Leptotene:

The chromosomes become visible, shorten and thick. The size of the nucleus increases and homologous chromosomes start getting closer to each other.

Zygotene:

First essential phenomenon of meiosis i.e., pairing of homologous chromosomes called synapsis starts. This pairing is highly specific and exactly pointed, but with no definite starting point(s). Each paired but not fused, complex structure is called bivalent or tetrad.

Pachytene:

The pairing of homologous chromosomes is completed. Chromosomes become more and more thick. Crossing Over. Each bivalent has four chromatids, which wrap around each other. Non-sister chromatids of homologous chromosomes exchange their segments due to chiasmata formation, during the process called crossing over.

Importance.

In this way reshuffling of genetic material occurs which produces recombinations.

Duration.

Pachytene may last for days, weeks or even years, whereas leptotene and zygotene can last only for few hours.

Diplotene:

The paired chromosomes repel each other and begin to separate. Separation however, is not complete, because homologous chromosomes remain united by their point of interchange (chiasmata).

Each bivalent has at least one such point, the chromatids otherwise are separated.

Diakinesis:

During this phase the condensation of chromosomes reaches to its maximum. At the same time separation of the homologous chromosomes (started during diplotene) is completed, but still they are united at one point, more often at ends. Nucleoli disappear.