The longest phase of prophase I is the pachytene stage. Prophase I is the first stage of meiosis and is divided into five sub-stages: leptotene, zygotene, pachytene, diplotene, and diakinesis. Pachytene is the third sub-stage of prophase I and is the longest of all.

Pachytene Stage:
During the pachytene stage, the homologous chromosomes pair up to form bivalents. Each bivalent contains four chromatids, two from each chromosome. This process is called synapsis. The paired homologous chromosomes exchange genetic material during a process called crossing-over, which results in the creation of new combinations of genetic material. This genetic recombination is important for the diversity of offspring.

Other stages of Prophase I:
Leptotene Stage: During the leptotene stage, the chromatin fibers condense to form visible chromosomes.

Zygotene Stage: During the zygotene stage, the homologous chromosomes start to pair up.

Diplotene Stage: During the diplotene stage, the homologous chromosomes start to separate, but the sister chromatids remain attached at the centromere. This is also the stage where the chiasmata, the sites where the crossing-over occurs, are visible.

Diakinesis Stage: During the diakinesis stage, the nuclear envelope breaks down, and the spindle fibers start to form. The homologous chromosomes are fully separated, and each bivalent is attached to the spindle fibers.

Conclusion:
In conclusion, the pachytene stage is the longest sub-stage of prophase I. During this stage, the homologous chromosomes pair up to form bivalents, and crossing-over occurs, resulting in new combinations of genetic material. This process is essential for the diversity of offspring in sexually reproducing organisms.

May be diplotene because it last for months to years in oocytes of vertebrates.