In which sub-stages of meiosis-I do the following events occur?

i) pairing of homologous chromosomes

ii) terminalization

iii) crossing over

iv) formation of spindle apparatus.

i) Pairing of homologous chromosomes – Zygotene

In zygotene step of prophase, I of meiosis I pairing of homologous chromosomes takes place and this process is called synapsis. In this homologous chromosomes come near each other and start pairing.

ii) Terminalisation- Diplotene

In this separation of the paired homologous chromosomes takes place except for the region where crossing over had taken place. Formation of X- shaped structures take place which is called chiasmata and they move towards terminals. Therefore, this process is called terminalisation.

iii) Crossing over- Pachytene

Crossing over refers to the process in which segments are exchanged between non sister chromatids of homologous chromosomes. In meiosis, I pachytene is the step in which crossing over takes place.

In this tetrads are clearly visible.

iv) formation of spindle apparatus- Diakinesis

This is the last step of prophase I in meiosis I. During this step there is complete dissolution of nuclear membrane and nucleolous and they become disappear. In this step we left with spindle apparatus.

The following figure explains all the concepts: