Repressible operon system is usually found in (i)____ pathways. The pathway's end product serves as a (ii) ______ to activate the repressor, turn off enzyme synthesis and prevent overproduction of the end product of the pathway. Genes for this operon are usually switched (iii)_____ and the repressor is synthesised in an (iv)_____ .

Options

A. `{:("(i)","(ii)","(iii)","(iv)"),("anabolic","corepressor","on","inactive"):}`
B. `{:("(i)","(ii)","(iii)","(iv)"),("anabolic","inducer","off","active"):}`
C. `{:("(i)","(ii)","(iii)","(iv)"),("catabolic","inducer","off","active"):}`
D. `{:("(i)","(ii)","(iii)","(iv)"),("catabolic","corepressor","on","inactive"):}`

Answer

**Step-by-Step Solution:**

1. **Identify the type of pathways where the repressible operon system is found:**
- The repressible operon system is typically associated with anabolic pathways. Anabolic pathways are those that synthesize larger molecules from smaller ones. In this context, the TRP operon, which synthesizes the amino acid tryptophan, is a classic example.

**Hint:** Think about whether the pathway builds up or breaks down molecules.

2. **Determine the role of the end product in the operon system:**
- The end product of the anabolic pathway (in this case, tryptophan) serves as a co-repressor. A co-repressor is a molecule that binds to a repressor protein, activating it and allowing it to bind to the operator region of the operon. This binding prevents the transcription of the genes involved in the synthesis of that end product.

**Hint:** Consider how the presence of the end product affects the operon's activity.

3. **Identify the state of the genes in the operon:**
- In a repressible operon system, the genes are usually switched on. This means that in the absence of the end product (co-repressor), the genes are actively transcribed to produce the enzymes necessary for the synthesis of the end product.

**Hint:** Think about whether the genes are active or inactive when the end product is not present.

4. **Determine the form in which the repressor is synthesized:**
- The repressor in a repressible operon is synthesized in an inactive form. This means that it does not bind to the operator unless it is activated by the co-repressor (the end product). When the co-repressor is present, it binds to the repressor, activating it and allowing it to inhibit gene expression.

**Hint:** Consider how the repressor behaves in the absence of the end product.

**Final Answers:**
(i) Anabolic pathways
(ii) Co-repressor
(iii) On
(iv) Inactive

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