Meiosis: Prophase I
Figure 8. Prophase l of meiosis.The homologous chromosomes form tetrads. Notice the arrangement of the genes in each tetrad. During chromosome replication in interphase, the chromosomes are copied and later form a replicated chromosome with two identical sister chromatids. Those replicated, homologous chromosomes come together in prophase I to form a tetrad. Image by M. Hanneman, 2021. CC BY-NC-SA 4.0.
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 (Fig. 8) can be recombined so that they are not always inherited together. The tetrad or bivalent formed during synapsis remains assembled as prophase progresses. The tetrad therefore moves as a unit to the center of the cell.