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Chapter 18: Q18.1-2CC. (page 368)

Describe the binding of RNA polymerase, repressors, and activators to the lac operon when both lactose and glucose are scarce. What is the effect of these scarcities on transcription of the lac operon?

Short Answer

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RNA polymerase is the enzyme that mediates the process of transcription of a gene. The cAMP binds to the receptor protein called cAMP receptor protein. During the absence of glucose, this complex binds with the lac operon. It activates the transcription of the lac operon.

The absence of lactose can block the transcription process due to the binding of the repressor to the lac operator.

Step by step solution

01

Description of activators and repressors

The activators are the molecules that trigger the transcription process in a particular gene. The repressor is the molecules that block the transcription of a specific gene.

02

Step 2: Description of RNA polymerase

RNA polymerase is the enzyme that acts on the DNA molecule and converts the DNA into RNA. Prokaryote has a polymerase with the catalytic core along with the other subunits. Mainly, the RNA polymerase binds to the promoter region of the RNA polymerase.

03

Effect of the scarcity of glucose and lactose in the lac operon

Lac operon helps the cell to metabolize the lactose that is consumed. The operon contains the gene coding for the enzyme beta-galactosidase. The presence of substrates such as lactose and glucose makes the digestion process of lactose easier.

During the absence of glucose, cAMP will bind to the CRP, activating the lac operon since CAMP acts as a substitute for glucose.

In the absence of lactose, the lac operator is blocked by the lac repressor that stops the transcription process.

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Most popular questions from this chapter

Suppose you compared the nucleotide sequences of the distal control elements in the enhancers of three genes that are expressed only in muscle cells. What would you expect to find? Why?

How does the binding of the trp corepressor to the trp repressor alter repressor function and transcription? What about the binding of the lac inducer to the lac repressor?

Once mRNA encoding a particular protein reaches the cytoplasm, what are four mechanisms that can regulate the amount of the protein that is active in the cell?

The flashlight fish has an organ under its eyethat emits light, which servers to startle predators and attract prey, and allows the fish to communicate with other fish. Some species can rotate the organ inside and then out, so the light appears to flash on and off. The light is actually emitted by bacteria (of the genus Vibrio) that live in the organ in a mutualistic relationship with the fish. (The bacteria receive nutrients from the fish). The bacteria must multiply until they reach a certain density in the organ (a "quorum"; see Chapter 11.1), at which point they all begin emitting light at the same time. There is a group of six or so genes, called lux genes, whose gene products are necessary for light formation. Given that these bacterial genes are regulated together, propose a hypothesis for how the genes are organized and regulated.

The diagram below five genes, including their enhancers, from the genome of a certain species. Imagine that pink, blue, green, black, grey and dark blue activator proteins exist that can bind to the approximately colour-coded control elements in the enhancers of these genes.

(a) Draw an X above enhancer elements (of all the genes) that would have activators bound in a cell where only gene five is transcribed. Identify which coloured activators would be present.

(b) Draw a dot above all enhancer elements that would have activators bound in a cell where the green, blue, and yellow activators are present. Identify which gene(s) would be transcribed.

(c) Imagine that genes 1, 2, and 4 codes for nerve-specific proteins, and genes 3 and 5 are skin-specific. Identify which activators would have to be present in each cell type to ensure transcription of the appropriate genes.

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