Re: Can crossing over ever occur between the chromatids of one chromosome? If s

Hi Eva,

This can and does happen, at least in some species.  The “chromatids of one chromosome” are 
referred to as “sister chromatids”.  Crossing over, or recombination, between sister chromatids is 
known as “sister chromatid exchange”.  As you indicate, these chromatids are identical copies, the 
result of DNA replication.  Since they are identical, you are correct in stating that it would not 
produce any new genetic variation or combinations.  There is a brief explanation of the process of 
sister chromatid exchange and how it is detected here: http://www.bookrags.com/research/sister-chromatid-exchange-wog/

Why does it happen?  It plays a role in a process known as gap repair.  This is when a cell uses one 
chromatid as a template to repair damage to the other chromatid.

During meiosis, when there is a lot of recombination going on between non-sister chromatids (i.e. 
the chromatids of homologous chromosomes), sister chromatid exchange is also taking place.  
Why?  Perhaps because the cell would need to a special mechanism in place to prevent it from 
happening.  While the recombination molecular machinery is doing its work, there is probably no 
way for the cell to easily distinguish between sister and non-sister chromatids.  Since sister 
chromatid exchange does not have any net effect, why should the cell have a special mechanism in 
place to prevent it? Since it is apparently inconsequential, and has not been directly demonstrated 
in many organisms, it is not typically portrayed in diagrams of meiosis.

Alex Brands