What must occur before a bivalent can be classified as ready for metaphase?

For a bivalent to be classified as ready for metaphase, the dissolution of the synaptonemal complex is a critical step. The synaptonemal complex is a protein structure that forms between homologous chromosomes during prophase I of meiosis. It facilitates the pairing of homologous chromosomes and is essential for their proper alignment and interaction during the early stages of meiosis.

As prophase I progresses, particularly during the transition from the zygotene stage to the pachytene stage, the synaptonemal complex aids in holding sister chromatids together. However, to prepare for metaphase alignment on the metaphase plate, this complex must dissolve, allowing the homologous chromosomes to be positioned correctly. This dissolution ensures that the chromosomes become free to move and orient themselves correctly for separation.

While chiasma formation and crossing over are important for genetic recombination and contribute to genetic diversity, they are not the factors that directly indicate readiness for metaphase. The displacement of chromatids pertains more to the movement during anaphase than to the readiness state for metaphase itself. Thus, the establishment of the readiness of a bivalent for metaphase is contingent upon the dissolution of the synaptonemal complex.