D2.2 Gene expression (HL only)

• Gene expression = the process by which information in a gene produces an effect on the phenotype.

• The most common sequence is transcription → translation → functional protein product.

• A gene only affects phenotype if it is expressed.

• Exam idea: connect DNA sequence, RNA production, protein function, and observable trait.

This diagram shows the overall pathway of gene expression in eukaryotes, linking DNA, transcription, RNA processing, translation, and the final protein product. It is useful for seeing where regulation can occur before a phenotype appears. Source

Regulation of transcription

• Transcription is a major control point in gene expression.

• Regulation happens when proteins bind to specific base sequences in DNA.

• Key DNA regions:

  • Promoter = where RNA polymerase and transcription machinery assemble.

  • Enhancer = DNA sequence that increases transcription when bound by regulatory proteins.

• Key proteins:

  • Transcription factors bind DNA and help activate or repress transcription.

• High-yield exam idea: if transcription is reduced, less mRNA is made, so usually less protein is produced.

• Do not confuse gene present in genome with gene actively expressed.

This image shows how transcription factors regulate gene expression by interacting with DNA and transcription machinery. It is helpful for linking promoters, regulatory proteins, and changes in transcription rate. Source

Control after transcription: mRNA lifespan

• Cells can regulate translation by controlling mRNA degradation.

• mRNA may last from minutes to days in human cells before being broken down by nucleases.

• Short-lived mRNA → brief protein production.

• Longer-lasting mRNA → more opportunities for translation.

• Exam idea: gene expression is not controlled only at transcription; it can also be controlled by how long mRNA survives.

Epigenesis and cell differentiation

• Epigenesis = development of different patterns of cell differentiation in a multicellular organism.

• Cells with the same genome can develop different structures and functions because they express different genes.

• Epigenetic changes alter phenotype without changing the DNA base sequence.

• Therefore:

  • Genotype stays the same.

  • Phenotype can change.

• High-yield link: differentiation depends on different patterns of gene expression, not different DNA in each body cell.

Genome, transcriptome, and proteome

• Genome = all of the genetic information / all DNA in a cell.

• Transcriptome = all RNA transcripts being produced in a cell at a given time.

• Proteome = all proteins present in a cell at a given time.

• No cell expresses all of its genes.

• Different cell types have the same genome but different transcriptomes and proteomes.

• The pattern of gene expression determines cell specialization.

• Exam comparison:

  • Same genome across most body cells.

  • Different transcriptome/proteome depending on cell role and conditions.

This diagram compares the genome, transcriptome, and proteome. It helps show why cells with the same DNA can still produce different RNAs and proteins, leading to different cell functions. Source

This diagram summarizes major histone modifications involved in epigenetic regulation. It is useful for understanding how chemical changes to histones can switch transcription on or off. Source

This image shows DNA methylation, an epigenetic modification in which methyl groups are added to DNA. For IB Biology, the key idea is that promoter methylation represses transcription and therefore reduces gene expression. Source

Environmental effects on gene expression

• The environment can alter gene expression.

• Required example: air pollution can alter methyl tags on DNA.

• Environmental change can therefore affect phenotype through epigenetic modification.

• This does not require a change in base sequence.

• Exam link: environment may influence gene activity both in cells and in the whole organism.

This image shows how environmental conditions can influence phenotype through epigenetic changes. It is a strong visual reminder that changes in gene expression can occur without changing DNA sequence. Source

This diagram shows the lac operon, a classic example of how an external biochemical signal such as lactose can regulate gene expression in bacteria. It helps explain how regulatory proteins interact with the promoter, operator, and structural genes. Source

Exam connections and common traps

• Gene expression explains how genes influence phenotype; mutation is not required for different cells to behave differently.

• Different cell types in one organism usually have the same genome but different transcriptomes and proteomes.

• Promoter methylation usually means less transcription.

• Epigenetic change alters expression, not base sequence.

• Environmental factors can change gene expression through epigenetic mechanisms.

• Monozygotic twins are useful because genotype is controlled, so environmental effects are easier to identify.

• If asked about inheritance, distinguish carefully between genetic inheritance and epigenetic inheritance.