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Molecular Biology, 4/e
Robert F. Weaver
Homologous Recombination

The Holliday Model

What are the consequences of crossing over? Crossing over is a crucial step in meiosis in which two homologous chromosomes become physically attached to one another. The points of attachment are known as chiasmata. Portions of the chromosome are exchanged between two chromatids. Crossing over can occur between sister chromatids. As sister chromatids are genetically identical these crossovers have no genetic effect. When crossing over involves chromatids on different homologous chromosomes then new genetic combinations can occur. Genes on the same chromosome can be inherited independently as a result of crossing over.

View the animation below, then complete the quiz to test your knowledge of the concept.



1

According to the Holliday model, the first step of recombination involves a nick in
A)only one DNA strand of each of the two homologous chromosomes.
B)only one DNA strand of only one of the two homologous chromosomes.
C)both DNA strands of each of the two homologous chromosomes.
D)both DNA strands of only one of the two homologous chromosomes.
E)either one or both strands of each of the two homologous chromosomes.
2

Arrange the following in the proper order in which they occur during recombination.

1. Holliday junction formation
2. Strand invasion
3. Branch migration
A)1, 2, 3
B)2, 1, 3
C)2, 3, 1
D)3, 2, 1
E)3, 1, 2
3

After isomerization, cleavage by an endonuclease results in
A)two nonrecombinant chromosomes, each containing a heteroduplex region.
B)two nonrecombinant chromosomes, without heteroduplex regions.
C)two recombinant chromosomes, each containing a heteroduplex region.
D)two recombinant chromosomes, without heteroduplex regions.
E)two chromosomes that are unchanged from the original chromosome.
4

The Holliday junction consists of a single DNA strand containing genetic material from two different chromosomes.
A)True
B)False
5

The Holliday model can be used to explain crossing over in meiosis.
A)True
B)False








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