Prophase I (
meiosis):
Chromatin condenses and chromosomes come into view in pairs. Once contact is made at any point between the two homologues, paring extends in a zipperlike fashion along the length of the chromatids in a process called synapsis. Pairing of homologous chromosomes actually involves four chromatids, a complex of chromatids called a tetrad.
- Crossing over: exchange of segments of one chromosome with corresponding segments from its homologous chromosome.
- As prophase progresses, the homologues begin to pull away from each other, except at the crossover points (chiasmata.) At chiasmata, the homologues remain in close association until the end of prophase. Then, the chiasmata seem to slip off the ends of the chromosomes.
- Although the homologues chromosomes have moved slightly apart, they are still paired.
- The nuclei and nuclear envelope disappear towards the end of this stage.
Prophase II (meiosis):
Nuclear envelopes (if present) disappear and new spindle fibers begin to form.