Methylation, Histone Marks and Heritable Expression States
Chemical marks on DNA and on histone tails are copied when a cell divides, so a pattern of gene activity can be passed to daughter cells without any change in sequence. The marks are readable, writable and erasable, which is what makes them a control system.
What a learner can do afterwards
- Explains how a methylation pattern survives replication.
- Distinguishes an epigenetic change from a mutation by what is altered and what is not.
- Gives one case where an environmental input leaves a lasting mark on expression.
1 · Read
Epigenetic marks gate access to DNA without changing its letters. Methylation of cytosine, usually at CG sites, recruits binders that compact the region, while acetyl on histone tails loosens wrapping and favors firing. A mutation rewrites a letter permanently, but an epimutation only changes how the same letters get read, which is why plain sequencing spots one and stays blind to the other.
Methylation patterns survive division through maintenance copying. After replication each daughter duplex is half marked, with the old strand marked and the new one bare. A maintenance enzyme recognizes exactly that half marked state and marks the new strand to match, so daughters inherit the same settings with no fresh instruction.
That copying is why a passing signal can echo for generations of cells. A diet, stress, or toxin signal rewrites marks in a progenitor, and maintenance copying preserves them in descendants long after the signal fades. Coat color studies in mice and imprinting disorders show the same lesson: transient input, lasting expression state.
To separate the two kinds of change, ask what changed: letters mean mutation, while access marks with intact letters mean epigenetics.
Marks gate access, maintenance copying inherits them, and the sequence underneath never changes.
2 · Watch
Take it off screen
Where it sits
This opens up
Nothing builds on it yet.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.