Methylation, histone marks and heritable expression states
DNA methylation and some chromatin states can help maintain patterns of gene activity through cell divisions without changing the DNA base sequence. Maintenance depends on cellular machinery, and marks can also be lost or removed.
What a learner can do afterwards
- Explains how maintenance methylation can restore a CpG pattern after replication and how histone-state maintenance differs.
- Distinguishes an epigenetic change from a mutation by what is altered and what is not.
- Interprets a hypothetical environmental signal followed by a persistent expression change, distinguishing a possible mechanism from proof of cause.
1 · Read
A heritable epigenetic expression state persists through cell division without requiring a change in the DNA base sequence. A mutation changes that sequence by replacing, adding or removing bases. Both kinds of change can occur together. Marks can be added or removed, and not every mark persists. You have studied small RNAs acting on mRNA; here we examine DNA and the proteins that package it.
DNA wraps around histone proteins to form chromatin. In mammals, CpG means a C followed by G along one DNA strand. Promoter CpG methylation often supports repressor recruitment and lower transcription. Histone acetylation often favors accessible chromatin and transcription. Histone methylation can favor activity or repression, depending on the modified site. A mark's effect depends on context.
At a CpG site methylated on both strands, replication leaves each daughter molecule with one methylated old strand and one unmethylated new strand. The maintenance enzyme DNMT1 helps methylate the new strand to match. Histone states are maintained differently: parental histones are reused on daughter DNA, and enzymes modify newly added histones. Maintenance can be incomplete, and marks can be removed. Persistence is not guaranteed.
A report giving only the DNA base sequence does not measure DNA methylation or histone marks. Those features need suitable measurements of their own. Some sequencing methods can detect modified bases, so do not assume that every sequencing method is blind to methylation. Finding no sequence change does not rule out epigenetic regulation.
In a hypothetical culture, a brief nutrient signal is followed by increased promoter methylation and reduced transcription. Both remain through several divisions after the signal ends. Maintenance methylation could help preserve the pattern. This does not prove that methylation caused low transcription. Compare untreated cells and test whether changing the marks changes transcription. Persistence in daughter cells does not prove inheritance by an organism's offspring.
The last two cells describe observations after division. Their association does not establish that the marks caused low transcription.
Transcription factors bind regulatory DNA and affect transcription. They can also recruit enzymes that modify chromatin. Low promoter accessibility can limit activator binding even when the activator is abundant. Methylation alone does not prove that access is blocked. Cells with matching DNA sequences can differ in expression for several regulatory reasons; sequence identity alone does not identify the cause.
Cells can maintain expression states through division without changing the DNA sequence. DNA methylation and histone states use different maintenance processes; neither always persists.
Lesson, questions and flashcards written for this dataset. Textbook records were consulted for factual background; their prose and figures are not reproduced.
Readings and flashcards written for this dataset. Linked video: DNA and chromatin regulation, credited to Khan Academy in the source catalog; video terms are unverified.
More to read
Sequence, packaging and gene activity
A DNA sequence records the order of bases. Chromatin describes DNA packaged with proteins, including histones. Changes in packaging and chemical modifications can affect gene regulation without replacing, adding or removing DNA bases.
In mammalian cells, methylation at promoter CpG sites often supports repression. Histone acetylation commonly favors DNA accessibility and transcription. Histone methylation has different effects at different sites. These associations do not mean that one mark fixes the output of every gene.
Transcription factors bind regulatory DNA and influence transcription. They can recruit enzymes that change chromatin, while chromatin accessibility can affect factor binding. Regulation is therefore not a fixed sequence in which marks decide eligibility and factors act only afterward.
A mutation changes the DNA base sequence. Epigenetic regulation can alter expression without such a change, and an expression state may persist through cell division. A mutation and an epigenetic change can also occur together.
Check what a measurement actually reports. A base-sequence-only report does not describe histone marks or DNA methylation. Other methods can measure those features, and some sequencing methods detect modified bases. The absence of a mutation does not establish the absence of epigenetic regulation.
Two ways cells maintain chromatin information
Start with a CpG site methylated on both DNA strands. Replication gives each daughter DNA molecule an old strand and a new strand. The old strand retains its mark, while the corresponding site on the new strand initially lacks one.
Maintenance methylation can restore methylation on both strands. DNMT1 helps add methyl groups at corresponding sites on the newly made strand. The old mark is not moved to the new strand, and the DNA base order does not need to change.
Histone-state maintenance involves different material. Histones are proteins, not DNA strands. Parental histones are reused on daughter DNA, and enzymes modify newly added histones. This is not direct copying of histone marks from one DNA strand to another.
These processes can help preserve a chromatin state, but maintenance is not perfect or inevitable. Marks can be lost, removed or changed. The presence of a mark at one time does not show that it will remain through every later division.
Following a state through a lineage of dividing cells tests cellular inheritance. It does not by itself show that the state passes through reproduction to an organism's offspring.
What a lasting response would show
Consider a hypothetical cell-culture experiment. A brief nutrient signal is followed by increased methylation at a promoter and reduced transcription of its gene. The signal is removed. After several cell divisions, both differences remain. This example describes an imagined experiment, not published results.
Maintenance methylation could explain why the promoter pattern persists. The observations do not prove that methylation caused the reduction in transcription. The nutrient signal might also have changed other regulators.
A comparison culture without the signal helps distinguish a response to the treatment from changes that occur in both cultures. Altering the promoter marks and measuring transcription can help test their causal role. Such a test also needs controls for other effects of the intervention.
Measure the proposed steps separately. Methylation, promoter accessibility, activator binding and transcription are different features. An abundant activator need not bind well to a poorly accessible promoter. Conversely, methylation alone does not establish that access is blocked.
If a response lasts through cell divisions, the result concerns that cell lineage. It does not demonstrate inheritance by an organism's offspring or establish that every environmental signal leaves a lasting epigenetic state.
Explore more
2 · Watch
Take it off screen
Where it sits
Learn first
Optional connections
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.