temperature (' C)
FIGURE 6*16 Dependence of DNA denaturation on C + C content and an salt concentration. The greater the G + C content, the higbef the temperature ml<sl be in denature the DNA strand. DNA from different sources was dissolved in solutions of low (red line) and high (green line) concentrations of sail at pH 7.0. The points represent the temperature at which the DNA denatured, graphed against the G + C content. (Source: Data from Mermur J and Doty P. 1962. Journal of Molecular Bio lag}1 b . 120. Copyright © 1962, with permission from Riseuier Science.)
groups which carry a negative charge. These negative charges arc close enough across the two strands that if not shielded, they tend to cause the strands to repel each other, facilitating their separation. At high ionic strength, the negative charges are shielded by cations, thereby stabilising the helix. Conversely, at low ionic strength the unshielded negative charges render the helix less stable.
It was initially believed that all DNA molecules ai-e linear and have two free ends. Indeed, the chromosomes of eukaryotic cells each contain a single (extremely long) DNA molecule. But now we know that some DNAs are circles. For example, the chromosome of the small monkey DNA virus SV40 is a circular, double-helical DNA molecule of about 5,000 base pairs. Also, most (but not all) bacterial chromosomes are circular; B coli has a circular chromosome of about 5 million base pairs. Additionally, many bacteria have small autonomously replicating genetic elements known as pJasmids, which are generally circular DNA molecules.
Interestingly, some DNA molecules are sometimes linear and sometimes circular. The most well-known example is that of the bacteriophage A, a DNA virus of E. coli. The phage A genome is a linear double-stranded molecule in the virion particle. However, when the A genome is injected into an E. coli cell during infection, the DNA circularizes. This occurs by base-pairing between single-stranded regions that protrude from the ends of the DNA and that have complementary sequences, also known as "sticky ends."
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