In E. coli, the trp operon is being transcribed because tryptophan is scarce. A supply of tryptophan then arrives and binds the trp repressor protein. What happens next?

An E. coli cell has lactose available, but plenty of glucose too, so cyclic AMP stays low and the CAP activator does not bind beside the lac promoter. What level of lac transcription results?

A mutant E. coli strain carries a change in the lac operator sequence, so the lac repressor protein can no longer attach there. The strain is grown with no lactose in the medium. What is observed?

A second mutant makes a lac repressor that still grips the operator but can no longer bind the inducer. Lactose is added to the medium. What happens to the lac genes?

An E. coli culture has been growing on lactose and making the lac enzymes. The lactose is then used up completely. What happens to the operon?

A strain of E. coli has lost the gene coding for the trp repressor protein altogether. The cells are grown in a medium rich in tryptophan. What do the trp genes do?

A liver cell and a nerve cell taken from the same person carry an identical set of genes, yet they build different proteins and do different jobs. What accounts for the difference between them?

John Gurdon transplanted the nucleus of a specialized intestinal cell from a tadpole into a frog egg whose own nucleus had been removed, and the egg developed into a complete tadpole. What does this establish?

Researchers compare the DNA sequences and the mRNA populations of a pancreas cell and a skin cell taken from one mouse. What should they expect to find?

Early in a female mammal's development, one of the two X chromosomes in a cell coils into a dense packed body and stays that way in every descendant of that cell. What is the effect on that cell line?

In pancreas cells the promoter of one gene carries many methyl groups; in liver cells, which make that gene's protein in quantity, the same promoter carries almost none. What does the comparison indicate?

In a skin cell, enzymes strip acetyl groups from histone tails and a stretch of chromatin coils so tightly that nothing can reach the DNA inside it. At which stage is control being applied?

In a muscle cell nucleus, some segments of a new RNA transcript are cut out and the remaining pieces joined in a different combination than in a heart cell, so one gene yields two proteins. Where is control applied?

A short RNA molecule made by the cell pairs with a matching stretch of an mRNA that has already reached the cytoplasm, and the protein-building machinery stalls on that message. Where is control being applied?

A polypeptide is fully assembled but does nothing until an enzyme attaches phosphate groups to it, after which it becomes active. Where is control being applied?

Two mRNA molecules carry the same message, but one has a much shorter poly-A tail and is dismantled by cytoplasmic enzymes within minutes while the other lasts hours. Where is control being applied?

A protein released at one end of an insect embryo forms a gradient, concentrated at that end and sparse at the other. Nuclei along the embryo meet different concentrations of it. How does this produce different body regions?

Estrogen crosses the membrane of a target cell and binds a receptor protein inside it. The hormone and receptor together then attach to particular sequences in the DNA. What do they do there?

A pale seedling grown in darkness is moved into light. Within hours its cells begin making chlorophyll-binding proteins and the tissue turns green. What has the light done?

In a fruit fly carrying a mutation that switches on the Antennapedia gene in the head, legs grow from the sockets where antennae belong. What does this show about the gene's normal job?