How does chromosomal crossing over occur in tightly wound DNA?

[Yuqing]

In most of the books I've read, chromosomal cross over has been described as simply "segments of the chromosome breaking off and reattaching", but I have a bit of difficulty with this explanation. Chromosomes are densely packaged DNA that have been wound and wound again. How can DNA which has been so tightly wound and packaged be cut and rejoined so simply? Most of the searches of the recombination process shows recombination occurring between two simple strands of DNA which should not be accessible when DNA is packaged as chromosomes. Does the chromosome uncondense at the part crossing over in order to recombine?

 

[Kalirren]

The short answer is yes, the chromatin DNA must unwind from its nucleosome-bound state in order to engage in homologous recombination.