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bacterial strain x is resistant to pencillin and senstitive to gentamycin. bact strain y is resistant to gentamycin and sensitive to ampicillin. bact strain x and y are grown in mixed culture in medium without antibiotics, then the culture is plated on medium containing both ampicilin and gentamycin. bacterial colonies grow on the plates. in a second experiment, dnase is added to the mixed culture medium. when this is mixed, culture is plated on a medium contaiing both antiobiotics, no colonies grow. assuming that bacterial cells are impermeable to dnase, which of the following processes best explains these observations?
a conjugation
b mutation
c transduction
d transformation
e transposition
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D.... The DNAse is hydrolyzing any extracellular DNA that can be taken up by transformation and undergo homologous recombination
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ans is D..
can someone explain.
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You need first to understand the difference between these terms
Bacteria can acquire new DNA mainly by three different mechanisms:
1) Transformation: in which the bacteria acquire the DNA from the environment.
2) Conjugation: which means acquisition of DNA by cell-cell contact between bacteria.
3) Transduction: is the process by which DNA is transferred from one bacterium to another by a bacteriophage intermediate (viruses that infect bacteria).
Transposition: these are mobile genetic elements (DNA sequences) that can move around between different locations in the genome of the same cell (don't move outside the cell)
So looking again to the question, what happened is when both bacterial strain x and y are grown in mixed culture in medium without antibiotics, then the culture is plated on medium containing both ampicilin and gentamycin. bacterial colonies grow on the plates. this required that both bacteria had some sort of DNA exchange between them, which conveyed gentamycin resistance to strain x and penicillin resistance to strain y. This can only occur either by transformation or by conjugation (because there is no mentioning of any viral transduction, and it is not transposition because it occurred between different cells).
In the second experiment, DNase is added to the mixed culture medium and culture is plated on a medium containing both antibiotics, no colonies grow, which means that the DNase (an enzyme that cleaves DNA) cleaved any DNA transformed in the environment between the two bacteria and had no effect on the DNA of both bacteria (because bacterial cells are impermeable to DNase as mentioned in the stem of the question) , so if the exchange of DNA happened by conjugation (by cell-to-cell contact), both bacteria should have acquired the resistance to antibiotics and survived. But what happened was transformation, so the DNase cleaved the exchanged DNA in the environment and no exchange of resistance genes DNA occurred, so both colonies died.
So the answer is: D transformation
