Question

You have found a strain of microorganism, Schizosaccharomyces spp that is very sensitive to mutagenic compounds. When grown in the presence of known mutagens this organism produces many mutant progeny. This microbe, initially isolated from beer-stained carpeting from Zach’s apartment, is obviously deficient in repairing damage to DNA. You perform a series of experiments to determine which repair system is defective.
A. You first take two identical cultures and expose them to 30 minutes of irradiation. You place one of the cultures in the dark and expose the other culture to blue light. Then, you take samples from each culture and select for drug-resistance mutations. The dark culture exhibits a high rate of mutation to resistance; the culture exposed to blue light has a much lower rate of mutation to resistance. Keeping the purpose of these experiments in mind, what conclusions about repair can you draw from these results? Please briefly explain your answer.
B. You perform another experiment. An E. coli strain and the Schizosaccharomyces strain are grown separately in a medium containing 3H-thymidine (radioactive label) for several cell generations. They then are switched to a cold medium (non-radioactive) and exposed to UV light. After a very short time, DNA obtained from the wild-type E. coli strain is examined. Small fragments of radioactive DNA are present the intact chromosomal DNA, while radioactive, contains stretches of cold DNA. When DNA obtained from the Schizosaccharomyces strain is examined, only genome-size pieces of intact chromosomal DNA are found. These contain stretches of cold DNA in an otherwise radioactive DNA molecule; small. fragments of radioactive DNA are not found. What can you conclude about repair from these results? please complain.

Answer 1

The repair system in exposing the dark cultures exhibited higher rate of mutations to drug resistance. However, teh cultures exposed to blue light showed lower mutation rate to drug resistance. This indicates that the strains when exposed to radiations, are less resistant to drugs as their mutations is low because there is lysis in their DNA when exposed to the UV rays or the blue light.The repair is defective as the culture is lysed and needs to be ligased.

The Griffith transformation experiment showed that the wild type strain is obtained after a result of transformation of the strain from rough to smooth i.e from avirulent to virulent strain which is infectious. The cold DNA in otherwise radioactive DNA, there is no evidence of the presence of small fragments of radioactive DNA. The transformation led to the formation of cold DNA with intact chromosomal gene. There is no lysis which is separation of DNA fragments and the integration of a DNA molecule n the presence of the enzyme ligase. The transformation occurred has left the strain unaffected by the radioactivity.