Using molecular dynamics simulations, we show here that growing plectonemes 
resulting from transcription-induced supercoiling have the ability to actively 
push cohesin rings along chromatin fibres. The pushing direction is such that 
within each topologically associating domain (TAD) cohesin rings forming 
handcuffs move from the source of supercoiling, constituted by RNA polymerase 
with associated DNA topoisomerase TOP1, towards borders of TADs, where 
supercoiling is released by topoisomerase TOPIIB. Cohesin handcuffs are pushed 
by continuous flux of supercoiling that is generated by transcription and is 
then progressively released by action of TOPIIB located at TADs borders. Our 
model explains what can be the driving force of chromatin loop extrusion and how 
it can be ensured that loops grow quickly and in a good direction. In addition, 
the supercoiling-driven loop extrusion mechanism is consistent with earlier 
explanations proposing why TADs flanked by convergent CTCF binding sites form 
more stable chromatin loops than TADs flanked by divergent CTCF binding sites. 
We discuss the role of supercoiling in stimulating enhancer promoter contacts 
and propose that transcription of eRNA sends the first wave of supercoiling that 
can activate mRNA transcription in a given TAD.