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BIO 240 notes.doc

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University of Toronto St. George

BIO 240 Lecture 414102010 060900Xray crystallography supports a zigzag model for the stacking of nucleosomes in the 30nm fiberCryoelectron microscopy of longer strings of nucleosome supports a solenoidal structure with intercalated nucleosomes in the 30nm fiberThe causes of nucleosomes to stack so tightly on each other in a 30nm fiberThe nucleosome to nucleosome linkages formed by histone tails most notably the H4 tail constitute one important factorAdditional histone 1 to 1 ratio with nucleosome cores known as histone H1A change in the exit path in DNA is crucial for compacting nucleosomal DNA so that it interlocks to form the 30nm fiberIn eukaryotes proteincoding genes are usually composed of a string of alternating introns and exons associated with regulartory regions of DNACells contain dozens of different ATPdependent chromatin remodeling complexes which are specialized for different roles ATPdependent chromatin remodeling complexes due to presence of these arrangement of nucleosomes on DNA can be highly dynamic changing with needs of the cellAs genes are turned onoffchromatin remodeling complex brought to specific regions of DNA to influence chromatin structureMost important influence on nucleosome positioning presence of other tightly bound proteins on DNAExact position of nucleosomes along stretch of DNA depends mainly on presence and nature of other proteins bound to the DNAThe DNA in eucaryotes is tightly bound to an equal mass of histones which form repeated arrays of DNAprotein particles called nucleosomes The nucleosome is composed of an octameric core of histone proteins around which the DNA double helix is wrapped Nucleosomes are spaced at interuals of about 200 nucleotide pairs and they are usually packed together with the aid of histone Hl molecules into quasiregular arrays to form a 30nm chromatin fiber Despite the high degree of compaction in chromatin its structure must be highly dynamic to allow access to the DNA There is some spontaneous DNA unwrapping and rewrapping in the nucleosome itself how euer the general strategy for reuersibly changing local chromatin structure features ATPdriuen chromatin remodeling complexes Cells contain a large set of such complexes which are targeted to speciflc regions of chromatin at appropriate times The remodeling complexes collaborate with histone chaperones to allow nucleosome cores to be repositioned reconstituted with dffirent histones or completely remoued to expose the underlying DNA
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