AP BIOLOGY

Detailed explanation-1: -When lactose is available, some molecules will be converted to allolactose inside the cell. Allolactose binds to the lac repressor and makes it change shape so it can no longer bind DNA. Allolactose is an example of an inducer, a small molecule that triggers expression of a gene or operon.

Detailed explanation-2: -When lactose is present, allolactose binds to the lac repressor, causing an allosteric change in its shape. In its changed state, the lac repressor is unable to bind tightly to its cognate operator.

Detailed explanation-3: -Transcription of the lac operon normally occurs only when lactose is available for it to digest. Presumably, this avoids wasting energy in the synthesis of enzymes for which no substrate is present. A single mRNA transcript includes all three enzyme-coding sequences and is called polycistronic.

Detailed explanation-4: -In the absence of lactose the lac repressor blocks the expression of the lac operon by binding to the DNA at a site, called the operator that is downstream of the promoter and upstream of the transcriptional initiation site.

Detailed explanation-5: -In the absence of lactose, the lac repressor, lacI, halts production of the enzymes encoded by the lac operon. The lac repressor is always expressed, unless a co-inducer binds to it. In other words, it is transcribed only in the presence of small co-inducer molecules.