1. Contrast constitutive genes from inducible genes. Give two examples for each.
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Question
1. Contrast constitutive genes from inducible genes. Give two examples for each.2. Contrast repressors and activators. How do they contribute to positive or negative gene regulation?
3. Transcription and translation are typically coupled in prokaryotes. Explain what this means. Why is this possible in prokaryotes and not eukaryotes?
4. Describe the structure of a typical operon. How many genes are found in an operon?
3. What is an inducible operon? A repressible operon?
4. What is the function of the trp operon?
5. Describe the regulation of the trp operon (a) in the presence of tryptophan and (b) the absence of tryptophan. Name all components involved and tell where they bind.
6. What is the function of the lac operon?
7. Describe how the lac operon is regulated by both lactose levels and glucose levels. Name all components involved and tell where they bind.
8. Prokarytoes have only one chromosome. Explain why a mutation thus has a more significant effect.
10. An operon with one regulatory region and three protein-coding genes will have how many promoters? Related: Explain how bacteria can make one mRNA molecule that contains the code for several proteins.
11. An operon has four protein-coding genes (for proteins A, B, C, D). Which proteins will be produced (a) if the promoter ahead of gene A is mutated? (b) if the stop codon of gene B is deleted; (c) if a repressor binds to the operator (regulatory DNA) for this operon.
Explanation / Answer
protein that binds to an enhancer (or activator binding region) and activates transcription from nearby promoter. A-DNA - a form of DNA found at low relative humidity, with 11 bp per helical turn. This form assumed in solution by RNA-DNA hybrid. Many enzymes that serve housekeeping functions, such as those involved in oxidative phosphorylation, citric acid cycle reactions, or glycolysis, are required at all times. In order to maintain a constant cellular level of these enzymes, they must be synthesized continuously since all proteins have finite lifetimes and are being degraded at all times. These are called constitutive enzymes. On the other hand, there are enzymes that are required only in special conditions. These enzymes are normally synthesized at very low levels, but their synthesis can be induced to proceed at much higher rates. For example, when lactose is added to E. coli bacteria growing on glycerol as their sole carbon source, the bacteria begin synthesizing ß-galactosidase, an enzyme that converts the indigestible disaccharide lactose into the metabolizable monosaccharides glucose and galactose. ß-galactosidase is called an inducible enzyme. The molecular mechanism of enzyme induction has been worked out by Jacob and Monod and is known as the operon hypothesis. Its mechanism of operation is now well established. THE LACTOSE OPERON When lactose is added to a culture of E. coli growing in the presence of glycerol, rapid synthesis of ß- galactosidase begins within about 2 minutes. Together with ß-galactosidase, two other enzymes, galactoside permease and thiogalactoside transacetylase, are induced. (The permease is required for uptake of lactose; the transacetylase's function in the cell is not known.) The genes for these three enzymes belong to a single operon; their structural genes lie next to each other on the chromosome and are transcribed into a single mRNA molecule. It is called a "polycistronic mRNA". (Cistron is a synomym for gene.) The expression of this operon is regulated by the interaction of a regulatory protein, the repressor, with the operator, a segment of DNA preceding the structural genes that encode the enzymes (Fig. 1). The mechanism of regulation of the lactose operon is as follows: The regulator gene i (which need not be located next to the operon it controls) codes for the repressor protein. The repressor has a very high affinity for the operator sequence o. RNA polymerase attaches to the promoter p, located just upstream of, and partially overlapping, the operator, but is prevented from binding if a repressor occupies the operator. Thus, in the presence of an active repressor, transcription of the entire operon is shut down. However, in the presence of the inducer lactose, the repressor forms a complex with the inducer. This results in a conformational change of the repressor, which in turn drastically reduces its affinity for the operator. As a result, RNA polymerase can now bind to the promoter and begin the synthesis of the Concept 2: Transcription and Translation in Cells In a prokaryotic cell, transcription and translation are coupled; that is, translation begins while the mRNA is still being synthesized. In a eukaryotic cell, transcription occurs in the nucleus, and translation occurs in the cytoplasm. Because there is no nucleus to separate the processes of transcription and translation, when bacterial genes are transcribed, their transcripts can immediately be translated. polycistronic mRNA coding for the three proteins of the operon Concept 2: Transcription and Translation in Cells In a prokaryotic cell, transcription and translation are coupled; that is, translation begins while the mRNA is still being synthesized. In a eukaryotic cell, transcription occurs in the nucleus, and translation occurs in the cytoplasm. Prokaryotic Cell Because there is no nucleus to separate the processes of transcription and translation, when bacterial genes are transcribed, their transcripts can immediately be translated. Eukaryotic Cell Transcription and translation are spatially and temporally separated in eukaryotic cells; that is, transcription occurs in the nucleus to produce a pre-mRNA molecule. The pre-mRNA is typically processed to produce the mature mRNA, which exits the nucleus and is translated in the cytoplasm.
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