Meiosis I: Prophase

During prophase, chromosomes condense, centrosomes move to opposite poles of the cell, the mitotic spindle assembles and the nuclear envelope and the nucleolus disappear.

During prophase, chromosomes condense, centrosomes move to opposite poles of the cell, the mitotic spindle assembles and the nuclear envelope and the nucleolus disappear.

During prophase, chromosomes condense, centrosomes move to opposite poles of the cell, the mitotic spindle assembles and the nuclear envelope and the nucleolus disappear.

During prophase, chromosomes condense, centrosomes move to opposite poles of the cell, the mitotic spindle assembles and the nuclear envelope and the nucleolus disappear.

During prophase, chromosomes condense, centrosomes move to opposite poles of the cell, the mitotic spindle assembles and the nuclear envelope and the nucleolus disappear.

During prophase, chromosomes condense, centrosomes move to opposite poles of the cell, the mitotic spindle assembles and the nuclear envelope and the nucleolus disappear.

Prophase of meiosis I consists of five stages:  leptotene; zygotene; pachytene; diplotene; and diakinesis.  Leptotene and zygotene are presented together, as are diplotene and diakinesis.

During leptotene, replicated DNA condenses into visible chromosomes consisting of paired, sister chromatids.  During zygotene, synapsis occurs resulting in the close alignment of homologous pairs of chromosomes.  The pairs are tightly bound to each other by protein bridges, the synaptonemal complex, forming bivalents of four chromatids.

During the pachytene stage of prophase, crossing over occurs in which fragments of homologous regions of maternal and paternal chromosomes are exchanged.  A site of crossing over, called a chiasma (plural, chiasmata), resembles an “X” and forms a connection between paired homologous chromosomes.  Crossing over leads to greater genetic diversity in the gametes.

During the diplotene stage, chromosomes further condense and the synaptonemal complex breaks down, leading to a separation of homologous chromosomes.  As the chromosomes separate, chiasmata become more apparent.  During diakinesis, chromosomes condense to their maximum thickness, the nuclear envelope disappears and the nucleolus fragments.