Gene control
Cambridge A-Level Biology (9700) · Unit 16: Inheritance · 10 flashcards
Gene control is topic 16.3 in the Cambridge A-Level Biology (9700) syllabus , positioned in Unit 16 — Inheritance , alongside Passage of information and The roles of genes in determining phenotype. In one line: Structural genes code for proteins that become part of the cell's structure or have metabolic/physiological roles. Regulatory genes control the expression of structural genes by coding for proteins that influence transcription (.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 10 flashcards — 5 definitions and 5 key concepts — covering the precise wording mark schemes reward. Use the 5 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Some cards in this deck are on other syllabus topics: 17.1 Variation. Each card is labelled with its topic.
The difference between a structural gene and a regulatory gene
Structural genes code for proteins that become part of the cell's structure or have metabolic/physiological roles. Regulatory genes control the expression of structural genes by coding for proteins that influence transcription (
What the Cambridge 9700 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Describe how structural genes differ from regulatory genes, and how inducible enzymes differ from repressible enzymes.
- Explain, using the lac operon, how a prokaryote genetically controls the production of proteins (the role of cAMP is not required).
- State that in eukaryotes, transcription factors are DNA-binding proteins that help control gene expression by raising or lowering the rate of transcription.
- Explain how gibberellin switches genes on by triggering breakdown of DELLA proteins, repressors that normally hold back factors that promote transcription.
Also in this deck: 17.1 Variation
- Explain, giving examples, that phenotypic variation can be caused by genetic factors, by environmental factors, or by both acting together.
- Explain the meaning of continuous variation and discontinuous variation.
- Explain the genetic causes of continuous and of discontinuous variation.
- Apply the t-test to two separate samples to compare their means (the formula is provided, as set out in the Mathematical requirements).
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9700 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in these topics
- › Clarify that 'regulatory genes' code for 'transcription factors' (proteins) which then bind to DNA to control gene expression.
- › For stem elongation, focus on how active gibberellin leads to the breakdown of DELLA proteins, allowing transcription factors to promote transcription.
Topic 16.3 · Gene control
What is the difference between a structural gene and a regulatory gene?
Structural genes code for proteins that become part of the cell's structure or have metabolic/physiological roles. Regulatory genes control the expression of structural genes by coding for proteins that influence transcription (
Topic 16.3 · Gene control
Distinguish between repressible and inducible enzymes, referencing their normal state.
Repressible enzymes are typically synthesized unless a repressor molecule binds, halting production (normally 'on'). Inducible enzymes are typically not synthesized unless an inducer molecule is present, triggering production (normally 'off').
Topic 16.3 · Gene control
Explain the role of the lac repressor in the absence of lactose.
In the absence of lactose, the lac repressor protein binds to the lac operator region of the lac operon. This binding physically blocks RNA polymerase from transcribing the structural genes, preventing the production of lactose-digesting enzymes.
Topic 16.3 · Gene control
How do transcription factors influence gene expression in eukaryotes?
Transcription factors are proteins that bind to specific DNA sequences, either increasing (activators) or decreasing (repressors) the rate of transcription of a gene. They affect the ability of RNA polymerase to bind to the promoter and initiate transcription.
Topic 16.3 · Gene control
Describe the mechanism by which gibberellin activates gene expression.
Gibberellin binds to a receptor, leading to the degradation of DELLA repressor proteins. These DELLA proteins normally inhibit transcription factors. Degradation releases these factors, promoting transcription of the gibberellin-responsive genes.
Topic 17.1 · Variation
What is phenotypic variation, and what are the potential causes?
Phenotypic variation refers to the differences in observable characteristics within a population. It can be due to genetic factors (different alleles), environmental factors (
Topic 17.1 · Variation
Define discontinuous and continuous variation.
Discontinuous variation shows distinct categories with no intermediate values (
Topic 17.1 · Variation
Explain the genetic basis of discontinuous variation.
Discontinuous variation is typically controlled by a single gene or a small number of genes, with each gene having a major effect on the phenotype. The alleles produce distinct phenotypes, and there is little environmental influence.
Topic 17.1 · Variation
Explain the genetic basis of continuous variation.
Continuous variation is typically controlled by many genes (polygenic inheritance), each having a small additive effect on the phenotype. Environmental factors also play a significant role in determining the final phenotype.
Topic 17.1 · Variation
What is the purpose of using a t-test?
A t-test is a statistical test used to determine if there is a significant difference between the means of two independent groups or samples. It helps to determine if the observed difference is likely due to a real effect or just random chance.
Review the material
Read full revision notes on Gene control — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 16 — Inheritance
Gene control sits alongside these A-Level Biology decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Gene control deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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