORDER CHROMATIN

STRUCTURE 160 H is tone HI Binds to the Linker DNA between

N uc leosomes 160 Nucleosome Arrays Can Form More Complex

Structures: the 30-nnn Fiber 161 The Historie N-Terminal Tails Are Required for the Formation of the 30-nm Fiber 162 Further Compaction of DNA Involves Large Loops of Nucleosomal DNA 162 Historie Variants Alter Nucleosome Function 163 REGULATION OF CHROMATIN

STRUCTURE 165 The interaction of DNA with the Histone Octamer Is Dynamic 165

Nucleosome Remodeling Complexes Facilitate Nucleosome Movement 166 Some Nuc leosomes Are Found in Specific Positions in vivo: Nucleosome Positioning 168 Modification of the N-Termtnal Tails of the Histondä Alters Chromatin Accessibility 169 Box 7-2 Determining ~Nnd?osome Position in the Cell 170

Specific Enzymes Are Responsible for Histone Modification 17} Nucleosome Modification and Remodeling Work

Together to Increase DNA Accessibility 174 NUCLEOSOME ASSEMBLY 175 Nucleosjonies Are Assembled Immediately after DNA Replication 175 Assembly of Nucleosomes Requires

Histone "Chaperones" 176 Swiiwiary 179 BiHiogTUÍíhy ISO

CHAPTfcR 8

The Replication of DNA 181 THE CHEMISTRY OF DNA SYNTF1ESIS

DNA Synthesis Requires DeOxynucleoside Triphosphates and a PrimenTemplatt: Junction DNA Is Synthesized by Extending the 3' End of the Primer 183 Flydrolysis of Pyrophosphates Is the Driving Force for DNA Synthesis 183 THE MECHANISM OF DNA POLYMERASE 184 DNA Polymerases Use a Single Active Site to Catalyze DNA Synthesis 184 DNA Polymerases Resemble a Hand that Grips the PrimenTemplate Junction 186 DNA Polymerases Are Ptocessive Enzymes 188 Exonucleases Proofread Newly Synthesized DNA THE REPLICATION FORK 192 Roth Srrands of DNA Are Synthesized Together at the Replication Fork 192 The Initiation of a New Strand of DNA Requires an RNA Primer 193

182 RNA Primers Must Be Removed to Complete DNA Replication 194

182 DNA Helicases Unwind the Double Helix in Advance of the Replication Fork 194

Single-Stranded Binding Proteins Stabilize Single-Stranded DNA Prior to Replication 195

Dvx 8-1 De terming the Po/arir}1 of a DNA Hehcase 196

Topoisotnerases Remove Supercoils Produced by DNA Unwinding at the Replication Fork 198

Replication Fork Enzymes Extend rhe Range of DNA Polymerase Substrates 199

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