During meiosis I, the stage characterized by the appearance of...

During the pachytene stage of prophase I in meiosis, the bivalent chromosomes clearly appear as tetrads.

This stage is characterized by the appearance of recombination nodules, which are the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes.

Crossing over is an enzyme-mediated process and the enzyme involved is called recombinase.

Answer: Pachytene