Meiosis I

Meiosis I: Definition & Steps with Diagrams

1. Prophase I
2. Metaphase I
3. Anaphase I
4. Telophase I and cytokinesis, or cleavage of the cytoplasm, producing two daughter cells.

Although not an official part of meiosis I, interphase is also important because DNA replication happens in this stage.

Interphase:

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.

Prophase I:

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 (Fig. 1).

Prophase I is an essential step because, unlike in mitosis, genetic information is being swapped between the homologous chromosomes, increasing the genetic diversity among gametes. This process is known as crossing over and happens towards the end of prophase I.

The homologous chromosomes line up parallel to one another (Fig. 1). The synaptonemal complex is a protein structure formed to keep the homologous chromosomes together during crossing over. The two homologous chromosomes together include four chromatids: the original ones and their copies, which is why they are called a tetrad. Under a microscope, the point where the chromosomes crossing over can be seen is called the chiasma.

This means that the DNA inherited from one parent is mixed with the DNA inherited from the other, creating chromosomes that are different from somatic cells (body cells). Crossing over allows gametes to be genetically different from the ones of the parents, thereby increasing genetic variation in a population.

• During prophase I, homologous chromosomes form a tetrad (of four chromatids), a protein structure held together by a synaptonemal complex.
• In the tetrad, they swap genes in a process called crossing over.
• Chiasmata (singular: chiasma) are the points where the actual chromosomes are crossing over and can be seen under a microscope.
• Crossover events during meiosis I increase the genetic variation of gametes.

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).

Anaphase I:

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:

Telophase I is the last stage of meiosis I (Fig. 4), 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.

Differences between Meiosis I and Mitosis

Now that we have discussed the details of meiosis I, you may realize some similarities between this stage of meiosis and mitosis. For the most part, the machinery and steps we discussed in meiosis are the same for mitosis, i.e. centrosomes, spindle fibers (microtubules), and lining up at the metaphase plate. However, important differences between meiosis I and mitosis are highlighted in Table 1.

Table 1: Differences between mitosis and meiosis I.