Eukaryotic DNA Is Complexed with Histones

DNA in eukaryotic chromosomes exists in a highly compacted form known as chromatin, a complex of DNA with a great variety of proteins. Five proteins called histones are present in large amounts (see table 25.2) and are believed to form a regularly repeating structural motif. The remaining proteins are present in smaller amounts and are irregularly distributed. 

The histones present in chromatin are of five major types HI, H2a, H2b, H3, and H4 (table 25.2). The lysine-rich histone HI is not present in the nucleosome core particles, as evidenced by its release on extensive nuclease treatment and the finding that HI is the only histone that readily exchanges between free and DNA-bound histone. HI may play a key role in the conversion of chromatin to the highly compacted chromosome that occurs immediately before cell division. The other eight histones, two each of the other four histones, form the protein core of the nucleosome. These protein octamers do not come apart even when chromosomes duplicate. 

An illustration of the coiled-coil structure of the nucleosome is presented in figure 25.18. The 140 bp of DNA 

Salt bridges between positively charged basic amino acid side chains of histones and the negatively charged DNA phosphates play a major role in stabilizing the DNA-histone complex. Indeed, treatment of chromatin with concentrated NaCl (1-2 m), which is known to disrupt electrostatic bonds, causes a complete dissociation of DNA and histone in the nucleohistone complex. 

The organization of genes within a typical eukaryotic chromosome is far more complex and less well understood than in prokaryotes. It is highly likely that a much lower percentage of the DNA is informational in complex eukaryotes than in prokaryotes. E. coli contains about 3,000 genes the 

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Physical Structure of the Bacterial Chromosome The Genetic Map of Escherichia coli Eukaryotic DNA Is Complexed with Histones Organization of Genes within Eukaryotic... 

A number of differences between eukaryotes and prokaryotes affect the processes of replication, transcription, and translation, in addition to the content of their DNA. Eukaryotic DNA is complexed with histones, and prokaryotic DNA is not. In eukaryotic cells, the process of transcription, which occurs in the nucleus, is separated by the nuclear envelope from the process of translation (protein synthesis from the mRNA template), which occurs in the cytoplasm. Because prokaryotes lack nuclei, the processes of transcription and translation occur simultaneously. Transcription of bacterial DNA requires only one promoter per operon. In contrast, human DNA requires one promoter for each gene. 

How does supercoiling take place in eukaryotic DNA Eukaryotic DNA is complexed with histones and other basic proteins, but less is known about proteins bound to prokaryotic DNA. 

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