Meiosis I is known as the reduction division stage of meiosis because after meiosis I, the two cells create half the parent cell’s genetic material. The whole process of meiosis requires one DNA replication event and two cell divisions. Before meiosis I, in interphase, the DNA duplication event occurs. Then, meiosis I contains one cell division event, with the second taking place in meiosis II.
Steps of Meiosis I
- Prophase I
- Metaphase I
- Anaphase I
- Telophase I and cytokinesis, or cleavage of the cytoplasm, producing two daughter cells.
Interphase is the part of the cell cycle in which the cell is not in mitosis or meiosis. It is split up into three parts: G1, S, and G2. G1 is the growth phase. The genetic material is duplicated during the S phase to prepare for mitosis or meiosis. Further preparation happens in the G2 phase.
During prophase I of meiosis I, as in the prophase stage of mitosis, the nuclear envelope dissolves, the spindle fibers begin to form, and the chromosomes condense in preparation for movement and cell division .
Metaphase I: During metaphase I of meiosis I, like in mitosis, the chromosomes line up in the middle of the cell at the point known as the metaphase plate. Unlike in mitosis, however, the homologous chromosomes line up side by side in the center and are separated in this first part of meiosis (Fig. 2). Spindle fibers attach to the homologous chromosomes at the centromere and allow sister chromatids to stay together.
After meiosis I, each daughter cell will have one copy and its duplicate (sister chromatid) of each chromosome. Eventually, after meiosis II, the sister chromatids will be separated, and each daughter cell will have one copy of each chromosome (they will be haploid).
In anaphase I of meiosis I, the spindle fibers attach to the homologous chromosomes at the kinetochore, a region of the centromere, and pull them towards opposite poles of the cell (Fig. 3). Sister chromatids remain intact. Spindle fibers not attached to the chromosomes help push the centrosomes and cell poles away from each other.
Telophase I is the last stage of meiosis I, and the nuclear membrane begins to reform. In animal cells, the cleavage furrow forms, whereas the cell plate forms in plant cells. Telophase I is followed by cytokinesis, or the cleavage of the cell membrane, which results in two haploid daughter cells with a copy of each chromosome (n+n, but not 2n). They have two copies of “the same” alleles (not exactly due to crossing over), but not two different alleles for each gene.
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