Four different uracil of Neurospora, a eukaryotic mold, are tested for growth on

Question

Four different uracil of Neurospora, a eukaryotic mold, are tested for growth on methionine and methionine precursor (compounds A-D). The data are shown in the following table. A plus sign (+) means growth. Diagram the uracil pathway, showing the order of the intermediates in the pathway (in the rectangles) and the step at which each mutant is blocked (above the arrow). In the late 1950s, Meselson and Stahl grew bacteria in a medium containing "heavy" nitrogen (^35N) and then transferred the bacteria to a medium containing^14N ("lighter" nitrogen) and allowed some of the bacteria to undergo one round of replication and some bacteria to undergo two rounds of replication. They then performed density gradient centrifugation and looked to see where the DNA settled in the test tube. In the diagram below, draw bands indicating the density of the DNA before replication, after one round of replication and after two rounds of replication. The above represent the actual results from Meselson and Stahl's experiment. What would the results have looked like if replication were conservative instead of semiconservative?

Explanation / Answer

Ans Q1.The four different uracil auxotrophs of neurospora are tested for growth on methionine and methionine precursors (compound A-D)

Met 2 mutant is blocked at step where methionine precursor B is added. Then Met 4 mutant is blocked at step where methionine precursor A and B is added. Then Met 3 mutant is blocked at step where methionine precursor A, B and D is added. Then Met 1 mutant is blocked at step where methionine precursor A, B, C and D is added.

Ans Q2.Meselson and Stahi's classical experiment to show semi conservative mode of DNA replication grew grew bacteria in a medium containing heavy nitrogen isotope so all DNA became heavy. So before replication single band will be formed in all heavy category. After one round of replication one strand will be heavy and one with newly synthesized light strand so all of them will band in intermediate density, and the DNA after two generations (two rounds of replication) showed equal amounts of intermediate and light strands of DNA. So will form two bands, one in intermediate hybrid DNA band and second in light uppermost band in equal quantity of banding density.

If replication is assumed to conservative then old and new mixing would not have occured i.e. before first round all heavy then after one round equal amount of banding in lower heavy and upper light must have been observed and after two rounds the light upper band intensity would have been double the intensity of lowest heavy and no intermidiate would have ben formed or found (No hybrid middle band).

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