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.
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
Official 2025-2027 spec · A2 LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- describe the differences between structural genes and regulatory genes and the differences between repressible enzymes and inducible enzymes
- explain genetic control of protein production in a prokaryote using the lac operon (knowledge of the role of cAMP is not expected)
- state that transcription factors are proteins that bind to DNA and are involved in the control of gene expression in eukaryotes by decreasing or increasing the rate of transcription
- explain how gibberellin activates genes by causing the breakdown of DELLA protein repressors, which normally inhibit factors that promote transcription
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 Gene control
- › Use 'independent assortment' when discussing the random alignment of homologous pairs at the equator; use 'crossing over' for the exchange of alleles between non-sister chromatids.
- › Define haploid (n) as one set of chromosomes and diploid (2n) as two sets of chromosomes, applicable to all organisms.
- › In Hardy-Weinberg problems, always find q first by using the frequency of the homozygous recessive (q squared).
- › Recognize that a 9:3:3:1 ratio from a dihybrid cross confirms that genes are on separate autosomes and assort independently.
- › Use comparative language or a table where each row addresses one specific feature for both subjects being compared.
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 (
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').
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.
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.
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.
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 (
Define discontinuous and continuous variation.
Discontinuous variation shows distinct categories with no intermediate values (
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.
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.
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.
More 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.
How to study this Gene control deck
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