Next the researchers repeated the experiment in part c but instead of mixing the ax and ay strains, they put one strain on each side of a glass u-tube. the u-tube contains a glass filter between the two strains of bacteria that prevents the cells from interacting directly.

Bactria that are AmpS cannot grow in the presence of ampicillin. If a strain of bacteria has a mutation in a gene that is necessary to make an essential nutrient (for example, leu-), then the strain requires that nutrient to be provided in the medium.

Bactria that are AmpR are resistant to ampicillin and can grow in the presence of ampicillin. If a strain of bacteria is wild-type for a gene that is necessary to make an essential nutrient (for example, leu+), then the strain can grow in media that lacks that nutrient.

When gene transfer occurs between bacteria, an auxotropic strain such as AY, that cannot grow on media lacking bio and trp, can become prototropic. Prototropic bacteria do not need nutrients added to minimal medium in order to grow.

Bacterial gene transfer by conjugation requires physical contact between cells of the two strains. The glass filter in this experiment prevents cell-cell contact. Without gene transfer, the AY strain remains bio- trp- and cannot grow on any medium that lacks bio and/or trp.

Explanation: