IB DP Biology: Gene expression and epigenetics (HL) (DP Biology) – Revision Notes

These notes condense section D2.2, Gene expression, of the IB Biology guide (first assessment 2025). Everything here is HL only. For full explanations and worked examples, read the study guide, then test yourself with the practice questions. The course hub, the printable checklist and the free 10-minute diagnostics help you plan the rest of your revision.

Key definitions

Term	Meaning
Gene expression	The process by which gene information has effects on phenotype
Promoter	DNA sequence next to a gene where RNA polymerase attaches
Enhancer	DNA sequence where regulatory proteins bind to raise transcription of a gene
Transcription factor	Protein that binds a specific base sequence and helps or blocks transcription
Nuclease	Enzyme that breaks down mRNA
Epigenesis	Process by which cells of a multicellular organism develop different patterns of differentiation
Genome	All the DNA of a cell
Transcriptome	All the mRNA in a cell at a given time
Proteome	All the proteins in a cell
Epigenetic tag	Chemical group (such as a methyl group) on DNA or histones that changes expression without changing the base sequence
Monozygotic twins	Twins from one zygote, with the same genome

The pathway in one line

Gene → transcription → mRNA → translation → polypeptide → protein function (for example an enzyme) → phenotype. Control can act at several points: whether transcription starts, how long mRNA lasts, and how the protein works.

Control points at a glance

Level	Mechanism	Effect
Transcription	Transcription factors at promoters and enhancers	Raise or lower the rate
Transcription	Methylated cytosine in a promoter	Represses; gene downstream silenced
Transcription	Methylated amino acids in histones	Repress or activate, depending on site
After transcription	Nucleases degrade mRNA	Shorter mRNA life, less translation

Genome, transcriptome, proteome

• Cells of one organism share the same genome.
• No cell expresses all its genes.
• The pattern of expression decides the cell type.
• So the transcriptome and proteome differ between cells even though the genome is the same.
• The proteome does not exactly mirror the transcriptome: mRNA lifetime and translation also matter.

Epigenetic inheritance in steps

1. A tag (DNA methylation or histone modification) is in place on a gene.
2. The tag stays in place during mitosis, so daughter cells show the same expression pattern.
3. If tags stay in place through meiosis, offspring could show the changed phenotype.
4. The base sequence never changed, so genotype is unaltered.

Tigons and ligers in steps

1. Most epigenetic tags are removed from ovum and sperm, resetting the zygote.
2. Not all are removed, so some tags carry information about the parent of origin.
3. Retained tags on growth-related genes differ between lion and tiger.
4. Lion father and tiger mother give ligers; tiger father and lion mother give tigons.
5. The same genes are expressed differently, giving different phenotypes.

State it as an outline, with careful wording (“it is proposed that”).

Method boxes

Half-life from mRNA data

1. Find the time for the amount to halve.
2. Check against a second pair of values.
3. For the fraction left after time t: (1/2) raised to the power t / half-life.

Percentage change (new − old) / old × 100. Quote it with a unit, such as %.

Fold change new / old. State which is divided by which.

Interpreting a methylation or twin data set

1. Quote data for both groups with units.
2. Calculate the difference or ratio.
3. Link the trend to the biology (more methylation, less expression).
4. Add one limitation: correlation is not cause, group sizes, confounding variables.

Small worked reminders

Half-life. mRNA falls from 200 to 100 units in 4 h. After 12 h (three half-lives) it is 200 × (1/2)³ = 25 units.

Genes expressed. A cell transcribes 7 500 of 20 000 genes (data supplied): 7 500 / 20 000 × 100 = 37.5%.

Fold change. Gene activity of 18 with a drug and 90 without: the drug lowers it by (90 − 18) / 90 × 100 = 80%, or 5-fold.

Environmental effects

Example	What changes	Link to the guide
Air pollution	Methyl tags on DNA	Environment alters expression through tags
Hormone	A steroid hormone-receptor complex acts as a transcription factor	One hormone example
Lactose (bacteria)	Lactose-derived molecule binds repressor; genes are transcribed	One biochemical example
Tryptophan (bacteria)	Tryptophan binds repressor; genes are blocked	Alternative biochemical example

Twin studies

• Monozygotic twins share a genome, so differences in expression come from the environment.
• Older pairs, and pairs who spent more years apart, may show larger differences in methylation.
• Check whether the data follow the same twins over time or compare different groups.

Hormone and bacterial examples in steps

Steroid hormone (one hormone example)

1. The hormone passes through the cell-surface membrane, because it is lipid-soluble.
2. It binds a specific receptor protein.
3. The hormone-receptor complex enters the nucleus.
4. It binds a specific base sequence and acts as a transcription factor.
5. Transcription of the target genes rises, so more of the protein product is made.

Only cells with the receptor respond. This is why a hormone reaches every cell but changes only some.

Lactose in bacteria (one biochemical example)

1. With no lactose, the repressor sits on the operator and RNA polymerase is blocked.
2. Lactose arrives and a lactose-derived molecule binds the repressor.
3. The repressor changes shape and leaves the operator.
4. RNA polymerase transcribes the genes and the enzymes for lactose use appear.

Tryptophan in bacteria (the alternative biochemical example)

1. Without tryptophan, the repressor cannot bind and the genes are transcribed.
2. With tryptophan, the repressor binds the amino acid and now attaches to the operator.
3. Transcription stops, so the cell stops making what is already supplied.

Learn one clearly rather than three vaguely. The guide asks for one hormone and one biochemical.

Data interpretation reminders

Reporter-gene data with methylation. If activity falls when the promoter is methylated and rises when a methyl-removing drug is added, the intervention supports a causal role for methylation. If the drug restores only part of the activity, say that some methyl groups probably remain, or that other factors are involved.

Cells with and without a receptor. If cells lacking the receptor show no response, the receptor is needed. If cells with the receptor respond only when the hormone is present, the hormone is needed too.

Mutant repressor data. A strain with high activity whether or not the inducer is present has lost control. The repressor no longer blocks the operator.

Twin data. Differences that rise with age fit the idea that environments add up over time. A cross-sectional study (different pairs at each age) cannot track change within one pair.

Air pollution data. A higher methylation level in a polluted district fits environmental alteration of tags, but check age, smoking and sample size before claiming cause.

Command words for this unit

Command word	What to do here
State	One fact: for example, the three stages of gene expression
Outline	Brief main points: for example, how tigons and ligers differ
Explain	Give reasons: for example, why methylation lowers expression
Compare	Give similarities and differences, using both sides in each sentence
Calculate	Show the working and include units
Suggest	Apply the ideas to a new situation, and hedge where the data allow

Must-know distinctions

Compare	First	Second
Genome vs transcriptome	DNA, same in nearly all cells	mRNA present, varies
Mutation vs epigenetic change	Sequence altered	Sequence unaltered
Promoter vs enhancer	Where RNA polymerase attaches	Where factors bind to raise transcription
Lactose vs tryptophan	Releases the repressor	Activates the repressor
Promoter methylation vs histone methylation	Represses	Represses or activates

Quick self-test

1. State the three common stages of gene expression.
2. Name the enzymes that degrade mRNA.
3. What is the effect of methylating cytosine in a promoter?
4. mRNA starts at 160 units with half-life 2 h. How much is left after 6 h?
5. A cell transcribes 5 000 of 20 000 genes. What percentage is that?
6. Why do the transcriptomes of two cell types differ when the genome is the same?
7. Why can histone methylation not be called always repressive?
8. Why are monozygotic twins useful for studying the environment?
9. A gene’s activity is 40 without a drug and 10 with it. Give the percentage decrease.
10. What does a lactose-derived molecule do to the repressor in E. coli?
11. Why are tigons and ligers different in phenotype?
12. State what stays unchanged in epigenesis.

Answers

1. Transcription, translation, function of the protein product.
2. Nucleases.
3. It represses transcription, so the gene downstream is not expressed.
4. Three half-lives: 160 × 1/8 = 20 units.
5. 5 000 / 20 000 × 100 = 25%.
6. Different genes are transcribed in each cell type.
7. It can repress or activate, depending on the amino acid and site.
8. They share a genome, so differences in expression come from the environment.
9. (40 − 10) / 40 × 100 = 75%.
10. It binds the repressor, which releases the operator, so the genes are transcribed.
11. Retained epigenetic tags differ, so gene expression differs depending on the parent species.
12. The DNA base sequence (genotype).

Where marks are usually lost

• Saying epigenetic changes are mutations or alter the DNA sequence.
• Saying all tags are erased in gametes.
• Writing that the genome differs between cell types.
• Quoting a half-life without checking that the amount halves each time.
• Giving a percentage change with the wrong baseline (divide by the old value).
• Claiming a correlation proves that pollution or methylation causes an effect.
• Writing “the gene is switched off” without saying which step (transcription) is affected.
• Mixing up the repressor action of lactose and tryptophan.
• Forgetting a unit or the words “fold” or “%”.

Official syllabus

IB Biology guide, first assessment 2025, International Baccalaureate Organization, section D2.2 Gene expression (additional higher level).