Questions: Cell Differentiation and Lineage Specification

Question 1

A skeletal muscle cell and a hepatocyte (liver cell) are isolated from the same individual and their complete DNA sequences are determined. What do you expect to find?

• A. The muscle cell has additional muscle-specific genes absent from the liver cell's genome
• B. Both cells have identical DNA sequences, but different subsets of genes are expressed in each
• C. The liver cell has fewer total genes, having irreversibly deleted unused developmental genes during differentiation
• D. The muscle cell has amplified copies of myosin and actin genes to support their high expression

Answer: (B)

Differentiation operates entirely through differential gene expression, not by changing DNA sequence. Every somatic cell carries the complete genome. Muscle cells express myosin and actin not because they have extra copies, but because lineage-specific transcription factors (like MyoD) activated those genes and epigenetic marks keep their chromatin open, while genes for alternative fates are silenced. This is the fundamental insight of differentiation biology, and Yamanaka's iPSC experiments confirm it: the complete developmental program remains encoded in differentiated cells.

Question 2

A researcher introduces the transcription factor MyoD into cultured skin fibroblasts. According to the logic of master regulatory transcription factors, what is the most likely result?

• A. Nothing — transcription factors alone cannot override a cell's established differentiated state
• B. The fibroblasts begin expressing muscle-specific genes (myosin, actin, creatine kinase) and take on muscle-like characteristics
• C. The fibroblasts dedifferentiate to a pluripotent stem cell state before redifferentiating as muscle cells
• D. MyoD causes global chromatin opening that indiscriminately activates all genes

Answer: (B)

MyoD is a master regulator that sits at the top of the muscle differentiation transcriptional cascade. By binding to muscle-gene promoters and enhancers, it can single-handedly activate the entire battery of muscle-specific genes. This is precisely what makes these factors 'master regulators' — one TF triggers a coordinated program. The fibroblasts don't need to pass through a stem cell state; MyoD directly converts them to a muscle-like fate. This experiment, performed by Davis et al. in 1987, was a landmark demonstration of how differentiation is a regulatory state, not an irreversible cellular identity.

Question 3

As a cell differentiates, it permanently loses the DNA sequences encoding genes needed for alternative cell fates.

• T. True
• F. False

Answer: False

Differentiation does not involve genetic loss. All somatic cells retain the complete genome. Genes needed for alternative fates are silenced through epigenetic mechanisms — DNA methylation, repressive histone marks (like H3K27me3), and chromatin condensation — not deleted. This distinction is critical: it explains why Yamanaka could reprogram differentiated adult cells back to pluripotency by introducing four transcription factors. If genes had been deleted, reprogramming would be impossible. The genome retains all developmental instructions; differentiation is a regulatory state layered on top of an unchanged sequence.

Question 4

The epigenetic marks that enforce a cell's differentiated state are heritable: when a differentiated cell divides, its daughter cells inherit the same chromatin state and continue expressing the same specialized genes.

• T. True
• F. False

Answer: True

Maintenance enzymes copy epigenetic marks to newly synthesized DNA strands and histones during cell division. DNA methyltransferase 1 (DNMT1) copies CpG methylation patterns, and histone-modifying complexes propagate activating and repressive marks. This inheritance mechanism is what gives differentiated cell identity stability over many cell generations — a liver cell divides to produce more liver cells — without requiring the original differentiation signals to be present. The self-reinforcing nature of this system is what makes the differentiated state stable, though not irreversible.

Question 5

Yamanaka's experiment showed that four transcription factors can convert a differentiated adult cell into an induced pluripotent stem cell. What does this reveal about the nature of the differentiated state?

Model answer: It reveals that differentiation is a regulatory state imposed on top of an unchanged genome, not a permanent alteration of the DNA sequence. A differentiated cell still contains the complete genome with instructions for every cell type. What distinguishes cell types is a layer of epigenetic programming — patterns of chromatin accessibility and repression — that routes gene expression to a cell-type-specific program. Yamanaka's four factors (Oct4, Sox2, Klf4, c-Myc) work by erasing or overriding this epigenetic layer, resetting chromatin to a permissive state in which all developmental programs are again accessible. The difficulty and inefficiency of reprogramming (most cells fail) reflects the stability of epigenetic marks, not genetic irreversibility.

The broader implication is that the complete potential for every cell type is preserved in every cell — differentiation doesn't reduce a cell's genetic repertoire but restricts which parts of it are expressed. This understanding is foundational for regenerative medicine, where the goal is to harness pluripotent cells (iPSCs or embryonic stem cells) to generate specific tissues, and for understanding cancer, where dedifferentiation can contribute to uncontrolled proliferation.