Chromosomes replicate as in mitosis. Each duplicated chromosome consists of two identical sister chromatids attached at their centromeres. Centriole pairs in animal cells also replicate into two pairs. This is a longer and more complex process than prophase of mitosis. During this process, homologous chromosomes come together as pairs.
Chromosomes condense further until they are distinct structures that can be seen with a microscope.
Since each chromosome has two chromatids, each homologous pair in synapsis appears as a complex of four chromatids or a tetrad . In each tetrad, sister chromatids of the same chromosome are attached at their centromeres. Chromosomes thicken further and detach from the nuclear envelope. As prophase I continues, the cell prepares for nuclear division. Centriole pairs move apart and spindle microtubules form between them.