Chapter07_SSM - 65781_CH07_137_151.qxd 12:58 PM Page 137...

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137 Chapter 7: Molecular Organization of Chromosomes Chapter Summary Most organisms have a genome composed of double-stranded DNA. (It is RNA or single-stranded DNA in some viruses.) Some chromosomes and plasmids in prokaryotic organisms are circular DNA molecules, whereas others are linear DNA molecules. Circular DNA molecules are invariably supercoiled. In bacteria the supercoiling is negative (right-handed twists) whereas in archaeans it is positive (left-handed twists). The chromosomal DNA of eukaryotes is nearly always linear and has little supercoiling except in regions where genes are actively being transcribed. Among metazoans, there is no correlation between the genome size of an organism and its evolutionary or metabolic complexity (the C-value paradox). The DNA of both prokaryotic and eukaryotic cells and of viruses is never in a fully extended state, but rather is folded in an intricate way, which reduces its effective volume. In viruses, the DNA is tightly folded but without bound protein molecules. In bacteria, the DNA is folded to form a multiple-looped structure of independently supercoiled domains called a nucleoid, which includes several proteins that are essential for folding. In eukaryotes, the DNA is compacted into true chromosomes, which contain several proteins and which are large enough to be visible by light microscopy during the mitotic phase of the cell cycle. The DNA–protein complex of eukaryotic chromosomes is called chromatin. The protein component of chromatin consists primarily of five distinct proteins: histones H1, H2A, H2B, H3, and H4. The last four histones aggregate to form an octameric protein that contains two molecules of each. DNA is wrapped around the histone octamer, forming a structure called a nucleosome core particle. This wrapping is the first level of compaction of the DNA in chromosomes. Each nucleosome unit contains about 200 nucleotide pairs, of which about 145 are in contact with the core particle. The remaining 55 nucleotide pairs link adjacent nucleosomes. Histone H1 binds to the linker segment and draws the nucleosomes nearer to one another. The DNA in its nucleosome form is further compacted into a helical fiber, the 30-nm chromatin fiber. In the interphase nucleus, the chromatin fiber is organized into 100-kb chro- matin loops and these into 1-Mb chromatin domains, which occupy distinct positions defining a territory for each chromosome. In chromosome condensation during the cell cycle, the 30-nm chromatin fiber folds to produce a highly compact visible chromosome. The result is that a eukaryotic DNA molecule,
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138 whose length and width are about 50,000 μm and 0.002 μm, respectively, is folded to form a chromosome with a length of about 5 μm and a width of about 0.5 μm.
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This note was uploaded on 09/29/2011 for the course GENETICS 380 taught by Professor Glodowski during the Spring '08 term at Rutgers.

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Chapter07_SSM - 65781_CH07_137_151.qxd 12:58 PM Page 137...

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