You are given a flask with liquid media containing a population of S. pombe cell
ID: 210844 • Letter: Y
Question
You are given a flask with liquid media containing a population of S. pombe cells have a temperature sensitive mutation of a gene that causes complete loss of function of the gene product at the restrictive temperature. The flask had been incubating at the permissive temperature before it was given to you, and the cells in the flask were replicating asynchronously with respect to the stage of the cell cycle.
The mutated gene in this strain of yeast encodes an essential subunit of a DNA polymerase required for DNA replication, but not DNA repair. (Assume that the cell only has one DNA polymerase that performs DNA replication and that cells paused in DNA replication can restart and complete this process).
Which FACS plot (see attached figure) best represents how cells in this flask would appear following growth after 2 hours in culture at the restrictive temperature followed by 60 min in culture at the permissive temperature ?
(Note, assume that FACS plot #1 shows analysis of a population of S. pombe replicating asynchronously (i.e. each cell within a population is at a random stage of the cell cycle) in culture. In the rich media assume the yeast have a cell cycle time of 100 min, with G1=50 min; S = 25 min; G2 = 15 min and M = 10 min).
#1 #2 #3 cells in G phase #4 #5 #6 cells in Gz and Mphases cells in Sphase relative amount of DNA per cell arbitrary units)Explanation / Answer
The mutation will hinder the DNA replication at restrictive temperature so the DNA replication will be blocked.
When cells are kept at restrictive temperature for 2 hr, the DNA replication will be halted. Later on when shifted to permissible temperature for 60 minute it will again start replicating.
The picture no one shown that there is normal replication going on asynchronously.
In second the replication is halted and almost 4 stranded dna is more than 2 stranded dna during to some short of blocking mechanism
Third is synchronised culture replication simultaneously
Fourth is synchronised culture but replication is blocked
Fifth 2 stranded dna is more than 4 stranded dna normally replicating in asynchronously
Sixth replication blocked some where between 2 and 4 stranded dna due to non replication
The last one best explains the above said problem
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