Each cell in our body contains two metres of DNA. All this DNA is packaged into a nucleus that is 6 micrometres across (0.006mm). The DNA is wrapped up around histone proteins, which are then further organised into larger structures.
Despite all this packaging, the interior of a nucleus is rather ordered. This is because when the cell divides the chromosomes cannot be intertwined and tangled up, otherwise they would break apart during division. Humans have 46 chromosomes in all cells except gametes. Each chromosome is found in a particular location or territory of the nucleus.
Each chromosome is made up of DNA which contains genes. Some of these genes will be expressed and some of them will be switched off. For instance the cells in your stomach need to express the gene that makes stomach acid, but probably don’t need the one that makes your eyes blue. Genes that are switched on tend to associate together whilst those that are switched off tend to hang out near the periphery of the nucleus.
Sometimes the section of DNA that recruits transcription machinery to a gene is quite far away in the DNA sequence. In order for the machinery to transcribe the gene, the two distant pieces of DNA need to be brought together. proteins called cohesins can bring the two pieces of DNA together by forming a loop. This can promote long range interactions.
This illustration was created for a talk given by Wendy Bickmore about the structure of DNA within the nucleus, and how long range interactions influence gene expression.