Structural genes vs regulatory genes

· Structural genes code for proteins that have a direct function in the cell, e.g. enzymes, membrane proteins or transport proteins.
· In the lac operon, the structural genes are lacZ, lacY and lacA, which code for proteins involved in lactose metabolism.
· Regulatory genes code for proteins that control the expression of other genes.
· A regulatory gene may code for a repressor protein or a transcription factor.
· Key exam contrast: structural gene = codes for functional product; regulatory gene = codes for control protein.

Repressible and inducible enzymes

· Repressible enzymes are normally produced, but their production can be switched off when the product is no longer needed.
· Repressible systems usually stop wasteful production when enough product is already present.
· Inducible enzymes are normally not produced, but their production can be switched on when the substrate is present.
· The enzymes of the lac operon are inducible enzymes because they are produced when lactose is present.
· Exam phrase: inducible enzyme production prevents waste of resources because enzymes are only made when needed.

Lac operon: key parts

· The lac operon is a group of genes in a prokaryote that controls production of enzymes for lactose metabolism.
· Regulatory gene lacI codes for the lac repressor protein.
· Promoter = DNA region where RNA polymerase binds to start transcription.
· Operator = DNA region where the repressor can bind.
· Structural genes = lacZ, lacY and lacA.
· lacZ codes for β-galactosidase, which hydrolyses lactose.
· lacY codes for lactose permease, which helps lactose enter the cell.
· Knowledge of cAMP is not expected for CIE 16.3.

This diagram shows how the lac repressor blocks transcription when lactose is absent, and how lactose prevents the repressor from binding so transcription can occur. It is useful for linking promoter, operator, repressor and structural genes in one visual. Source

Lac operon when lactose is absent

· The regulatory gene lacI is expressed and produces an active repressor protein.
· The repressor binds to the operator.
· This blocks RNA polymerase from transcribing the structural genes.
· lacZ, lacY and lacA are not transcribed.
· No mRNA is made for lactose-metabolising enzymes.
· Therefore, β-galactosidase and lactose permease are not produced.
· Exam phrase: lactose absent → repressor active → operator blocked → transcription prevented.

Lac operon when lactose is present

· Some lactose is converted into allolactose, which acts as the inducer.
· Allolactose binds to the repressor, changing its shape.
· The repressor can no longer bind to the operator.
· RNA polymerase binds to the promoter.
· The structural genes lacZ, lacY and lacA are transcribed.
· mRNA is translated to produce enzymes for lactose uptake and breakdown.
· Exam phrase: lactose present → allolactose binds repressor → operator free → transcription occurs.

This source clearly shows the logic of an inducible operon: lactose indirectly inactivates the repressor, allowing transcription. For CIE, focus on the lactose/repressor/operator mechanism and ignore detailed cAMP/CAP regulation because it is not required. Source

Explaining the lac operon in exam answers

· Always state whether lactose is present or absent.
· Name the repressor, operator, promoter, RNA polymerase and structural genes.
· Explain the consequence for transcription, not just enzyme production.
· Use cause-and-effect wording: binds, blocks, changes shape, allows transcription.
· Avoid saying lactose “switches on genes” without explaining that lactose/allolactose affects the repressor.
· Do not include unnecessary detail about cAMP because it is outside the specified CIE requirement.

Gene control in eukaryotes: transcription factors

· Transcription factors are proteins that bind to DNA.
· They control gene expression by changing the rate of transcription.
· Some transcription factors increase transcription by helping RNA polymerase bind or work more effectively.
· Other transcription factors decrease transcription by preventing transcription machinery from working.
· Eukaryotic gene control is important because different cells can contain the same genes but express different sets of genes.
· Exam phrase: transcription factors regulate gene expression by increasing or decreasing transcription.

This diagram shows how DNA-binding proteins can control whether transcription starts in a eukaryotic cell. It helps students visualise that transcription factors do not code for the protein product directly; they control the rate of transcription. Source

Gibberellin and DELLA protein repressors

· Gibberellin is a plant hormone involved in gene activation.
· DELLA proteins are repressors that normally inhibit factors that promote transcription.
· When DELLA proteins are present, transcription-promoting factors are inhibited.
· When gibberellin is present, it causes the breakdown of DELLA protein repressors.
· This releases the transcription-promoting factors so they can activate transcription.
· Result: target genes are expressed, leading to plant growth responses such as stem elongation.
· Exam phrase: gibberellin activates genes by causing breakdown of DELLA repressors.

Common exam mistakes to avoid

· Do not confuse regulatory genes with regulatory DNA regions such as the promoter and operator.
· Do not say the operator codes for a protein; the operator is a DNA binding site.
· Do not say lactose binds directly to the operator; lactose/allolactose binds to the repressor.
· Do not describe the lac operon as eukaryotic; it is a prokaryotic gene control system.
· Do not include cAMP/CAP details unless asked in another context; they are not expected here.
· Do not say transcription factors are always activators; they may increase or decrease transcription.