IB Biology · Theme D: Continuity and change · HL only
D2.2 Gene expression
A one-page summary of D2.2 Gene expression, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
How is gene expression changed in a cell?
How can patterns of gene expression be conserved through inheritance?
What D2.2 covers
Controlling transcription and translation
- D2.2.1Genes only matter once they’re expressed
- D2.2.2Promoters and transcription factors switch genes on
- D2.2.2Enhancers reach genes from a distance
- D2.2.3Degrading mRNA controls how much protein gets made
Epigenesis and the epigenome
- D2.2.4One genome, many cell types: epigenesis
- D2.2.5Genome, transcriptome, proteome: what’s shared, what’s not
- D2.2.6Methyl tags: one mark that always represses, one that can go either way
- D2.2.7Tags can outlive the cell that made them
Gene expression responds to the environment
- D2.2.8Air pollution can reach into the genome
- D2.2.9Tigons and ligers: nearly the same DNA, different phenotypes
- D2.2.10What monozygotic twins reveal about environment and gene expression
- D2.2.11A hormone example: oestradiol switches on target genes
- D2.2.11A biochemical example: lactose availability in bacteria
D2.2 Gene expression: summary
Gene expression
- Transcription (DNA→mRNA), translation (mRNA→polypeptide), then protein function. A gene only affects phenotype once expressed.
Controlling transcription
- Promoters and enhancers (DNA looping) are bound by transcription factors. mRNA degradation (nucleases) tunes translation after transcription.
Genome vs transcriptome vs proteome
- Same genome, different expressed subset. No cell expresses all its genes — that subset, not the genome, makes a cell type what it is.
Epigenetic tags
- Promoter DNA methylation always represses. Histone methylation can repress or activate. Neither changes the base sequence.
Inheritance & the environment
- Tags survive mitosis/meiosis (epigenetic inheritance). Most, not all, tags are erased in ovum/sperm — tigons/ligers. Air pollution and MZ twins show environmental effects.
External factors
- Oestradiol (hormone, intracellular receptor) and lactose availability in bacteria (biochemical) — the two required examples. No operon names needed.
Key terms
- Gene expression
- The mechanism by which information carried in a gene produces an effect on phenotype.
- Transcription factor
- A protein that binds a specific DNA base sequence to regulate the rate of transcription of a nearby gene.
- Promoter
- A DNA base sequence, close to a gene, where transcription is initiated.
- Enhancer
- A DNA base sequence that increases the rate of transcription of a gene when bound by an activator transcription factor.
- Nuclease
- An enzyme that breaks down mRNA, a post-transcriptional control on translation.
- Epigenesis
- The development of patterns of cell differentiation in a multicellular organism through selective gene expression.
- Epigenetic tag
- A chemical modification, such as DNA or histone methylation, that affects gene expression without altering the DNA base sequence.
- Genome, transcriptome, proteome
- The complete set of DNA sequences; of RNA transcripts present; of proteins present, in a cell at a given time.
- Epigenetic inheritance
- The passing of a heritable change in gene expression to daughter cells or offspring without any change to the DNA nucleotide sequence.
Sample exam questions
Three of the 36 multiple-choice questions for D2.2. Try each one before opening the answer.
Question 1. Which sequence correctly lists the most common stages by which gene expression produces an effect on phenotype?
- Translation → transcription → DNA replication
- Transcription → translation → function of the protein product
- DNA replication → transcription → mRNA degradation
- Function of the protein product → translation → transcription
Show the answer
Answer: B. Gene expression proceeds from transcription (DNA to mRNA) to translation (mRNA to polypeptide) to the function of the resulting protein product, such as an enzyme, which is what actually produces a phenotypic effect.
Question 2. A gene with a completely normal, error-free base sequence produces no observable effect on phenotype in a particular cell. What is the most likely explanation?
- The gene must be a pseudogene
- The gene's promoter has mutated
- The gene is located on a different chromosome
- The gene has not been expressed in that cell
Show the answer
Answer: D. A gene only affects phenotype once it is expressed. A normal base sequence that is never transcribed and translated in a given cell produces no protein product there, and therefore no phenotypic effect in that cell.
Question 3. An enhancer sequence increases the rate of transcription of a gene it controls, even though it lies far away from that gene on the chromosome. What best explains this?
- The enhancer is transcribed into a separate regulatory mRNA that then acts on the promoter
- The enhancer physically moves along the chromosome until it reaches the promoter
- DNA looping brings the enhancer-bound activator protein into contact with the promoter complex
- Enhancers only work on genes located immediately next to them
Show the answer
Answer: C. Because DNA is a flexible molecule, it can loop so that a distant enhancer, with an activator transcription factor bound to it, comes into physical contact with the promoter and the transcription machinery there, increasing the rate of transcription.
Practise D2.2
Study notes, every question and full markschemes for D2.2 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.