The melting temperature Tm of DNA in 0'2M NaCl can be calculated from the following formula:

1
The melting temperature (Tm) of DNA in 0.2M NaCl
can be calculated from the following formula Tm
(oC) 69.3 0.41 (GC). What can be concluded
from the melting curve of a sample of DNA with a
GC content of 50 if the Tm was found to be
75oC? A. The thermal denaturation was carried
out in lt 0.2M NaCl. B. The thermal denaturation
was carried out in pH gt11. C. The thermal
denaturation was carried out in the presence of
histones. D. The thermal denaturation was carried
out in the presence of magnesium ions. E. The
thermal denaturation was carried out in the
presence of DNA endonucleases. (44)
The above formula would predict that a DNA with a
GC content of 50 would have a Tm of 90oC if
denatured in 0.2M NaCl, since 69.3 0.41(50)
90oC. Since the Tm was only 75oC, the result
suggests that the NaCl concentration was lower
than 0.2M, thereby destabilizing the DNA duplex
and allowing a lower melting temperature. A pH
gt11 would itself completely denature the DNA.
Positive charges from histones or magnesium ions
would stabilize the DNA, not destabilize it. DNA
endonucleases would cut up the DNA but would not
necessarily change the melting temperature.
The ability of denatured, single-stranded DNA to
form duplexes of double-stranded DNA depends
primarily on the A. sequence complementarity
between the two strands. B. base composition
(GC) of the DNA strands. C. melting temperature
of the renatured duplexes. D. presence of
repetitive sequences in the DNA. (60)
The key to renaturation of DNA (or RNA-DNA
hybridization) is the complementarity between the
two strands this governs the ability of the
strands to reanneal. Base composition or melting
temperature (also dependent on base composition)
will not govern the renaturation process. The
presence of repetitive sequences may speed up the
process but the actual renaturation still depends
on sequence complementarity.