Assume that you grew a culture of E. coli for many generations in medium contain
ID: 165160 • Letter: A
Question
Assume that you grew a culture of E. coli for many generations in medium containing 15N (from the ammonium ion), a heavy isotope of nitrogen. You extract DNA from a portion of the culture and determine its density to be 1.723 gm/cm3 (call this sample A). You then wash the remaining E. coli cells and grow them for one generation in 14N, and extract the DNA from a portion of the culture (call this sample B). You let the culture grow for one more generation in 14N, and extract the DNA (call this sample C). Each sample of DNA (A, B, and C) is then subjected to ultracentrifugation. Label the centrifugation profiles for the below given diagram. (Note: Assume that unlabeled [14N] DNA has a density of 1.700 gm/cm3.)
semiconservative replication conservative replication Absorbance (260mm) B 66 1.70 density 1.74 Absorbance B 1.66 1.70 density 1.74Explanation / Answer
It is called as density gredian centrifugation, which is highly dependen on the density of th ebiological molecules,procedure in which separating of particles such as viruses or ribosomes or molecules such as DNA in which the sample is placed on a preformed gradient.
Upon centrifugation either by rate zonal or equilibrium procedures, the macromolecules are forming one band in the gradient and can be collected as a pure fraction.
it has two types
Isopycnic centrifugation: In this techniq the density gradient column encompasses the whole range of densities of the sample particles.
therefor it seperates the molecules based on the density. it formnig band in the form of same molecules is concentrated in each side.
Rate zonal centrifugation:in this type of centrifugation, the sample is applied in a thin zone at the top of the centrifuge tube on a density gradient.
in above problem the method is not defind for ther centrifugation.
conservative raplication means teo separate DNA in supercoil
while semiconservative means each contain two one of origanal DNA.
In abobe problem
Zone A: it defining the higher dense molecules it is not quantitative. the no. of such DNA as compared to othe is lower but the density it is higer than the others.
Zone B it suggesting that the two diffferent molecules are be therw in the sample whose density is same among one as the A and the othe region has lower density as comared to zone A it suggesting the different kind of molecules present in b or we can say that it is mixture of A and the other molecules.
Zone C It suggesting that density it lies in below 1.74 but when it conservative supercoil structure impreove the density and thats why it shows two separate region. among them one will be similar to A and the othe will be similar to B.