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FIGURE 14.10 Timeline of Eukaryotic Recombination in Yeast

Site-specific double-stranded breaks in DNA appear 60-90 minutes after DNA replication. The breaks disappear as hybrid molecules are made during the zygotene phase of meiosis. Resolution of the hybrids then occurs during pachytene. Recombinant molecules appear approximately 120 minutes after the appearance of double-stranded breaks. Therefore, eukaryotic recombination occurs in a span of approximately 2 hours.

During meiosis in eukaryotic cells, frequent recombination occurs between pairs of homologous chromosomes.

stranded breaks appear in eukaryotic chromosomes during the first stage of meiosis, known as leptotene and the paired chromosomes are joined together during the next stage (zygotene) to form the hybrid junction structures needed for recombination (Fig. 14.10). It is assumed that these resemble the Holliday junction of bacteria but the details are obscure. Resolution of the crossovers then occurs during the third stage of meiosis (pachytene). Finally, the crossovers dissociate, releasing recombinant chromosomes in the final stage of meiosis (diplotene).

rEVERSIBLE CLEAVAGE

Spo11 Spo11

Spo11 Spo11

Dissociation of complex

FIGURE 14.11 Spo11 Promotes Double Strand Breaks

Spo11 binds and cleaves double-stranded DNA in yeast. After cleaving the DNA, Spo11 is replaced by a nuclease that generates the single-stranded region. The two DNA fragments are held together by other proteins that are not shown here.

Spo11

Spo11

Spo11

Removal of Spo11 followed

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