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| Bacterial Chromosome |
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| In bacteria, which being prokaryotic organisms, the entire hereditary material is packed into a single, irregularly folded compact mass called nucleoid or genophore or bacterial chromosome. It is short and simple consisting of a single DNA molecule. The DNA is in the form of a double helix which forms a closed ring or circle with no free ends. It is permanently attached to a mesosome, an infolding of the plasma membrane. The bacterial chromosome lacks a protein coat and it is in direct contact with the cytoplasm, since a nuclear membrane is absent. A small amount of protein, mainly in the form of an enzyme called RNA polymerase, may be found associated with the bacterial chromosome. |
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| Since a single copy of the chromosome occurs in a cell, the bacterial cells are haploid. The amount of DNA is much less and codes for fewer proteins compared to an eukaryotic cell. In the bacterium Escherichia coli the 1100 Mm long DNA is packed into a space of just 1 Mm due to intense coiling. It has about 2500 genes. |
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| fig. 19.2 - Bacterial Chromosome |
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| In addition to the nucleoid, a bacterial cell may show the presence of extra chromosomal DNA molecules called plasmids. Like the bacterial chromosome, plasmids are double stranded circular DNA molecules which can replicate and function independently. The plasmids mainly carry genes responsible for characteristics like fertility, antibiotic resistance and production of bacteriocin (a protein that kills closely related bacteria). The plasmids can be easily isolated from or introduced into the bacterial cells. They can be integrated with desired genes. Hence, plasmids are of immense use in genetic engineering. |
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