-EARLY; Chromosomes begin to condense, and homologs loosely pair along their lengths by a zipper-like protein structure the synaptonemal complex; this state is called synapsis.
-Crossing over, a genetic rearrangement between non-sister chromatids involving the exchange of corresponding segments of DNA molecules, begins during pairing and synaptonemal complex formation, and is completed while homologs are in synapsis.
-IN THIS PICTURE; Synapsis has ended with the disassembly of the snaptonemal complex in midprophase, and the chromosomes in each pair have moved apart slightly.
-Each homologous pair has one or more x-shaped regions called chiasmata. A chiasmata exists at the point where a cross over has occured. It appears as a cross because sister chromatid cohesion still holds the 2 original sister chromatids together even in regions beyond the cross over point, where 1 chromatid is now part of the other homolog.
-Centosome movement, spindle formation and nuclear envelope break down occur as in mitosis.
-LATER; Microtubules from one pole or the other attach to 2 kinetochores, protein structures at the centromeres of the 2 homologs.The homologous pairs then move toward the metaphase plate.
-Pairs of homologous chromosomes are now arranged at the metaphase plate, with 1 chromosome in each pair facing each pole.
-Both chromatids of one homolog are attached to kinetochore microtubules from one pole; those of the other homolog are attached to microtubules from the opposite pole.
-Break down of proteins responsible for sister chromatid cohesion alon(?) chromatid arms allows homologs to separate.
0The homologs move toward opposite poles, guided by the spindle apparatus.
-Sister chromatid cohesionpersists at the centromere, causing chromatids to move as a unit toward the same pole.
Telophase 1 and Cytokinesis
2 haploid cells form; each chromosome still consists of 2 sister chromatids.
-At the begining of telophase 1, each half of the cell has a complete haploid set of duplicated chromosomes. Each chromosome is composed of 2 sister chromatids; one or both chromatids include regions of nonsister chromatid DNA.
-Cytokinesis(divison of the cytoplasm) usually occurs simultaneously with telophase 1, forming 2 haploid daughter cells.
- In animal cells a cleavage furrow forms.
- In some species chromosomes decondense and nuclear envelopes forms.
- No chromosome duplication occurs between meiosis 1 and meiosis 2