bacterial drug resistance
you tried but that is not the ans. the ans is D. This the explanation.
a) conjugation is bacterial sex. in conjugation DNA is transferred directly by cell-to-cell contact through sex pili. so in this case DNA is secured by cell wall and membrane from action of DNAse.
b) mutation can occur. but it is very rare to bacteria obtain antibiotic resistance due to mutation...
c) transduction occurs when a virus that infects bacteria, bacteriophage, carries a piece of bacterial DNA from one bacterium to another.. in this case DNA also secured by virus capsid or envelope..
e) transposition... transposons are mobile geneticx elements. they insert into the DNA of phages, plasmids, and bacterial chromosomes. they do not replicate independently but are copied during their host's DNA tracscription. transposons can move drug resistance to the plasmids... but all these occur inside of the bacteria, and DNA is secured by bacterial membrane and cell wall, from action of DNAse..
D) TRANSFORMATION. naked DNA fragments from one bacterium, released during cell lysis, bind to the cell wall of anothe bacterium. the recipient bacterium must be competent, which means that it has structures on its cell wall that can bind the DNA and take it up intracellularly. Recipient competent bacteria are usually of the same species as the donor. the DNA that has been brought in can then incorporate itself into the recipient's genome if there is enough homology between strands (another reason why this transfer can only occur between closely related bacteria).
in this case when two strains of bacteria grow together in one medium, they exchange antibiotic resistance by means TRANSFORMATION and become both resistant to both antibiotics (gentamicin and ampicillin)... if two strains were cultured in the medium containing DNAse, transformation can't occur, b/c DNAse cleaves DNA fragments outside the cells, so both strain remain susceptible to another antibiotic, and both strain can't grow in medium containing both antibiotics..
they discribe two experiments... in first two bacterial strains X and Y were grown in mixed culture in medium without antibiotics, without DNAse....
in the second experiment they added DNAse...
in first experiment (without DNAse) transformation occured and bacterial strains obtained cross resistance to antibiotic to which they were before susceptible...
in the second experiment (with DNAse) no transformation occure and both strains were susceptible in medium containing both antibiotics.