Question

For the lac genotypes shown in the following table, predict whether the structural genes (Z) are constitutive, permanently repressed, or inducible in the presence of lactose. Genotype Constitutive Repressed Inducible I⁺O⁺Z⁺ x I⁻O⁺Z⁺ I⁻OᶜZ⁺ I⁻OᶜZ⁺/F'O⁺ I⁺OᶜZ⁺/F'O⁺ IˢO⁺Z⁺ IˢO⁺Z⁺/F'I⁺

Accepted Answer

Step 1: Understand the components of the lac operon genotype notation: 'I' refers to the repressor gene, 'O' is the operator site, and 'Z' is the structural gene encoding β-galactosidase. Superscripts indicate the allele type: '+' is wild type, '⁻' is a loss-of-function mutation, 'ᶜ' is a constitutive operator mutation, and 'ˢ' is a super-repressor mutation. F' indicates a plasmid-borne gene. Step 2: Recall the functional effects of each mutation: I⁺ produces a functional repressor that binds the operator; I⁻ produces no repressor; Iˢ produces a repressor that cannot be inactivated by lactose; O⁺ is a normal operator; Oᶜ is a mutated operator that cannot bind repressor; Z⁺ is a functional structural gene. Step 3: For each genotype, determine whether the repressor can bind the operator and whether lactose can induce the operon. For example, if the repressor is absent (I⁻), the operon is constitutive; if the operator is mutated (Oᶜ), the repressor cannot bind, leading to constitutive expression; if the repressor is super-repressor (Iˢ), the operon is permanently repressed. Step 4: Consider the presence of F' plasmids carrying different operator or repressor alleles. Since the lac operon is cis-regulated by the operator, mutations in the operator on the chromosome affect only the chromosome's operon, while repressor mutations on F' plasmids can act in trans and affect both operons. Step 5: Using these principles, classify each genotype as constitutive, permanently repressed, or inducible by analyzing the interaction between the repressor and operator alleles, and the presence or absence of functional lactose induction.

비슷한 문제에 대한 검증된 영상 답변:

주요 개념

Lac Operon Structure and Function

Role of the Repressor Protein (lacI) and Operator (O) Mutations

Inducibility and Constitutive Expression in the Presence of Lactose