Question

1. Suppose the synthesis of an amino acid was accomplished by four enzymes encoded by four genes that formed an operon. Draw a model of the operon showing how it could be controlled by negative regulation both at the level of transcription initiation and elongation via attenuation.

Answer 1

I have attached lac operon model having 4 genes. See attachment for better understanding.

Answer:

The Lac operon is controlled  by two signals.

Initiation

Glucose and lactose  concentrations control the initiation of  transcription of the Lac operon through  their effects on the Lac repressor protein  and Cap.

Lactose absent (Repression)

When lactose is absent, the Lac  repressor binds the Lac operator and shuts  off expression of the operon.

Lactose present (Transcription)

Addition  of lactose increases the intracellular  concentration of a related compound,  allolactose. allolactose binds to the  Lac repressor, causing it to undergo a  conformational change that releases its grip  on the operator DNa.

Glucose absent (Repression)

When  glucose is absent, cyclic aMp (red triangle)  is produced by the cell and Cap binds to  DNA.

Glucose present (Transcription)

LacZ, the first gene of the operon,  encodes the enzyme b-galactosidase, which  breaks down lactose to galactose and  glucose.