Page 46 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

6.5 Homologous Recombination at the DNA Level 205

Crossover Pathway
Step 4 Formation of a double Holliday junction. New DNA added to the invading 3′ tail (blue dots at the top) enlarges the
D-loop until the single-stranded bases on the displaced strand can form complementary base pairs with the 3′ tail on the non-
sister chromatid. New DNA added to this latter tail (blue dots at the bottom) re-creates the DNA duplex on the bottom chromatid.
At each side of the original break, the 3′ end of the newly synthesized DNA becomes adjacent to a 5′ end left after resection,
and DNA ligase forms phosphodiester bonds to rejoin DNA strands without the loss or gain of nucleotides. The resulting
X-shaped structures are called Holliday junctions after Robin Holliday, the scientist who first proposed their existence as a key
intermediate in recombination.

Invading strand
5' 3'

3' 5'
D-loop
Holliday junction
Ligase
Holliday junction

5' 3'

3' 5'

Step 5 Branch migration. The two invading strands tend to zip up by base pairing with the complementary strands of the parental
double helixes they invade. The DNA double helixes unwind in front of this double zippering action, moving in the direction of the ar-
rows in the figure, and two newly created heteroduplex molecules rewind behind it. Branch migration thus lengthens the heteroduplex
region of both DNA molecules from tens of base pairs to hundreds or thousands.

Heteroduplex
5' 3'

3' 5'
Direction of migration
5' 3'

3' 5'
Heteroduplex

(continued)