Cells in multicellular organisms share essentially the same genome, yet look and function differently. This happens because different cell types express different subsets of genes, producing distinctive tissue-specific proteins.

Core idea: same DNA, different proteins

Cell differentiation results from differential gene expression—turning some genes “on” and others “off” in a cell-type-specific pattern. The proteins produced (or not produced) shape cell structure and function, creating specialised tissues.

A key implication is that many differences among tissues reflect differences in which proteins are present, not differences in the DNA sequence.

Tissue-specific proteins: what they are and why they matter

A tissue-specific protein is made at high levels in some cell types and at low/undetectable levels in others.

This tissue expression graph illustrates how a gene’s expression can be high in a limited set of tissues and low or absent elsewhere. It provides a simple visual model for “tissue-specific” expression patterns, which often translate into tissue-enriched protein abundance. Differences like these are the quantitative basis for many observable differences among tissues. Source

These proteins create observable differences among tissues.

Tissue-specific proteins commonly fall into functional categories:

• Structural proteins that determine shape and mechanical properties (e.g., cytoskeletal or extracellular matrix components)

• Transport proteins that enable selective movement of substances (channels, carriers)

• Enzymes that support specialised metabolic pathways

• Signalling/communication proteins (receptors, ligands) that tune cells to particular cues

• Adhesion proteins that organise cells into tissues with specific architectures

How gene expression patterns produce differentiation

This diagram summarizes eukaryotic gene expression from transcription through RNA processing to translation. It highlights where regulation can occur along the pathway, reinforcing the idea that differential control of these steps changes which proteins are produced. Because proteins directly shape cell structure and function, changing expression patterns can drive observable differentiation. Source

For a gene to influence phenotype, its information must be expressed into a functional product. Differentiation depends on coordinated control of multiple genes so a cell builds a consistent “toolkit” of proteins.

Key features of differentiation-related gene expression:

• Selective expression: only certain genes are transcribed and translated in a given tissue.

• Quantitative differences: the same gene may be expressed at different levels across tissues, changing protein abundance.

• Timing and stability: some proteins are produced transiently during differentiation, while others persist to maintain tissue identity.

Observable outcomes arise when tissue-specific proteins:

• Alter cell morphology (e.g., elongated vs. flattened cells) by changing cytoskeleton or membrane composition

• Establish specialised organelles or membrane domains (e.g., secretion-focused architecture)

• Enable tissue-specific physiology (e.g., rapid contraction, selective absorption, electrical excitability)

• Promote tissue organisation through cell–cell and cell–matrix interactions

Differentiation at the tissue level

Tissues are collections of cells with shared expression programs that yield characteristic functions.

This heatmap shows gene expression values across multiple samples, with color indicating relative expression level and dendrograms grouping similar genes and samples. It models how a tissue can be characterized by a reproducible expression “signature,” where many genes are consistently high or low together. Such patterns are a standard way to visualize differential gene expression that underlies tissue-specific protein production. Source

Tissue specificity often emerges from:

• Shared protein expression signatures across the tissue’s cells

• Division of labour among related cell subtypes, each expressing overlapping but distinct protein sets

• Maintained identity as cells continue expressing key proteins needed for their role

Because differentiation depends on expressed proteins, changes to expression patterns can change phenotype:

• Reduced expression of a key tissue-specific protein can impair tissue function.

• Ectopic expression (expression in the “wrong” tissue) can disrupt normal structure or signalling relationships.

Common AP Biology connections to emphasise

Within this syllabus focus, the essential reasoning chain is:

• Gene expression → tissue-specific proteins → distinct cell structures/functions → observable differentiation

• A multicellular organism’s diversity of cell types is therefore explainable by which genes are expressed in each cell type and the proteins produced as a result.