Five Substages of Prophase I
Prophase I has so many processes happening that it is usually separated into five stages. They are listed, in order, below with their explanations.
During this stage, the chromosomes begin to condense and become visible. Researchers also believe that homologous pair searching begins also at this stage.
The chromosomes continue to become denser. The homologous pairs have also found each other and begin to initially align with one another, referred to as 'rough pairing'. Lateral elements also form between the two homologous pairs, forming a synaptonemal complex.
Coiling and shortening continues as the chromosomes become more condense. A synapsis forms between the pairs, forming a tetrad.
The sister chromatids begin to separate slightly, revealing points of the chiasma. This is where genetic exchange occurs between two non-sister chromatids, a process known as crossing over.
The chromosomes continue to pull apart, but non-sister chromatids are still loosely associated via the chiasma. The chiasma begin to move toward the ends of the tetrad as separation continues. This process is known as terminalization. Also during diakinesis, the nuclear envelope breaks down and the spindle fibers begin to interact with the tetrad.