How do cells with identical DNA become different cell types?
Topic 6.6 Gene Expression and Cell Specialization: explain how differential gene expression produces specialized cell types from one genome.
A focused answer to AP Biology Topic 6.6, covering differential gene expression, cell differentiation, the role of signalling and transcription factors, stem cells, and how one genome builds many cell types, with a worked example.
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What this topic is asking
The College Board (Topic 6.6) wants you to explain how differential gene expression produces specialized cell types from a single genome, including the roles of signalling, transcription factors and cell differentiation.
One genome, many cell types
How cells differentiate
Stem cells
The control points are the same regulatory mechanisms covered in the regulation topic: transcription factors and epigenetic changes (such as DNA methylation and histone modification) keep the genes for other cell types switched off, so a liver cell does not start making nerve-cell proteins. Once a cell has differentiated, these patterns are usually stable and are copied to its daughter cells when it divides, which is why a tissue keeps making the same cell type.
This explains a deep idea: a multicellular organism is built not by giving cells different genes, but by giving them the same genes and different instructions about which to use. The same principle links back to the cell-communication pathways of Unit 4, because the signals that trigger differentiation act through signal-transduction pathways that ultimately switch transcription factors on or off.
Try this
Q1. Define differential gene expression. [1 point]
- Cue. Different cell types expressing different subsets of the same genome, so they make different proteins.
Q2. Explain the role of transcription factors in cell differentiation. [2 points]
- Cue. Signals activate specific transcription factors, which switch particular genes on or off, committing the cell to a fate and producing that cell type's proteins.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2020 (style)4 marksSection II (long FRQ excerpt). A muscle cell and a nerve cell in the same organism contain identical DNA but have very different structures and functions. (a) Explain how this is possible. (b) Describe the role of transcription factors and signalling in producing different cell types during development.Show worked answer →
A 4-point explain-and-describe FRQ on differentiation.
(a) Explain (2 points): (1 point) the cells have the same genome but express different subsets of genes (differential gene expression); (1 point) so each cell makes a different set of proteins, giving it a different structure and function.
(b) Describe (2 points): (1 point) signals received during development switch on specific transcription factors in each cell; (1 point) those transcription factors turn particular genes on or off, committing the cell to a fate and producing the proteins specific to that cell type.
Markers reward identifying differential gene expression and linking signalling and transcription factors to cell fate.
AP 2017 (style)1 marksSection I (multiple choice). The process by which an unspecialised cell becomes a specific cell type with a particular structure and function is called: (A) replication. (B) differentiation. (C) mitosis. (D) translation.Show worked answer →
The correct answer is (B).
Differentiation is the process by which a cell becomes specialized by expressing a particular subset of its genes. Replication (A) copies DNA, mitosis (C) divides cells, and translation (D) builds proteins, but none of these alone makes a cell specialized.
Related dot points
- Topic 6.5 Regulation of Gene Expression: explain how gene expression is regulated in prokaryotes and eukaryotes, including operons and regulatory sequences.
A focused answer to AP Biology Topic 6.5, covering the lac and trp operons, promoters, regulatory sequences, transcription factors and epigenetic control, and how regulation lets cells respond to the environment, with a worked operon example.
- Topic 6.3 Transcription and RNA Processing: explain how RNA polymerase transcribes a gene into mRNA and how the primary transcript is processed in eukaryotes.
A focused answer to AP Biology Topic 6.3, covering RNA polymerase, the template strand, the differences between transcription and replication, and eukaryotic RNA processing (cap, tail, splicing), with a worked transcription example.
- Topic 5.5 Environmental Effects on Phenotype: explain how environmental factors can affect the phenotype produced by a given genotype.
A focused answer to AP Biology Topic 5.5, covering how temperature, nutrients, pH and other environmental factors influence phenotype, the genotype-by-environment interaction, and norms of reaction, with a worked example.
- Topic 4.3 Signal Transduction Pathways: explain how signalling pathways relay and amplify a signal to produce a response, and how mutations or chemicals that change the pathway affect the cell.
A focused answer to AP Biology Topic 4.3, covering relay molecules, phosphorylation cascades, signal amplification, the variety of cellular responses, and how mutations and chemicals alter pathways.
- Topic 4.1 Cell Communication: describe the ways cells communicate, including direct contact and chemical signalling over short and long distances.
A focused answer to AP Biology Topic 4.1, covering direct contact signalling, paracrine, autocrine, synaptic and endocrine signalling, and how signal type relates to distance and target.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)