In 1883, Wilhelm Roux proposed that the filaments observed when cell nuclei were stained with basic dyes were the bearers of the hereditary factors. Heinrich Wilhelm Waldeyer later coined the word chromosome ("colored body") for these filaments. The eukaryotic chromosome now is defined as a discrete unit of the genome, visible only during cell division, that contains genes arranged in a linear sequence. Eukaryotic organisms contain much more genetic information than prokaryotes. For example, the eukaryotic organism Saccharomyces cerevisiae (baker's yeast) contains 3.5 times more DNA in its haploid state than the prokaryotic Escherichia coli, while higher vertebrate cells contain more than 1,000 times the DNA.
The basic component of the eukaryotic chromosome is its DNA, which contains all of the genetic material responsible for encoding a particular organism. 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.
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