Size, Organization, and Topology of DNA

DNA molecules are very long. For example the DNA in a bacterial cell is contained within a single double-helical molecule, which, when spread out, is about 1,000 times as long as the diameter of the rod-shaped cell. This molecule carries all the genetic information of the cell and thus describes the genome (a single complement of the genetic material). 

The term chromosome refers to a physical or organizational unit within which part of or all the genome is contained. Thus, the E. coli genome is contained within just one chromosome, comprising a single DNA molecule. It has a size of 2.5 x 109 Da and contains approximately 4.6 x 106 base pairs. The size of DNA molecules is more commonly expressed in kilobase pairs (kb, 1,000 base pairs). The E. coli chromosome is 4,639 kb in size. Another feature of this particular molecule is that it is a closed, or circular, structure, i.e., there are no free ends. 

What variation in genome size, chromosome number, and DNA topology occurs among various organisms  

This information is summarized below for a number of commonly investigated viruses, bacteria, and eukaryotes (organisms whose cells contain nuclei). 

Note that in Example 7.17, the DNA of bacteriophage f X174 is single-stranded, not double-stranded. In this case, the genome size given in kb refers to the number of base pairs in an equivalent duplex form. In progressing from the simple viruses to eukaryotes, the amount of information in the 

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