D2.2.4 (HL)—Epigenesis

Epigenesis produces differentiated cell states by changing gene activity patterns without altering the underlying DNA base sequence during development processes.

Epigenesis is the development of organized patterns of cell differentiation from an initially undifferentiated zygote.

one zygote → repeated mitosis → cells receive different positional or signalling inputs → different genes remain active or inactive → different proteins accumulate → cells acquire specialized structures and functions

A pancreatic β cell expresses proteins for insulin production and secretion.
A skeletal-muscle fibre expresses abundant contractile proteins.
A neuron expresses receptors, ion channels and synaptic proteins suited to signalling.

Differentiation usually changes which genes are expressed, not which genes the cell possesses. Most somatic cell types retain essentially the same genome.

Cells produced from the same zygote usually share the same genome, but regulatory states make different subsets of that genome accessible and active.

Same underlying genome	Different active output	Functional consequence
insulin gene present	abundant insulin mRNA and protein in pancreatic β cells	blood-glucose regulation
collagen genes present	high collagen expression in many connective-tissue cells	extracellular support
ion-channel genes present	neuron-specific channel and receptor profile	electrical signalling

regulatory state → selected transcriptome → selected proteome → specialized structure and function

A cell type does not normally lose every gene it does not use. Inactivity and absence are different claims.

shared genome → cell-specific regulatory and epigenetic state → cell-specific transcriptome → cell-specific proteome → differentiated structure and function

Process	What remains continuous?	What can change?
mitosis in a differentiated lineage	much of the expression state	reversible tags and regulatory inputs
environmental response	DNA base sequence usually remains	signalling, transcription and sometimes epigenetic marks
gamete and early-embryo reprogramming	genome and selected imprints	most parental epigenetic patterns are reset

For any epigenetic claim, ask where the tag is, how it changes access or protein binding, whether it persists through mitosis or gametes, and whether the evidence shows correlation or causation.

Concept essentials

Differentiated cells can contain the same genome but express different genes.
Epigenesis creates stable patterns of gene activity during development.
Epigenetic regulation does not require a DNA base-sequence change.
Cell type depends on which genes are active or inactive.

D2.2.4 (HL)—Epigenesis

Epigenesis Builds Specialized Cell Types from One Zygote

A Shared Genome Can Produce Distinct Cell Identities

Epigenetic Regulation Balances Memory with Resetting

Concept essentials