Concept explainers
Suppose you are analyzing a haploid E. coli strain and discover that the genes of the lac operon are never transcribed and that the bacteria cannot grow in lactose-containing medium. Provide two possible genotypes for your bacteria that would explain this
To determine: Two possible genotypes for a strain of haploid Escherichia coli bacteria in which the genes of the lac operon are never transcribed and the bacteria cannot grow in the lactose-containing medium.
Introduction: E. coli has a lactose operon, also called the lac operon. It is an inducible operon. This operon helps in the digestion and transportation of lactose. By genetic analysis of the lac operon of E. coli of various mutants, a lot of information has been retrieved.
Explanation of Solution
The lac operon under normal conditions can be represented by the genotype I+P+O+Z+ where the Z product will be expressed in the presence of lactose. The I gene represents the repressor gene, P represents the lac promoter gene, O represents the lac operator gene and the Z gene represents the protein product that is formed.
Two possible genotypes for a strain of haploid E. coli bacteria in which the genes of the lac operon are never transcribed and the bacteria cannot grow in the lactose-containing medium are as follows:
- If the I gene has a mutation such that it shows the suppressor genotype, a repressor will be formed in every case. This repressor will bind to the operator in every case, that is whether a lactose inducer is present or absent. Thus, the transcription of the lac genes are suppressed and will never be transcribed. The genotype of such a strain of bacteria can be represented as IsP+O+Z+.
- If the P gene has a mutation, the promoter will be non-functional. When the promoter is non-functional, the RNA polymerase enzyme will not be able to identify the promoter. Thus, the transcription of the lac genes are suppressed and will never be transcribed. The genotype of such a strain of bacteria can be represented as I+P-O+Z+.
Thus, two possible genotypes for a strain of haploid E. coli bacteria in which the genes of the lac operon are never transcribed and the bacteria cannot grow in the lactose-containing medium are IsP+O+Z+ and I+P-O+Z+.
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Chapter 20 Solutions
Becker's World of the Cell (9th Edition)
- Suppose that E. coli sustains a mutation in its gene for the lac operon repressor such that the repressor is ineffective. What effect would this have on the bacterium’s ability to catabolize lactose? Would the mutant strain have an advantage over wild-type cells? Explain your answer.arrow_forwardA strain of E. coli has the genotypes shown below at the lac operon, where I = regulator gene, P = promoter, O = operator, Z = βgalactosidase gene, and Y = permease gene. The superscript+ indicates a wild-type allele,c indicates a constitutive mutation, and− indicates a defective mutation. For each genotype, indicate whether the enzyme will be synthesized or not synthesized when lactose is present or absent by placing a + for synthesis occurring and a − for synthesis not occurring in the appropriate blank. Explain your reasoning for each answer.arrow_forwardGiven the following genotypes, explain, by answering the questions in each number, how the mutation (identified by a (-) superscript) will affect E. coli grown in lactose medium. Will there be a complete set ofgene products? (Yes/No) Will the lac operon be turnedon/off? Will the cell survive? (Yes/No) a. i + p + o + z - y + b. i + p - o + z + y + c. i + p + o - z + y +arrow_forward
- The following shows the genotype of a partial diploid bacterial cell - where one chromosomal region containing the lac operon in E,coli is given, and the other fragment is from a plasmid carrying another lac operon from another source. The two are separated by a slash (/). The possible answers indicate with a ʺ+ʺ or a ʺ-ʺ whether β-galactosidase would be expected to be produced at induced levels under two circumstances: 1) first in the absence of lactose and 2) second in the presence of lactose. (Assume that glucose is not present in the medium.)Genotype F: I+ Oc Z-/ Fʹ I- O+ Z+ KEY:I+ = wild-type repressorI- = mutant repressor (unable to bind to the operator)Is = mutant repressor (insensitive to lactose)O+ = wild-type operatorOc = constitutive operator (insensitive to repressor)arrow_forwardThe lac operon consists of three structural genes, lacZ, lacY and lacA that are transcribed as a single polycistronic mRNA. You are given a new strain of Escherichia coli with the following lac operon genotype: p+0°Z•Y*A +// P*O*Z*Y+ A- (i) Explain how the lac I gene affects gene expression. (ii) Explain the function of the lacP in the bacterial operon. (iii) Which part of the lac operon is cis-dominant? Explain.arrow_forwardA number of mutations affect the expression of the lac operon in E. coli. The genotypes of several E. coli strains are shown below. ("+" indicates a wild-type gene with normal function and "-" indicates a loss-of-function allele.) Please predict which of the following strains would have the highest beta-galactosidase enzyme activity, when grown in the lactose medium. CAP It P+ O`Z+ CAP+ I P+ O+ Zt CAP+ It P- O+ Z+ CAP+ It P+ 0+ Zarrow_forward
- Suppose that E. coli sustains a mutation in its gene for the lac operon repressor making the repressor ineffective. How would this mutation affect the bacterium's ability to catabolize lactose? Would the mutant strain have an advantage over the wild-type strain? Explain your answer. (Minimum 150 words, the document will be checked for plagiarism)arrow_forwardImagine that a new strain of E. coli is discovered, and it contains the following novel genetic region including the lacI gene and the lac operon: *see image* Upstream of the promoter associated with the lac operon, there is a new operator element "Oν" (O-"nu"). This new operator has high sequence homology to O1, the DNA element that is bound with the highest affinity by the lac repressor. What would be the impact of this new genetic element regarding behavior of the operonin the absence (vs. the presence) of lactose and allolactose? Answer in terms of the protein levels of the lac repressor, as well as the proteins encoded by the genes lacZ/Y/A in the downstream region of the operon. Remember that proteins, once expressed/generated, will not persist forever, and that they have a natural lifetime (ending when they are degraded by the cell).arrow_forwardSuppose that E. coli sustains a mutation in its gene for the lac operon repressor making the repressor ineffective . How would this mutation affect the bacterium's ability to catabolize lactose? Would the mutant strain have an advantage over the wild-type strain? Explain your answer.arrow_forward
- The lac operon consists of three structural genes, lacZ, lacY and lacA that are transcribed as a single polycistronic mRNA. You are given a new strain of Escherichia coli with the following lac operon genotype: p+0°Z•Y*A +// P*O*Z*Y+ A- (i) Explain how the lac I ³ gene affects gene expression.arrow_forwardA number of mutations affect the expression of the lac operon in E. coli. Consider each genotype below and complete the table using “+” to indicate that the gene is expressed, and “−” to indicate that gene is not expressed.arrow_forwardUse the blanks on the left hand side to put the following statements in order (use numbers or letters to designate the order). All of the statements refer to the diauxic (sequential) growth of a bacterium on the carbon substrates glucose and lactose (see textbook Fig 4.11 on diauxic growth). Remember, the lac operon contains the genes necessary for the lactose catabolism. The glucose substrate is completely consumed, at which time the repressor protein on the lac operon unbinds from the DNA molecule. The lactose substrate is catabolized by the active B-galactosidase enzyme. Glucose, as the preferred carbon substrate, is first consumed by catabolic enzymes that are constitutive (always present). After completion of translation, the B-galactosidase enzyme undergoes folding to form into an active enzyme that can breakdown lactose into its constituent monosaccharides. The messenger RNA code from the lac operon is translated by ribosomal RNAS to form lac-related enzymes, such as…arrow_forward
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