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Figure 4. Model of the condensed bacterial chromosome. The circular DNA molecule (a) is folded into several domains (b). Seven domains are shown, but in the cell there are approximately fifty. These domains are held together by RNA and proteins. The DNA is further condensed by supercoiling (c). If the DNA backbone is nicked by a nuclease, the DNA domain opens (d). Adapted from Pettijohn, David E., 1996.

Figure 4. Model of the condensed bacterial chromosome. The circular DNA molecule (a) is folded into several domains (b). Seven domains are shown, but in the cell there are approximately fifty. These domains are held together by RNA and proteins. The DNA is further condensed by supercoiling (c). If the DNA backbone is nicked by a nuclease, the DNA domain opens (d). Adapted from Pettijohn, David E., 1996.

Minimal-gene-set concept. One of the interesting features of studying bacterial chromosomes has been the concept of the minimal number of genes a cellular life form would need to survive. (This excludes viruses and viroids, which need living cells of a host in which to carry out their life cycle.) We know from the sequence of the Mycoplasma genitalium chromosome, the smallest genome sequenced so far, that the upper limit of the minimal gene set is 480, as shown in Table 1. After the sequence of the Haemophilus influenzae chromosome was completed, a comparison of the genes that were identical (or highly conserved) in the two species led to an estimate of 256 as the minimal gene set. The National Institutes of Health scientist Eugene Koonin, with the availability of many more sequenced species, has also estimated a minimal size of about 250 genes. It may be possible in the future for scientists to construct a minimal life-form by removing nonessential genes from an organism such as M. genitalium. see also Antibiotic Resistance; Archaea; Chromosome, Eukaryotic; Conjugation; Escherichia coli (E. coli bacterium); Eubacteria; Human Genome Project; Operon; Replication.

Ralph R. Meyer

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