Lesson 3 of 5 / Genomes and gene regulation
Make a gene accessible
Why can two cells with the same DNA transcribe different genes?
In this lesson: Explain chromatin-level regulation through histone modification and DNA methylation.
About 6 min
The key ideaChromatin state changes access to DNA, allowing stable but potentially reversible differences in gene expression.
Explore the idea
Change access without changing the sequence
Compact chromatin can restrict factor access. DNA methylation at relevant regulatory sites is commonly associated with repression, including recruitment of proteins that favour a less accessible state.
Accessibility is a continuum, not a universal binary switch. Histone methylation can activate or repress depending on its site and context. The bead spacing is an explanatory model, not a measured chromatin fibre.
Explanation
Wrapping DNA around histones helps package it, but also changes access for transcription machinery. More open chromatin generally permits transcription-factor binding more readily than highly condensed chromatin. Packaging therefore participates in regulation rather than merely solving a storage problem.
Histone acetylation often reduces attraction between positively charged histone regions and DNA, favouring a more accessible chromatin state. Removal of acetyl groups often promotes a less accessible state. Other histone modifications can activate or repress depending on the residue and context, so histone methylation is not universally an on or off switch.
DNA methylation at relevant regulatory regions is commonly associated with reduced transcription, directly or through recruitment of proteins that promote repression. These changes alter how the sequence is used without necessarily changing the nucleotide sequence itself.
Differential expression can be spatial, with genes active in one tissue but not another, or temporal, changing during development or in response to signals. Chromatin provides one regulatory level; active transcription factors and downstream RNA or protein controls still matter. An open gene is not guaranteed to be expressed if required activators are absent.
Step by step
- 1
Separate sequence from state
The DNA bases may remain unchanged.
- 2
Explain accessibility
Connect chromatin arrangement to machinery reaching DNA.
- 3
Avoid absolute claims
Regulation depends on modification type and location.
Worked example
Work through the evidence
Two tissues share a gene sequence. The promoter is heavily methylated in one and its mRNA level is lower. What model is consistent?
One way to explain it
Methylation-associated repression reduces transcription in that tissue, contributing to differential expression. Correlation alone does not prove methylation is the sole cause.
Why this answer works
- Use the observed RNA level as the expression measure.
- Other regulatory differences may coexist.
Is this true? "An epigenetic change must replace one DNA base with another."
Chromatin and methylation changes can regulate expression without changing the base sequence.