AP Syllabus focus:
‘Differential regulation of genes leads to distinct patterns of gene expression, producing unique sets of cell products and specialized cell functions.’
Cells in a multicellular organism usually contain the same DNA, yet look and act differently.
This diagram contrasts relaxed euchromatin (transcriptionally permissive) with condensed heterochromatin (transcriptionally repressive). It highlights how histone acetylation (often via HATs) tends to open chromatin, while deacetylation (often via HDACs) promotes chromatin condensation. This directly supports the idea that cells with identical DNA can look and function differently because different regions of the genome are made accessible or inaccessible. Source
This is explained by differential gene expression: cells turn different genes on or off, producing different proteins and functions.
Core idea: same genome, different outputs
Differential gene expression creates cell diversity by controlling which genes are expressed, when they are expressed, and how much product is made.
Differential gene expression: The cell-specific regulation of gene activity that results in different sets and amounts of RNA and protein products in different cells, despite the same DNA.
Cell specialisation emerges because cell structure and behavior depend on its gene products (proteins and functional RNAs), not on DNA sequence differences.
How differential regulation produces specialisation
From gene expression patterns to cell function
A cell’s observable traits (phenotype) reflect:
Which genes are expressed
The timing and rate of expression
The stability and activity of gene products
Specialised cells maintain distinct internal “profiles,” including:
Enzymes that determine metabolism (e.g., detoxification vs energy storage)
Structural proteins that shape the cell (e.g., cytoskeletal components)
Membrane proteins that control signalling and transport (e.g., receptors, channels)
Secreted proteins for organism-level roles (e.g., hormones, extracellular matrix proteins)
Levels where regulation can differ
Differential regulation can change gene product levels at multiple points:
This diagram maps gene regulation checkpoints across the full pathway from DNA to functional protein. It emphasizes that gene expression can be tuned not only at transcription, but also during RNA processing/export, translation, and post-translational modification and degradation. This directly supports the idea that different cell types can maintain different protein “outputs” from the same genome. Source
Transcriptional control: whether a gene is transcribed into RNA, and at what rate
Post-transcriptional control: whether an RNA transcript is processed, retained, degraded, or made available for translation
Translational control: how efficiently ribosomes translate an mRNA into protein
Post-translational control: whether a protein is activated, modified, localised, or degraded
Different cell types may express the same gene at different levels, or express entirely different gene sets, creating distinct functional capacities.
Why specialised cells stay specialised
Stable expression states
Once a cell adopts a specialised identity, it often maintains a stable expression pattern across many cell divisions. Stability is supported by:
This NIH illustration summarizes major epigenetic mechanisms—DNA methylation and histone-tail modifications—and connects them to chromatin accessibility (gene active vs. inactive). It visually reinforces how cells can “lock in” expression states by making DNA more or less available to transcription machinery. This provides a mechanistic basis for long-term cell identity and cellular memory. Source
Self-reinforcing regulatory networks in which gene products influence the expression of other genes in a consistent direction
Cell memory mechanisms that keep key genes active or inactive long-term, ensuring daughter cells resemble the parent cell’s type
Responsiveness without losing identity
Specialised cells can still adjust expression in response to signals while remaining the same cell type. Typical features include:
Rapid upregulation of certain genes when stimulated
Limited changes constrained by the cell’s established regulatory state, preserving core identity functions
What AP Biology expects you to connect
Linking regulation to phenotype
Be able to explain, in words and with logic, how:
Distinct gene expression patterns lead to
Distinct sets of cell products (proteins/functional RNAs), causing
Specialised structures and functions among cell types in the same organism
Common misconceptions to avoid
Specialised cells generally do not have different genomes; they differ mainly in gene expression.
Not all regulation is “on/off”; many important differences are quantitative (more or less expression).
A cell’s function depends on both which proteins are present and how much is present, as well as protein activity and localisation.
FAQ
They compare RNA or protein profiles between cell types.
Common approaches include:
RNA-level comparisons (e.g., transcript profiling)
Protein-level comparisons (e.g., antibody-based detection)
These methods reveal which gene products are enriched or absent in each cell type.
Yes, but it is usually restricted and requires substantial regulatory changes.
Examples include experimentally induced changes where key regulatory factors are altered, shifting the cell’s expression programme to a new stable state.
Housekeeping genes encode products needed for basic survival in nearly all cells.
They support core processes such as energy production, membrane maintenance, and macromolecule synthesis, even while specialised genes differ between tissues.
A marker gene is mainly useful for identifying a cell type because it is consistently expressed there.
A functional gene directly contributes to the specialised role (for instance, encoding a protein required for secretion, contraction, or signalling).
Cells can express the same gene at different rates, producing different concentrations of mRNA and protein.
Small expression differences can cause large functional effects when the gene product is:
An enzyme in a rate-limiting step
A receptor controlling signalling sensitivity
A regulatory protein influencing many downstream genes
Practice Questions
Explain how two cells in the same organism can have different functions despite containing the same DNA. (2 marks)
Same DNA/genome in different cell types (1)
Different genes are expressed or expressed at different levels, producing different proteins/cell products that determine function (1)
Describe how differential gene expression leads to cell specialisation. In your answer, refer to patterns of gene expression and the resulting cell products. (5 marks)
Differential regulation creates distinct patterns of gene expression between cell types (1)
Different sets and/or amounts of mRNA are produced (1)
This leads to different sets and/or amounts of proteins/functional RNA products (1)
Proteins determine cell structure (e.g., structural proteins, membrane proteins) and biochemical capabilities (e.g., enzymes) (1)
Therefore cells develop specialised functions based on their unique gene product profile (1)
