Why Science? | DNase Hi-C, Sequence-Independent Fragmentation and Genome-Contact Evidence

Section 1 of 36

1. Begin with the fragmentation question

DNase Hi-C asks how genome-contact maps change when chromatin is fragmented by DNase I rather than at a fixed set of restriction sites. The method still measures ligation products from crosslinked material, so it is a proximity assay rather than a direct photograph of chromosomes.

Contents · Next section

Section 2 of 36

2. Did you know random cutting is not the same as uniform cutting?

DNase I is sequence-independent in the sense that it does not require one recognition motif. Chromatin accessibility, protein protection, enzyme concentration and reaction time still influence where cuts occur. The useful claim is broader site coverage, not perfectly equal sampling.

Contents · Previous section · Next section

Section 3 of 36

3. Separate molecular resolution from biological certainty

Smaller and more widely distributed fragments can localise contacts more finely. That technical resolution does not prove that every narrow peak is a stable regulatory interaction. Read depth, background, fragment recovery and independent replication determine whether a fine feature is trustworthy.

Contents · Previous section · Next section

Section 4 of 36

4. Keep the 3C logic visible

Cells are crosslinked, chromatin is fragmented, nearby ends are ligated and chimeric products are sequenced. DNase Hi-C changes the fragmentation and junction-marking route, but the interpretation still depends on the core 3C assumption that crosslinked proximity influences ligation frequency.

Contents · Previous section · Next section

Section 5 of 36

5. Understand why heterogeneous ends matter

DNase I can generate blunt ends and different overhangs. End repair and dA-tailing convert that mixture into substrates for controlled adaptor ligation. Incomplete repair can bias which fragments survive, so the enzyme steps need measurable checkpoints rather than a recipe-only description.

Contents · Previous section · Next section

Section 6 of 36

6. Use internal bridge adaptors as junction logic

Biotinylated internal adaptors mark ends before proximity ligation. After the library is sheared, streptavidin enrichment selects molecules that contain the intended junction architecture. Adaptor sequence and orientation also help computational pipelines recognise valid products and reject incomplete constructions.

Contents · Previous section · Next section

Section 7 of 36

7. Plan comparison groups before cutting

Matched cell state, viability, fixation, input, DNase exposure and replicate number belong in the design memo. If one treatment changes accessibility or nuclear fragility, it may also change fragmentation. Measure those shifts before calling them changes in three-dimensional organisation.

Contents · Previous section · Next section

Section 8 of 36

8. Crosslink without freezing artefacts into the story

Formaldehyde stabilises protein-DNA and DNA-proximal neighbourhoods. Under-fixation can lose contacts; over-fixation can impair digestion and repair. Use matched concentration, time, temperature and quenching, and record nuclear morphology for every biological replicate.

Contents · Previous section · Next section

Section 9 of 36

9. Titrate DNase I instead of choosing one dose blindly

Run a controlled digestion series and examine fragment-size distributions, DNA recovery and nuclei integrity. A dose that works in one cell type may over-digest another. Choose a window that creates useful fragments while preserving enough material for repair, adaptor ligation and proximity ligation.

Contents · Previous section · Next section

Section 10 of 36

10. Preserve nuclei through fragmentation

In situ workflows aim to keep chromatin inside intact nuclei so ligation favours genuinely proximal fragments. Free DNA and broken nuclei increase random collisions. Microscopy, supernatant DNA, cis/trans balance and distance-decay curves help reveal damage that raw library yield can hide.

Contents · Previous section · Next section

Section 11 of 36

11. Repair mixed DNA ends reproducibly

Polymerase-based end repair converts heterogeneous DNase products into compatible ends, followed by dA-tailing for adaptor ligation. Record enzyme lots, incubation and cleanup. A condition-specific end-repair failure can alter fragment recovery before any spatial biology enters the analysis.

Contents · Previous section · Next section

Section 12 of 36

12. Ligate the internal adaptor before proximity partners

Adaptor ligation marks fragment ends with a known sequence and biotin. Remove free adaptor carefully because carry-over can consume ligase, create short products and dominate enrichment. Quantify adaptor-containing molecules and invalid orientations before advancing the library.

Contents · Previous section · Next section

Section 13 of 36

13. Join proximal fragments in intact nuclei

Proximity ligation connects adaptor-marked ends that remain near one another after crosslinking and digestion. DNA concentration, nuclear integrity and ligase access affect background. Negative controls and junction-class accounting help distinguish successful in-nucleus ligation from random solution products.

Contents · Previous section · Next section

Section 14 of 36

14. Purify, shear and enrich the intended junctions

After crosslinks are reversed, DNA is purified and sheared for sequencing-library construction. Streptavidin beads enrich biotin-bearing junction molecules. Report recovery, insert-size distribution, bead carry-over and unique complexity so enrichment is not confused with repeated sequencing of a few products.

Contents · Previous section · Next section

Section 15 of 36

15. Practise with an invented DNase Hi-C table

These fictional values teach evidence review; they are not performance benchmarks.

Contents · Previous section · Next section

Section 16 of 36

16. Trim adaptors before mapping fragments

The bridge-adaptor sequence is experimental evidence as well as technical sequence. Identify it with declared mismatch rules, split the read correctly and preserve orientation. Over-aggressive trimming can shorten genomic tags until repeats and low-complexity regions become ambiguous.

Contents · Previous section · Next section

Section 17 of 36

17. Map with fragment length and mappability in mind

Short DNase fragments can improve localisation yet reduce unique mapping. State reference genome, aligner, minimum tag length, mapping-quality threshold and blacklist policy. Report multimappers instead of silently converting hard regions into blank space.

Contents · Previous section · Next section

Section 18 of 36

18. Classify every junction before filtering

Count valid pairs, dangling ends, self-circles, unligated adaptors, same-fragment products, duplicates and interchromosomal pairs. A transparent funnel explains how raw reads became a contact map and exposes which technical failure consumed the library.

Contents · Previous section · Next section

Section 19 of 36

19. Remove duplicates without erasing real depth

Identical coordinates may be amplification duplicates, especially in a shallow-complexity library. At very high depth, independent molecules can also collide on the same ends. Declare the duplicate rule, show library saturation and confirm conclusions across reasonable alternatives.

Contents · Previous section · Next section

Section 20 of 36

20. Build distance-aware contact expectations

Nearby loci meet more often than distant loci even without a special loop. Normalisation should consider genomic distance, fragment recovery, mappability and coverage. Report effect sizes and uncertainty rather than presenting a thresholded matrix as a literal structure.

Contents · Previous section · Next section

Section 21 of 36

21. Use targeted capture for a different question

Hybridisation baits can enrich selected regions from a whole-genome DNase Hi-C library. Targeted DNase Hi-C offers deep local coverage but no longer samples every locus equally. Probe design, capture balance and off-target reads become additional sources of evidence and bias.

Contents · Previous section · Next section

Section 22 of 36

22. Challenge the phrase sequence-independent

DNase I does not require one restriction motif, but sequence composition and chromatin state can still influence cutting and recovery. Show cut-site distributions, accessibility relationships and fragment-size sensitivity before attributing improved coverage entirely to the enzyme choice.

Contents · Previous section · Next section

Section 23 of 36

23. Challenge accessibility-driven fragmentation

Open regulatory regions may be easier for DNase I to cut than compact chromatin. A treatment that changes accessibility can therefore change contact coverage. Compare marginal fragment coverage with normalised contact enrichment and, where possible, use an independent accessibility assay.

Contents · Previous section · Next section

Section 24 of 36

24. Challenge random ligation

Broken nuclei and free fragments can meet in solution and create convincing long-range or trans pairs. Inspect unexpected junction orientations, cis/trans balance, species-mixing controls where appropriate and replicate recurrence. Extraordinary interchromosomal contacts need especially strong validation.

Contents · Previous section · Next section

Section 25 of 36

25. Challenge copy-number and structural variation

Amplifications, deletions and rearrangements change both contact opportunity and mapping. Use a sample-aware reference and measure copy number in transformed or tumour-derived cells. A dense block may reflect extra DNA or a breakpoint rather than a newly formed regulatory domain.

Contents · Previous section · Next section

Section 26 of 36

26. Report comparisons at the biological-replicate level

Do not pool first and ask questions later. Show valid pairs, distance curves, coverage and headline contacts for every independent preparation. Matched downsampling helps separate biological differences from unequal sequencing depth and library complexity.

Contents · Previous section · Next section

Section 27 of 36

27. Validate the decisive contact through another route

Use conventional Hi-C, Capture-C, imaging, perturbation or expression measurement according to the claim. Agreement across fragmentation strategies is particularly valuable because it challenges DNase-specific cutting and repair biases.

Contents · Previous section · Next section

Section 28 of 36

28. Learn DNase Hi-C with paper strips

Cut identical printed sequences at fixed symbols for restriction digestion, then cut another set at randomly selected spaces for DNase fragmentation. Comparing coverage makes the design advantage visible while also showing why random does not guarantee uniform.

Contents · Previous section · Next section

Section 29 of 36

29. Build Primary Science process skills

Students can keep strip length constant, vary the cutting rule and count recovered regions. The safe model practises observation, classification, controlled variables and repeated trials without attempting any laboratory enzyme work.

Contents · Previous section · Next section

Section 30 of 36

30. Prepare for PSLE Science reasoning

An invented quality table can ask which library was over-digested, why nuclei integrity matters and which conclusion needs a control. The specialist name is enrichment; fair comparison and claim-evidence matching are the transferable skills.

Contents · Previous section · Next section

Section 31 of 36

31. Extend into Secondary and O-Level Science

DNase Hi-C connects enzymes, DNA structure, polymer chemistry, diffusion, probability, graphs and experimental controls. Students can trace one contact from biological question to fragment, junction, sequence read, matrix and bounded explanation.

Contents · Previous section · Next section

Section 32 of 36

32. Use the topic for school choices

Check current official school pages for practical science, computing, research and data-analysis opportunities. Ask how students learn to evaluate controls and uncertainty. One modern instrument, project or programme does not guarantee individual fit or admission.

Contents · Previous section · Next section

Section 33 of 36

33. See the career ecosystem without promises

Projects may involve molecular biology, sequencing, bioinformatics, statistics, software engineering and research governance. Roles and qualifications vary by organisation. Reliable maps come from teams that can question both wet-lab chemistry and computational assumptions.

Contents · Previous section · Next section

Section 34 of 36

34. Use questions for science tuition and enrichment

Ask why end repair is required, how DNase fragmentation differs from restriction digestion and which metrics reveal nuclear damage. These questions turn advanced genomics into evidence-reading practice for Primary Science, PSLE Science, Secondary Science and O-Level Science.

Contents · Previous section · Next section

Section 35 of 36

35. Did you know better coverage can expose more uncertainty?

Recovering fragments between restriction sites can reveal contacts that older maps could not resolve. It can also reveal how strongly conclusions depend on background models and mapping. Scientific progress often creates better questions before it creates simpler answers.

Contents · Previous section · Next section

Section 36 of 36

36. Conclude with a DNase Hi-C evidence checklist

Ask whether fixation and nuclei integrity were stable; DNase digestion was titrated; ends and adaptors were repaired and ligated correctly; junction classes were transparent; mapping and distance bias were modelled; replicates agreed; and decisive contacts survived an orthogonal challenge.

A publication-ready design memo should place fragmentation and comparison on the same page. Name the cell population, treatment, fixation, DNase titration, desired fragment range, internal adaptor, ligation route, biological replicates and primary map features. Predeclare minimum unique pairs, acceptable cis/trans balance and exclusion rules. If targeted capture will follow, separate the whole-genome quality criteria from bait-level performance so a weak base library is not disguised by deep local enrichment.

Quality control should travel with the sample. Record cell viability, nuclei appearance, DNase lot and units, digestion time, fragment distribution, recovered DNA, end-repair yield, adaptor ligation, free-adaptor removal, proximity ligation, enrichment and amplification. Randomise comparison groups across operators, plates and sequencing lanes. A treatment that alters chromatin accessibility or nuclear size should be measured independently because it can change cutting and ligation even when the underlying spatial relationship is stable.

The raw evidence funnel belongs beside the final map. Show reads entering adaptor recognition, valid split tags, uniquely mapped ends, junction classes, duplicates, cis/trans proportions and distance curves for each replicate. Preserve the coordinates and orientations that let another analyst rebuild pairs. Downsample libraries to matched unique-pair counts and examine saturation. More reads do not add evidence once sequencing repeatedly returns the same molecules.

Normalisation should acknowledge that DNase I changes fragment opportunities. Marginal coverage, fragment length, mappability, accessibility and genomic distance all influence observed pairs. Present raw counts, expected background, enrichment, uncertainty and multiple-testing control. Repeat headline analyses after matching coverage and changing bin size. A contact that survives several defensible views is stronger than a peak that appears only after one smoothing or threshold choice.

Figures should make chemistry visible before biology. Include nuclei, digestion titration, fragment profiles, adaptor and ligation checks, unique-complexity curves, replicate agreement, distance decay and cis/trans balance. Locus panels should place raw coverage below normalised contacts and use the same scale across conditions. A targeted view should declare its bait and capture efficiency. Label invented teaching data and exploratory examples plainly.

Stewardship includes consent, specimen provenance, fixation and enzyme lots, adaptor sequences, plate maps, raw reads, genome build, pair files, blacklist versions, normalisation, code, software environments and figure recipes. Human contact libraries contain sequence information and may reveal structural variants, requiring appropriate access. Safe practice covers biological material, fixatives, nucleases, polymerases, ligases, magnets, heat and amplified DNA under trained laboratory procedures.

The most useful next experiment addresses the dominant failure mode. Retitrate DNase when fragments are too short, improve nuclei handling when trans background rises, repeat end repair when adaptor recovery is uneven, add specimens when one library drives the result, or use a restriction-based contact assay when cutting bias remains plausible. Imaging tests spatial distance; perturbation plus expression tests regulation. Each follow-up should say what outcome would support or weaken the model.

Parents and students can use a three-column reading routine: what was physically done, what was counted and what was inferred. DNase Hi-C fragments and ligates chromatin, counts valid mapped junctions and infers enriched proximity. That separation keeps the method exciting without turning a contact map into a photograph. It also transfers directly to school practicals, where an instrument reading and the explanation for it are different kinds of statement.

One final safeguard is a decision table written before interpretation. For each proposed contact change, list the expected signature if three-dimensional organisation changed, the expected signature if DNase accessibility changed, and the observation that would leave the result unresolved. Pair that table with a matched input or orthogonal accessibility measurement, not as decoration but as part of the causal test. This habit shows learners that good science is not a hunt for the most colourful map. It is a disciplined comparison among explanations, with enough controls to let evidence rule some of them out.

A short preregistered analysis note can name the comparison, map resolution, exclusion rules and decision threshold before anyone inspects the final locus. That modest step reduces selective reporting and makes later revisions easy to explain.

Contents · Previous section · Continue to the Science Learning Hub