Chromosome Sequence And Diversity

Before we discuss the structure of chromosomes in detail, it is important to understand the features of the DNA molecules that form their foundation, The recent sequencing of the genomes of numerous organisms has provided a wealth of information concerning the makeup of chromosomal DNAs and how their characteristics have changed as organisms have increased in complexity.

Chromosomes Can Be Circular or Linear

The traditional view is that prokaryotic cells have a single, circular chromosome and eukaryotic cells have multiple, linear chromosomes (Table 7-1}. As more prokaryotic organisms have been studied, this view has been challenged. Although the most studied prokaryotes (such as E, coli and B. svbtiGs] do indeed have single circular chromosomes, there are now numerous examples of prokaryotic cells that have multiple chromosomes, linear chromosomes, or even both. In contrast, all eukaryotic cells have multiple linear chromosomes. Depending on the eukaryotic organism, the number of chromosomes typically varies from 2 to less than SO, but in rare instances can reach

TABLE 7-1 Variation in Chromosome Makeup in Different Organisms

Species

Number of chromosomes

Chromosome copy number

Form of chromosome(s)

Genome size (Mb)

PROKARYOTES Mycoplasma genitalium Escherichia cc/fK-12

Agrobactenum tumefaciens Sinorhhizobium melitoti

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