Which phase of cell division involves homologous chromosomes pairing up?

The phase of cell division in which homologous chromosomes pair up is Prophase I. During this stage, the chromosomes become visible as they condense, and each chromosome aligns with its homologous partner, forming a structure known as a tetrad. This pairing allows for genetic recombination through a process called crossing over, where segments of DNA can be exchanged between the homologous chromosomes. This exchange increases genetic diversity in the resulting gametes.

While other phases of cell division are important, Prophase I is specifically characterized by this crucial event of homologous chromosome pairing. Tetrad formation occurs as a result of this pairing but is not officially recognized as a distinct phase; instead, it is a part of the Prophase I process. The other phases mentioned do not involve the pairing of homologous chromosomes in the same way or focus on this significant preparatory step leading to meiosis.