Meiosis: Prophase I

This stage starts meiosis and is the same as prophase of mitosis with one important change. As the chromosomes condense they form groups of four chromatids called tetrads or bivalents. Close inspection reveals that each chromosome is replicated and consists of two sister chromatids. The two chromosomes in each cell that are homologous and have the same genes (but perhaps different alleles if the organism is heterozygous) will pair closely. This close association, or synapsis, allows the homologous chromosomes to crossover and exchange identical parts. The impact of crossing over is that genes that are on the same chromosome (ie. D and B in Fig.7) can be recombined so that they are not always inherited together. The tetrad or bilvalent formed during synapsis remains assembled as prophase progresses. The tetrad therefore moves as a unit to the center of the cell.

a tetrad is characterized by four chramtids in synapsis. Visually, a tetrad looks like two X's sitting next to each other or two V's, depending on where the centromere of the chromosomes is.
The tetrads line up along the center of the cell during metaphase I of meiosis.

Fig. 7 and 8. Prophase l and metaphase l of meiosis. (Images by D. Lee)