Fig.1

Comparison of mitosis and meiosis

The left side of the figure depicts the two meiotic cell divisions, meiosis I and II; the right side depicts mitosis. Both processes are illustrated with a diploid genome composed of two homologous chromosomes, one maternal and one paternal (shown in red and blue; 2n/4c; n = number of chromosomes, c = number of chromatids or DNA copies). In mitosis the chromatids of both chromosomes are separated and distributed to the two daughter cells (2n/2c). In the subsequent interphase the DNA is replicated, completing the complementary chromatid. This reinstates the starting conditions for the next mitosis. Meiosis consists of two sequential cell divisions that result in four daughter cells. In meiosis I, the homologous (maternal and paternal) chromosomes are paired, separated, and distributed to the two daughter cells (1n/2c). Because crossing-over may occur during the pro- and metaphases, segments can be exchanged between the homologous chromosomes and may end up in different daughter cells. The subsequent meiosis II proceeds like a mitosis in which the chromatids are distributed to the daughter cells (1n/1c). In this example, the diploid starting cell - a germline stem cell - results in four different haploid daughter cells. These gametes each possess one chromatid (1c) that is replicated during the subsequent inerphase (1n/2c). Upon fertilization the maternal and paternal gametes come together to form a diploid stem cell (2n/4c)