IB Biology · Theme D: Continuity and change · HL only
D2.2 Gene expressionHL only
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 lists the most common stages by which a gene affects the phenotype?
- Translation → transcription → replication
- Transcription → translation → protein function
- Replication → transcription → mRNA breakdown
- Protein function → translation → transcription
Show the answer
Answer: B. A gene is transcribed, the mRNA translated, and the protein then acts.
Question 2. A gene with a normal base sequence has no effect on the phenotype of a particular cell. What is the most likely explanation?
- The gene must be a pseudogene.
- The promoter of the gene has mutated.
- The gene is on a different chromosome.
- The gene is not expressed in that cell.
Show the answer
Answer: D. Genes affect the phenotype only when they are expressed.
Question 3. A gene codes for an enzyme in the pathway that makes a skin pigment. At which point does the gene affect the phenotype?
- When the enzyme makes the pigment
- When the gene is transcribed into mRNA
- When the mRNA leaves the nucleus
- When the gene is replicated
Show the answer
Answer: A. The phenotype changes when the protein product functions.
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.