The Holiday model was the first model to describe meiotic recombination. It is quite simple, but does not describe all observed results and so has been mostly discarded in favour of newer models. However, it does serve as a good introduction to recombination.
  1. The two chromosomes are next to each other
    
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    Chromosome 1
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    
    bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
    Chromosome 2
    bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
    
    
    (Note that each chromosome is a double helix)
  2. One strand from each chromosome is nicked and transfers to the other chromosome
    
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    Chromosome 1
    aaaaaaaaaaaaaaaa   aaaaaaaaaaaaaaaaaaaaaaaaaaa
                    \ /
                     X
                    / \
    bbbbbbbbbbbbbbbb   bbbbbbbbbbbbbbbbbbbbbbbbbbb
    Chromosome 2
    bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
    
  3. The branch may migrate, creating a region of heteroduplex DNA
    
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    Chromosome 1
    aaaaaaaaaaaaaaaabbbbbbbbbb   aaaaaaaaaaaaaaaaa
                              \ /
                               X
                              / \
    bbbbbbbbbbbbbbbbaaaaaaaaaa   bbbbbbbbbbbbbbbbb
    Chromosome 2
    bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
    

    (that is, there is now a section where part of each double helix is originally from the adjacent double helix)
  4. The Holiday junction is resolved in one of two ways. Both leave heteroduplex DNA, but only one is recombinant.

    (Type 1 - heteroduplex but non-recombinant)

    
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    Chromosome 1
    aaaaaaaaaaaaaaaabbbbbbbbbbaaaaaaaaaaaaaaaaaaa
    
    
    bbbbbbbbbbbbbbbbaaaaaaaaaabbbbbbbbbbbbbbbbbbb
    Chromosome 2
    bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
    
    

    (Type 2 - heteroduplex and recombinant)

    
    aaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbb
    Chromosome 1
    aaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbbb
    
    
    bbbbbbbbbbbbbbbbaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    Chromosome 2
    bbbbbbbbbbbbbbbbbbbbbbbbbbbaaaaaaaaaaaaaaaaaa
    
    

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