Genome structure / chromatin organization · BEGINNER LESSON
Loop anchor
A loop anchor is one DNA locus at an end of a chromatin loop, usually paired with another locus at the other end. In Hi-C and similar 3D genome maps, loop anchors are the positions that make a loop appear as a focal dot of frequent contact.
What it is
A loop anchor is one of the two DNA positions that define a chromatin loop. A chromatin loop is a physical contact between two distant parts of DNA that folds them close together in the cell nucleus. In interaction maps made from chromosome conformation capture methods such as Hi-C, these loop contacts often show up as small, focal enrichments, and the anchors are the paired loci at the ends of that contact. Sources describe loop anchors as often containing pairs of CTCF binding sites in a convergent orientation, meaning the CTCF proteins point toward each other at the two ends.
[1] UnionLoops: a workflow for calling chromatin loops across related Hi-C datasets with improved specificity, precision, and sensitivity [2] UnionLoops: a workflow for calling chromatin loops across related ... [3] TF-loop: deciphering the transcription factor regulatory ... - PMC
How loop anchors arise
The main mechanism described in these sources is loop extrusion by the cohesin complex. Cohesin can bind DNA and move along it, enlarging a loop until it meets boundary elements such as CTCF binding sites, where its movement is blocked or paused. That stalled position helps define the loop anchors. Some loops also occur at active promoter-enhancer interactions, where gene regulatory regions are brought together; these can affect local gene expression.
[1] UnionLoops: a workflow for calling chromatin loops across related Hi-C datasets with improved specificity, precision, and sensitivity [3] TF-loop: deciphering the transcription factor regulatory ... - PMC
How scientists detect them
Loop anchors are inferred from data rather than seen directly. Hi-C, Micro-C, ChIA-PET, and PLAC-seq measure how often pairs of DNA loci contact each other. In the resulting interaction matrix, a loop appears as a localized dot or spot. The two genomic positions that generate that dot are the loop anchors. Methods that compare related samples, such as UnionLoops, try to use repeated evidence across datasets to reduce sample-specific false positives and improve the positional precision of anchors.
[1] UnionLoops: a workflow for calling chromatin loops across related Hi-C datasets with improved specificity, precision, and sensitivity [2] UnionLoops: a workflow for calling chromatin loops across related ...
Limitations and caveats
Not every loop has the same anchor features. Many loop anchors contain convergent CTCF sites, but some loops do not use canonical CTCF anchors. Also, loop-calling can vary between samples and between computational methods, so an inferred anchor is a model-based result that depends on sequencing depth, data quality, and the algorithm used. The sources emphasize that single-sample callers can miss or misplace loops, which is why multi-sample workflows can improve specificity and consistency.
[1] UnionLoops: a workflow for calling chromatin loops across related Hi-C datasets with improved specificity, precision, and sensitivity [2] UnionLoops: a workflow for calling chromatin loops across related ... [3] TF-loop: deciphering the transcription factor regulatory ... - PMC
SEE THE IDEA
How a loop anchor is identified
Two distant positions on the genome can contact each other in 3D space.
Cohesin moves along DNA and helps enlarge a loop.
CTCF or other boundaries can stop or stall cohesin.
Hi-C or related data show a focal enrichment at the contact.
The two loci at the ends of that dot are the anchors.
cohesin-driven looping creates a focal contact signal, and the two end positions of that signal are called loop anchors
The anchor is not the whole loop; it is one end of a loop detected as a dot in 3D genome contact data.
TRY IT YOURSELF
Test your understanding
Choose an answer, then check the explanation.
PUT IT TOGETHER
Match each concept to its meaning
Drag a label onto its description, or select a label and then select a description. This also works with a keyboard.
4 pairs to match
Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.