Crossing Over Question

[John Carroll]

After checking assorted books on biology and biochemistry about
how crossing over takes place during meiosis, I find myself still
confused on a couple of points that an expert could undoubtedly 
clear up without breaking a sweat.  Here are the points of confusion:

	1. In the diagrams of crossing over that I've seen, one segment
	   of DNA on the paternal chromosome is traded for its 
	   counterpart on the maternal chromosome.  I assume that
	   this actually takes place at many points along the 	   
	   chromosomes, rather than just the one shown in diagrams,
	   but I haven't seen any text to confirm that suspicion or 
	   any diagrams showing multiple crossovers.  Does crossing
	   over actually occur at many separate places on the 
	   chromosome?  If so, roughly how many segments are likely,
	   on average,to be exchanged per chromosome?  And roughly how
           big are such segments, on average? 

	2. When DNA molecules are cut during the crossing over process,
	   are they cut only on linker strands (my guess) or only on
	   DNA attached to nucleosomes, or on both kinds of locations?

The world's fate doesn't hang on answers to these questions, as you
may have guessed, but they're driving me nuts.  All responses
appreciated.  Thanks in advance.


John Carroll
pp001356 at mindspring.com