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DNA is normally conceived of as a spiral ladder, but in eukaryotic cells (cells with nuclei) the DNA in the nucleus is strung around a series of spool-shaped proteins known as histones. Their chief functions are to compact and control the long threads of DNA. They compact the DNA by interacting with each other to form a structure like a compact spool. Two turns of DNA are wrapped around this spool, forming the subunits known as nucleosomes and decreasing the effective length of DNA eightfold. At high magnification these DNA-histone complexes look like a series of beads on a string. The complexes are further compacted by a factor of four by a linker histone that binds the DNA between the nucleosomes, organizing them into a coil. In this way a chromosome containing 20 million base pairs of DNA is organized into approximately 100,000 nucleosome core particles. Histones are also involved in controlling which sequences of DNA are turned on for transcription of RNA. When histones are chemically modified in certain ways, they may loosen their hold on the DNA and allow it to become accessible to proteins that activate transcription, or they may tighten their hold on the DNA and make it inaccessible. DNA itself may be chemically modified in the process known as DNA methylation, which is another mechanism for regulating gene expression. It is thought that the histones stay with the same sequences of DNA after cell replication, so the modifications of the histones and DNA allow the same sets of genes to be turned on and off in the daughter cells as in the parent cell. This is one way that multicellular organisms can make multiple types of cells (such as muscle, liver, and skin), even though the different types of cells all contain the same DNA in their nuclei. The histones are among the most well-conserved proteins known. There are only two minor changes in the amino acid sequences of the histone designated H4 in the pea and cow, for example. This near uniformity across species suggests that the entire surface of each histone is important to its function and that all plants and animals use histones for the same functions.