Gene expression — IB Diploma Biology HL
IB Biology HL D2.2: regulation of transcription and mRNA breakdown, genome vs transcriptome vs proteome, methylation, epigenetic inheritance and twins.
Revision notes
4 short notes, in the order of the specification. Each one in short:
Gene expression is the mechanism by which information in genes has effects on the phenotype, usually by transcription, translation and the function of a protein product such as an enzyme. Cells regulate it with transcription factors that bind to specific base sequences such as promoters and enhancers, and by controlling how quickly mRNA is broken down by nucleases.
Epigenesis is the development of patterns of differentiation in the cells of a multicellular organism. Every cell has the same genome, but each expresses only some genes, so transcriptomes and proteomes differ. Epigenetic tags such as methylation of cytosine in promoters and of histones change gene expression and phenotype without altering the base sequence.
Epigenetic inheritance is the passing of changes in gene expression to daughter cells or offspring without any change in the DNA base sequence. It happens when tags such as DNA methylation stay in place through mitosis or meiosis. The environment, such as air pollution, can alter tags, and most but not all tags are removed from gametes.
External factors such as hormones and nutrients change the pattern of gene expression in cells. Oestrogen binds to a receptor inside target cells, and the complex acts as a transcription factor. In E. coli, allolactose (made from lactose) binds to the lac repressor so it releases the operator, allowing genes for lactose uptake and breakdown to be transcribed.
Exam questions
6 exam-style questions (19 marks), each with its mark scheme.
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Quick check questions
What are the three most common stages in gene expression?
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Transcription, translation and the function of the protein product (such as an enzyme).
What is a transcription factor?
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A protein that binds to a specific base sequence in DNA, such as a promoter or enhancer, to regulate transcription.
What effect does methylation of cytosine in a promoter have?
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It represses transcription of the gene downstream.
Why do monozygotic twins become more different with age?
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They have identical genomes, but different environments change their epigenetic tags and so their gene expression.
Frequently asked questions
What is the difference between the genome, transcriptome and proteome?
The genome is all the genetic information of a cell, and it is the same in every cell of an organism. The transcriptome is all the RNA transcribed in a cell at one time, and the proteome is all the proteins it makes. These differ between cells because no cell expresses all of its genes.
What is epigenetics in simple terms?
Epigenetics is the study of changes in gene expression that do not change the DNA base sequence. Tags such as methyl groups on cytosine in promoters or on histones switch genes on or off. They alter the phenotype but not the genotype, and some can be passed to daughter cells or offspring.
How does DNA methylation affect gene expression?
Methylation of cytosine bases in the promoter of a gene represses transcription, so the gene downstream is not expressed. Methylation of amino acids in histones can either repress or activate transcription. In both cases the base sequence is unchanged; only the pattern of gene expression is altered.
Why are ligers bigger than tigons?
Ligers and tigons both have lion and tiger chromosomes, so the difference is epigenetic. Most, but not all, tags are removed from gametes, so imprinted growth genes keep parent-specific tags. A liger gets strong growth promotion from its lion father but weak restraint from its tigress mother, so it grows much larger than a tigon.
How does the lac operon work?
When lactose is absent, a repressor protein binds to the operator and stops RNA polymerase transcribing the genes for lactose uptake and breakdown. When lactose is present, it binds to the repressor and changes its shape, so the repressor leaves the operator. The genes are then transcribed and the enzymes are made.
Written and checked against the IB Biology HL specification · Updated October 2026