Genes are arranged in a linear array on the chromosome. A major distinction between eukaryotic and prokaryotic chromosomes is that eukaryotic chromosomes contain vast amounts of DNA between the genes. The function of most of this "extra" DNA is unknown. It contains repetitive sequences, functionless gene copies called pseudogenes, transposible elements, and other types of DNA.
Eukaryotic genes may be dispersed randomly throughout the chromosome or they may be specifically organized. A gene family is a set of genes that originated from the duplication and subsequent variation of a common, ancestral gene. Members of a gene family may be clustered on the same chromosome, as in the case of the globin genes. Gene duplication events also have resulted in gene clusters in which related or identical genes are arranged in tandem. Examples of gene clusters include the genes for rRNA and histone proteins.
Higher-Order Organization
The DNA of a eukaryotic cell must be constrained within the confines of the nucleus. In human cells, six billion base pairs are contained on the forty-six chromosomes of double-stranded DNA. This DNA has a total length of1.8 meters, yet it must fit into a nucleus with an average diameter of 6 micrometers. This feat is accomplished in part by the packaging of DNA into chromatin, a condensed complex of DNA, histones, and nonhistone proteins.
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