Why does heterochromatin stop transcription in IB Biology terms?

Heterochromatin is tightly packed chromatin, meaning the DNA is physically less accessible. In IB Biology markschemes, “RNA polymerase cannot access the promoter” is a high-value phrase because it links structure to mechanism. Transcription factors also struggle to bind their response elements when nucleosomes and higher-order folding hide the DNA. This matters because initiation is usually the key control point, so blocking initiation effectively silences the gene. It’s also why two cells with the same genome can behave differently: they keep different regions in euchromatin vs heterochromatin. When you answer, always connect packing to reduced binding and reduced initiation.

Does DNA methylation always silence genes?

In most IB Biology contexts, DNA methylation near promoters is described as reducing transcription, and that is the safe exam answer. Methyl groups can reduce transcription factor binding and recruit proteins that condense chromatin. The key is to mention both: direct interference at the promoter and indirect tightening via chromatin remodeling. It’s also an example of epigenetic regulation because the base sequence isn’t changed. Many questions want you to distinguish “mutation” from “epigenetic tag,” so say explicitly that methylation affects expression without altering the nucleotide order. If you want a clean, syllabus-matched explanation, revise D2.2.6 Methylation of the promoter and histones in nucleosomes.

Why must DNA unwind before transcription can begin?

RNA polymerase reads only one DNA strand as the template, so the bases must be exposed. In IB Biology, the expected logic is: closed double helix leads to bases hidden leads to polymerase can’t base-pair RNA nucleotides. Unwinding creates a transcription bubble where complementary RNA nucleotides can align with the template strand. Mention hydrogen bonding if the question hints at “A/T vs G/C” or “energy required.” This also links back to promoter regions, because initiation proteins help open the helix at the start site. A strong answer connects the physical opening to the chemical base pairing that builds mRNA.

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IB Biology: DNA Structure and Transcription Control

IB Biology

RevisionDojo Team

•Last updated: 19 Jul 2026•6 min read

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IB Biology: DNA Structure and Transcription Control

You can memorize every definition in IB Biology and still miss a mark because you answered the wrong question.

The examiner often isn’t asking, “What is transcription?” They’re asking something quieter: Why can transcription happen here, but not there? In other words, what about DNA structure makes a gene readable or effectively invisible?

In IB Biology, this is one of the cleanest “structure influences function” stories you’ll ever learn: the DNA code matters, but access to the code matters first.

IB Biology quick checklist: what structural features control transcription?

Use this as a fast exam scan. If the question says “influence,” mention at least three:

Chromatin state: euchromatin vs heterochromatin (accessibility)
Nucleosomes: positioning and remodeling around promoters
Epigenetic tags: DNA methylation and histone modifications
Promoter structure: sequence motifs and “strength”
Enhancers + DNA looping: 3D folding brings regulators together
Ease of unwinding: A/T rich regions open more easily than G/C rich ones

To ground the basics, revise transcription itself in 2.7 DNA replication, transcription and translation Notes.

Chromatin packing: the “door” to transcription

In eukaryotes, DNA isn’t left loose. It’s wrapped around histones, forming nucleosomes, and then folded into chromatin. This packaging isn’t just storage; it’s a gate.

Euchromatin is loosely packed, so RNA polymerase and transcription factors can physically reach the DNA.
Heterochromatin is condensed, so binding sites are blocked and transcription is unlikely.

If you need a crisp phrasing for essays, see How Chromatin Packing Controls Gene Expression and How Nucleosomes Control DNA Accessibility.

Nucleosomes and epigenetic tags: small changes, big consequences

A gene can have a perfectly good promoter and still be silent if nucleosomes sit in the wrong place.

Cells shift nucleosomes using remodeling complexes, and they “label” chromatin using epigenetic modifications:

Histone acetylation reduces histone to DNA attraction (more open chromatin, more transcription).
DNA methylation (often at CpG islands near promoters) typically reduces transcription by tightening chromatin and interfering with binding.

For the IB wording and examples, revise D2.2.6 Methylation of the promoter and histones in nucleosomes and the histone wrap itself in A1.2.13 Structure of a nucleosome (HL only).

Promoters, enhancers, and looping: structure as a meeting invitation

In IB Biology, treat the promoter as the start line. It’s a DNA region where RNA polymerase (plus transcription factors) assembles to begin transcription. Promoters vary in “strength” because their base sequence affects how easily proteins bind.

Enhancers add a structural twist: they can be far away on the DNA, but DNA looping brings enhancer-bound activators into contact with the promoter. This 3D folding is why distance on a linear diagram can be misleading.

Go deeper with How Promoters Control Transcription Start and IB Biology: How Activators & Repressors Control Genes. If you want the syllabus-aligned hub for HL gene expression, use D2.2 Gene expression (HL) Notes.

Unwinding the double helix: sequence affects “openness”

Even with a free promoter, transcription can’t start until the helix unwinds enough to expose the template strand.

Here’s the structural detail examiners love: A/T pairs have 2 hydrogen bonds, while G/C pairs have 3, so A/T rich regions separate more easily. That makes local DNA opening (during initiation) energetically easier.

For DNA structure recap, use 2.6 Structure of DNA and RNA.

Bring it home: use structure to predict transcription

If you can look at a scenario and say, “This promoter is exposed in euchromatin, nucleosomes are shifted, acetylation is high, methylation is low, and looping brings enhancers close,” you’re thinking like an IB Biology examiner.

To train that skill quickly, RevisionDojo is built for it: reinforce concepts with the D2.2 Gene expression (HL) - IB Questionbank, tighten definitions with Flashcards for D2.2 Gene expression (HL), and use Study Notes, AI Chat, Grading tools, Predicted Papers, Mock Exams, and Tutors when you want feedback that actually mirrors exam pressure.

In IB Biology, DNA isn’t just a sequence. It’s architecture. And transcription happens where the architecture lets it happen.