Cellular Location of DNA

Most of the DNA of animal cells is found in the nucleus, where DNA is the major constituent of the chromosomes. On the other hand, most of the RNA is located in the cytoplasm. Nuclear DNA exists as a thin, double helix only 2 nm wide. The double helix is folded and complexed with protein to form chromosomal strands approxim-ately 100 to 200 nm in diameter. Each chromosome contains a single DNA duplex. The human chromosomes vary in size the smallest contains approximately 4.6 X 10 base pairs of DNA, and the largest 2.4 X 10 base pairs. In contrast, the Escherichia coli chromosome has 4.5 x 106 base pairs. The DNA of die chromosomes is tightly packed and associated with both histone and nonhistone proteins. 

The amount of genomic DNA in a particular organism is roughly proportional to the complexity of the organism. Table shows the content of DNA in the genomes of several widely different organisms. The data are normalized to a haploid set of chro-mosomes, since some cells listed are haploid and others are diploid. The DNA content 

The chromosome structure is visible only during the mitotic portion of the cell cycle. The constituent parts of the chromosomes are nucleoprotein fibers called chromatin. When condensed, chromatin forms a microscop-ically visible chromosome-like structure. The chromosomes are composed of DNA, RNA, and proteins. The relative amounts of the three vary, but chromatin is primarily protein and DNA. 

The four histone groups that are composed of ho-mogeneous proteins, H2A, H2B, H3, and H4, make up the nucleosome core. Each core consists of two copies of the four histones. The double-stranded DNA is wrapped twice around each core in a left-handed superhelix. A superhelix is the name given to the additional helix made by the double-stranded, helical DNA as it is wrapped around the nucleosome core. A familiar superhelix in everyday life is a twisted spiral telephone cord. The nucleosome core of histones do not recognize specific DNA structures rather, they can bind to any stretch of DNA as long as it is not too close to a neighboring nucleosome. The order of contact of histones to the DNA is as follows  

Protein-protein interactions between the histone subunits are undoubtedly important in promoting formation of a nucleosome in which 146 base pairs of DNA are coiled around the outside of the histone core. One molecule of histone HI binds to an exterior region of each nucleosome, but histone HI is not needed to determine nucleo-some structure. The distance between nucleosomes is approximately 200 base pairs consequently, in electron micrographs, nucleosomes resemble evenly spaced beads on a string of DNA. Neutron and x-ray diffraction data are also consistent with this structure. 

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