9477 / 2027

Lesson 3 of 5 / Genomes and gene regulation

Make a gene accessible

Why can two cells with the same DNA transcribe different genes?

I can explain chromatin-level regulation through histone modification and DNA methylation.

  • I can explain eukaryotic regulation from chromatin to protein modification and degradation.Syllabus

    Syllabus 9477, Core 2(i). Explain eukaryotic regulation from chromatin to protein modification and degradation.

About 6 min

Make a guess. You are not marked.

A liver cell and a nerve cell from one person have identical DNA. Why do they make different proteins?

  1. Different genes are switched on in each.
  2. Mutations have changed their DNA differently.
  3. Each cell has lost the genes it does not need.
Show the answer

Different genes are switched on in each.

Chromatin state and transcription factors differ between cells, so different genes are transcribed.

The key idea

Chromatin state changes access to DNA, allowing stable but potentially reversible differences in gene expression.

Explore the idea

Change access without changing the sequence

More restricted factor accessSame DNA 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. 1

    Separate sequence from state

    The DNA bases may remain unchanged.

  2. 2

    Explain accessibility

    Connect chromatin arrangement to machinery reaching DNA.

  3. 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.

Watch out for this

A student says: "An epigenetic change must replace one DNA base with another." What is wrong with this?

Show the answer

Chromatin and methylation changes can regulate expression without changing the base sequence.

Check your understanding

Which interpretation of histone acetylation is appropriate?

  1. It generally tightens DNA-histone association and reduces access.
  2. It often increases chromatin accessibility.
  3. It adds methyl groups directly to promoter DNA.
Show the answer

It often increases chromatin accessibility.

This can facilitate transcription when other required factors are present.

Exam-style questions

Check your understanding

Which change would NOT usually make a gene more accessible for transcription?

  1. Removal of a repressor bound to a silencer
  2. Adding methyl groups to its promoter DNA
  3. Adding acetyl groups to nearby histones
  4. Loosening of nucleosome packing near the gene
Show the answer

Adding methyl groups to its promoter DNA

Promoter methylation is usually linked to reduced transcription. Acetylation and chromatin opening increase access.

Quick recall

Card 1 of 3. Answer in your head, then check.

Why can two cells with the same DNA transcribe different genes?

You can return to this lesson any time.