Answer: (1) Structural genes

Genes coding for the primary structure of enzymes in the operon model are structural genes, as they directly encode the enzyme proteins involved in metabolic pathways.

Option Breakdown

Structural genes
These are the coding sequences in an operon that transcribe into mRNA for synthesizing enzymes or proteins with specific functions, like beta-galactosidase in the lac operon. They determine the primary amino acid sequence of enzymes but do not regulate expression.

Repressor genes
Often called regulator genes (e.g., lacI in lac operon), these produce repressor proteins that bind the operator to block transcription when enzymes are unneeded, such as in high glucose conditions. They control but do not code for enzyme structure.

Regulatory genes
A broader term for genes (like repressor genes) that produce regulatory proteins or factors influencing operon activity via promoters or operators, without encoding the enzymes themselves.

Negative genes
Not a standard term in operon biology; it may vaguely refer to negative regulation components like repressors, but lacks precise definition and does not code for enzyme primary structure.​

The operon model revolutionized understanding of gene regulation in prokaryotes, with genes which code for primary structure of enzymes in operon model identified as structural genes. This mechanism allows efficient control of enzyme production for metabolic needs.

Operon Model Basics

An operon clusters structural genes under one promoter and operator for coordinated expression, as in Jacob and Monod’s lac operon model. RNA polymerase binds the promoter to transcribe structural genes into polycistronic mRNA when induced.

• Promoter: RNA polymerase binding site.​

• Operator: Repressor binding site to halt transcription.

• Structural genes: Encode enzyme primary structures (e.g., lacZ, lacY, lacA for lactose metabolism).

Role of Structural Genes

Structural genes provide the nucleotide sequence translated into enzyme polypeptides, forming their primary structure (amino acid chain). In the trp operon, genes like trpE to trpA code tryptophan synthesis enzymes. They differ from regulators by focusing solely on protein products, not control.

Other Gene Types

Regulatory genes (e.g., i gene) produce repressors for negative control, while operators are DNA segments, not protein-coding. “Negative genes” is not recognized terminology.

Exam Relevance

For competitive exams, remember: structural genes = enzyme-coding in operons. Example: Lac operon structural genes activate with lactose absence of repressor.