Dna Is Cut Into Short Lengths

Short, single-stranded piece of dna is TAKEN IN

FIGURE 18.07 Mechanism of Natural Competence

A cell that is naturally competent takes DNA into its cytoplasm by a protein-mediated process. First, the long molecule of double-stranded DNA is recognized by a receptor on the surface of the competent cell. A cell-surface endonuclease digests the DNA into small fragments. An exonuclease then degrades one strand of the DNA. The remaining single-stranded fragment is taken into the cytoplasm of the bacterium.

Natural competence is not merely due to random entry of DNA but involves the induction of a variety of genes whose products take part in DNA uptake. First DNA is bound by cell surface receptors (Fig. 18.07). Then the bound DNA is cut into shorter segments by endonucleases and one of the strands is completely degraded by an exonuclease. Only the resulting short single-stranded segments of DNA enter the cell. Part of the incoming DNA may then displace the corresponding region of the host chromosome by recombination.

Note that in the case of artificially induced competence, the mechanism is quite different. Double-stranded DNA enters the cell through a cell wall that is seriously

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