Answer each question in sentences or point form (as applicable). Use the number

Question

Answer each question in sentences or point form (as applicable). Use the number of marks allotted as a guide to how much detail your answer should include. Describe the Hershey-Chase experiment and indicate its findings. Differentiate between B-DNA and Z-DNA. Which form is more amenable to binding of transcription factors? How might this affect gene expression? Outline the process of DNA replication, and describe the roles of the enzymes involved. Define mutation. Why are mutation rates lower than the rate of spontaneous DNA damage? Describe mitosis, listing the events that occur in the cell in each phase of the process. Briefly describe the role of tumor suppressor genes in cancer formation. What is the result of a mutation in the p53 gene?

Explanation / Answer

1.

The experiments conducted by Hershey and Chase, based on the properties of protein (which has high sulphur content than do DNA) and DNA (which has high phosphorus content than does proteins) proved that, “the DNA is the substance of heredity.”

“Bacteriophage” is a type of virus, which infects bacteria; these bacteriophages carry hereditary information which they inject into the bacteria, making them infected with the virus particles. Electron micrographs allow observing how the bacteriophages attach to the bacteria by their tails. This “virus-bacteria” attachment can be broken by a kitchen blender.

In the first experiment, bacteria were cultured in growth medium containing a sulphur isotope 35S. The “protein” of bacteriophage was labelled with the 35S tracer and allowed to infect the fresh culture of unlabelled bacteria. This virus-bacteria complex was broken in a blender. And the bacteria were separated from virus containing fluid, the 35S content of each was measured separately, and the extracellular fluid was found to have most of the 35S. So, it indicates that the virus had not injected the protein into the bacteria.

This experiment was repeated using phosphorus isotope 32P. Now, the “DNA (but not protein)” of the bacteriophage is labelled with 32P. Again the fluid analysis was carried out, but this time, the isotope was found in the infected bacteria. So, this indicates that the viruses injected DNA into the bacterial cell but not the protein.

After Four minutes of blending, the extracellular concentration of 35S (sulphur isotope) is nearly 78% and that of the 32P (phosphorus isotope) is nearly 33%. So, as the time of time of blending increases, the extracellular concentration of 35S is increasing.

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