Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 77

The DNA molecule of a bacterium that carries most of its normal genes is commonly referred to as its chromosome, by analogy to the chromosomes of higher organisms. This name distinguishes the molecule from plasmid DNA, which in some cases can be almost as large as chromosomal DNA but usually carries genes that are not always required for growth of the bacterium (see chapter 4). Most bacteria have only one chromosome; in other words, there is only one unique DNA molecule per cell that carries most of the normal genes. There are exceptions, and it is estimated that 10% of bacteria have more than one chromosome, including Vibrio cholerae, the bacterium responsible for the disease cholera. Even in bacteria that contain multiple chromosomes, the second chromosome shows more characteristics of a plasmid than of a chromosome, particularly in how it initiates replication. There appear to be special molecular systems for managing multiple chromosomes (see Fournes et al., Suggested Reading).

As discussed below, when bacteria, such as E. coli, are reproducing very rapidly, new rounds of replication initiate before others are completed, temporally increasing the DNA content of the cells until cellular division returns the number of copies to one unit chromosome per cell. It is important to note, however, that these individual chromosomal DNAs are not unique since they are directly derived from each other by replication.

The structure of bacterial DNA differs significantly from that of the chromosomes of higher organisms. One difference is that the DNA in the chromosomes of most bacteria is circular in the sense that the ends are joined to each other (for exceptions, see Box 4.1). In contrast, eukaryotic chromosomes are usually linear with free ends. As discussed in chapter 4, the circularity of bacterial chromosomal DNA allows it to replicate in its entirety without using telomeres, as eukaryotic chromosomes do, or terminally redundant ends, as some bacteriophages do (see chapter 7). Even in cases where bacterial chromosomes are linear, they do not use the same mechanism, involving telomerases to replicate their ends, that is used by eukaryotic chromosomes. Another difference between the DNA of bacteria and eukaryotes is that the DNA in eukaryotes is wrapped around proteins called histones to form nucleosomes. Bacteria have the proteins HU, HN-S, Fis, and IHF, around which DNA is often wrapped, and archaea do have rudimentary histones related to those of eukaryotes. However, in general, DNA is much less structured in bacteria than in eukaryotes.