For some DNAs it is possible to separate the two strands, after denaturation, in a CsCl gradient. Part A What property of any DNA determines where it will band in a CsCl gradient? density, charge. polarity. solubility Part B What kind of DNA might have two strands that differ sufficiently in this property that they could be separated after denaturation? UT-rich DNA, GC-rich DNA, AT-rich DNA, AC-rich DNA Solution PART A: Density The experiments done by Meselson and Stahl’s supported the model of semiconservative replication of DNA. In their experiment, they obtained heavy DNA and light DNA by growing Escherichia coli (E. coli) bacteria with a radioactive nitrogen isotope 15N, and the more common isotope of nitrogen 14N respectively. In the present experiment, the bacteria grown in 15N medium were allowed to grow in 14N medium. Then the cultured bacteria were taken and the isolated DNA was dissolved and centrifuged caesium chloride solution (CSCl). Now, the concentration gradient is established in the test tube, and the DNA molecules move according to the gradient until they reach an equilibriumdensity with the solution. DNA with 15N strand is heavier than the DNA with 14N strand, so it reaches the bottom of the test tube. PART B: GC-rich DNA GC rich DNA contains high density, which also has high melting point..

1. For some DNAs it is possible to separate the two strands, after denaturation, in a CsCl gradient.
Part A What property of any DNA determines where it will band in a CsCl gradient? density,
charge. polarity. solubility
Part B What kind of DNA might have two strands that differ sufficiently in this property that
they could be separated after denaturation?
UT-rich DNA, GC-rich DNA, AT-rich DNA, AC-rich DNA
Solution
PART A: Density
The experiments done by Meselson and Stahl’s supported the model of semiconservative
replication of DNA. In their experiment, they obtained heavy DNA and light DNA by growing
Escherichia coli (E. coli) bacteria with a radioactive nitrogen isotope 15N, and the more common
isotope of nitrogen 14N respectively.
In the present experiment, the bacteria grown in 15N medium were allowed to grow in 14N
medium. Then the cultured bacteria were taken and the isolated DNA was dissolved and
centrifuged caesium chloride solution (CSCl). Now, the concentration gradient is established in
the test tube, and the DNA molecules move according to the gradient until they reach an
equilibriumdensity with the solution. DNA with 15N strand is heavier than the DNA with 14N
strand, so it reaches the bottom of the test tube.
PART B: GC-rich DNA
GC rich DNA contains high density, which also has high melting point.