A DNA fragment is introduced into the lacZ gene of a plasmid, which also contains an ampicillin resistance gene. What is the appearance of bacteria transformed with this plasmid if they are spread on plates containing ampicillin and X-gal

White colonies that are resistant to ampicillin.

Explanation:

The lac operon is an operon required for the metabolism of lactose in enteric bacteria such as Escherichia coli . It has three structural genes, a promoter, an operator and a regulator, all regulated by the availability of glucose and lactose. The lac repressor, a protein, senses lactose and blocks transcription of this operon. It acts as a repressor when lactose is present. A catabolite-activating protein (CAP), on the other hand, acts as a glucose sensor. The bacterium should express the lac operon only when lactose is available and glucose is not available. Thus, genes can always be transcribed, except when the Lac repressor protein is bound to the operon region, for which it has a high affinity (i.e. in the absence of lactose), where the Lac repressor protein maintains its high affinity for the operon region, preventing RNA polymerase from transcribing the structural genes. Thus, the system remains closed with consequent energy savings for the bacterium. In the presence of lactose, it binds to the Lac repressor protein and generates a conformational change that decreases its affinity for the operator region. Thus, the operator region is left free, RNA polymerase can freely transcribe the structural genes and the synthesized β-galactosidase (an enzyme) can degrade lactose to glucose plus galactose for energy.  

Thus, the lac z gene encodes the enzyme β-galactosidase, which catalyzes the hydrolysis reaction of lactose to glucose and galactose. In gene cloning experiments, a compound called X-gal is used as an indicator of cells expressing the β-galactosidase enzyme. X-gal is hydrolyzed by the enzyme to galactose and another compound that is oxidized giving an insoluble blue compound. Thus, if X-gal and a β-galactosidase inducer are dissolved in the medium of a culture plate where the transformed bacteria is found, colonies grown on the plate that possess a functional lac z gene (either because they were not transformed by the plasmid, or if they were but the plasmid does not have the cloned fragment or gene that disrupts the lac z gene) can be clearly distinguished by their blue coloration. If they have another gene inserted interrupting the lac z gene, they will not be able to produce the enzyme that degrades X-gal, resulting in white colonies since X-gal is not degraded giving that characteristic blue color.

The white, non-transforming colonies are eliminated by adding an antibiotic to the medium for which the plasmid provides resistance (in this case ampicillin), so that we can select the recombinant colonies that carry the vector with our sequence, simply by their color.

So, if bacteria are transformed with a plasmid (with ampicillin resistance) cloned with a gene that interrupts the lac z gene, the bacteria will be white because they do not synthesize the enzyme that degrades X-gal and will be resistant to ampicillin.

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