In a CsCl labeling experiment fully heavy DNA for bacterium MB100 has a buoyant
ID: 79742 • Letter: I
Question
In a CsCl labeling experiment fully heavy DNA for bacterium MB100 has a buoyant density of 1, 8 g/cc in CsCl and fully light DNA has a buoyant density of 1, 7 g/cc. After extended growth in heavy medium, MB100 is shifted to light medium a) What is the density or densities of DNA isolated after growth for 1 generation in light medium? what fraction of DNA found in each density b) what is the density, or densities, of the DNA after growth for 2 generation in light medium? What fraction of the DNA found in each density? c) The DNA is isolated after 1.0 generation is denatured and then banded in a CsCl gradient. How many peaks are seen, and what are the characteristics of the DNA found in each peak d) which of the above experiment (a, b, c, and/or d) demonstrate that DNA replication is overa semiconservative process? Why?Explanation / Answer
A- after one round of growth, the density of isolated DNA will be between 1.8g/cc and 1.7g/cc. All the DNA will be found between the 1.8g/cc and 1.7 g/cc density gradient of CsCl
b - after two round of growth, there will be two bands. One band will be at 1.7 g/cc and another band will be above 1.7 g/cc but below the 1.8 g/cc density gradient.
c- we will get two peaks after denaturing the DNA of one round of growth. DNA found at 1.8 g/cc will be Heavy DNA and DNA found in 1.7 g/cc will be light DNA.
d-A and B show that DNA replication is semiconservative. After round one of replication we got only one band which is in the middle of heavy and light DNA and after second growth cycle, we got two bands one at the 1.7 density gradient and another one between the 1.8 and 1.7 density gradient. The presence of one band in the first round of growth shows that DNA replication is semiconservative. Had DNA replication is not semi-conservative we would have got two bands one band at 1.8 density gradient and another one at 1.7 density gradient. Same is the case with the second round of replication. Had DNA replication is not semi-conservative we would have got two bands one band at 1.8 density gradient and another one at 1.7 density gradient. But we got two bands one at 1.7 density gradient and another one between the 1.8 and 1.7 density gradient.
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