Do you ever break down your tasks to make them easier to handle? Well, the same strategy can be used when creating sex cells. This is done through a process called meiosis, which is split into two parts: meiosis I and meiosis II. Today, we will focus on learning about meiosis I.

Meiosis I the reduction stage it the material in the cells by half compared to the parent cell. Meiosis itself requires one DNA replication and two cell divisions, with the replication event occurring before meiosis I during interphase. Meiosis I contains one cell division event and the second one happens during meiosis II.

During meiosis I, two daughter cells are produced that each contain half the genetic information of the parent cell. The cell goes through four phases: Prophase I, Metaphase I, Anaphase I, and Telophase I, which leads to the production of the two daughter cells. Although not officially part of meiosis I, DNA replication occurs during interphase, which is also an important stage.

So there you have it, meiosis I in a nutshell.

Interphase:

Interphase is an important part of the cell cycle where the cell is not in mitosis or meiosis. It is divided into three parts: G1, S, and G2. The G1 phase is the growth phase, and during the S phase, the genetic material is duplicated in preparation for mitosis or meiosis. The G2 phase further prepares the cell for division.

If want to learn more about these stages, you can check out our articles on Mitosis and Meiosis or the Comparison between Mitosis and Meiosis. They provide in-depth information on the general stages of cell division.

Prophase I:

During prophase I of meiosis I, the nuclear envelope dissolves, the spindle fibers start to form, and the chromosomes condense to prepare for cell division (Fig. 1). Homologous chromosomes contain the same genes but come from different parents. In other words, they have different variations the is from mitosis genetic information is exchanged between homologous chromosomes, which increases diversity among gametes. This process is called crossing over and happens towards the end of prophase I.

The homologous chromosomes line up next to each other (Fig. 1). The synaptonemal complex is a protein structure that holds the homologous chromosomes together during crossing over. The two homologous chromosomes together have four chromatids, which is why they are called a tetrad. The point where the chromosomes cross over is called the chiasma.

Crossing over means that the genetic material from one parent mixes with the other parent's material, creating chromosomes that are different from somatic cells. This allows gametes to be genetically diverse, increasing variation in a population.

In summary, crossing over is the process by which homologous chromosomes exchange genes during meiosis.

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Prophase I is a crucial stage in meiosis where homologous chromosomes come together to form a tetrad, consisting of four chromatids held together by the synaptonemal complex. The tetrad allows for the exchange of genetic material between homologous chromosomes through a process called crossing over. The actual points where the chromosomes cross over are called chiasmata, and they can be observed under a microscope.

Crossover events during meiosis I increase the genetic diversity of gametes. This is because crossing over allows for the exchange of genetic information between homologous chromosomes, resulting in a unique combination of genes in each gamete. Thus, prophase I plays a crucial role in ensuring genetic diversity in sexual reproduction.

Metaphase I:

metaphase I of meiosis I, the chromosomes align in the middle of the cell at a point called the metaphase plate, similar to what happens in mitosis. However, in meiosis I, homologous chromosomes line up side by side in the center and are separated in this stage (Fig. 2). Spindle fibers attach to the homologous chromosomes' centromeres and allow sister chromatids to remain together.

After meiosis I, each daughter cell will have one copy of each chromosome and its duplicate (sister chromatid). Eventually, after meiosis II, the sister chromatids will be separated, and each daughter cell will have one copy of each chromosome, making them haploid.

In summary, during metaphase I of meiosis I, homologous chromosomes line up side by side, and spindle fibers attach to their centromeres. After meiosis I, each daughter cell will have one copy and its duplicate of each chromosome, and after meiosis II, the sister chromatids will be separated, resulting in haploid daughter cells.

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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.

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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.

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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.

Study tip: Check out our article on Mitosis to review!

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Meiosis I - Key Takeaways Meiosis I consists of fours stages: prophase I, metaphase I, anaphase I, and telophase I plus cytokinesis. Known as the reduction division, meiosis I produces two daughter cells, each with half the chromosome number of the parent cell and its copies (n + n). During prophase I of meiosis, the homologous chromosomes form a tetrad. Held together by a protein structure known as a synaptonemal complex, the chromosomes swap genes in a process known as crossing over. Crossing over increases the genetic variation of gametes and overall genetic diversity within a population. During metaphase I, homologous chromosomes are separated. Sister chromatids remain intact during meiosis I. Meiosis I differs from mitosis in that during meiosis I crossing over occurs and homologous chromosomes are separated, resulting in a reduction of the chromosome number.

Meiosis I

What is the difference between meiosis I and meiosis II?

During meiosis I, which is known as the reduction division, homologous chromosomes are separated, creating two daughter cells with half the genetic information of parent cells, plus a copy. During meiosis II, sister chromatids are separated in the two daughter cells from the end of meiosis II, separating identical chromatids and producing four haploid daughter cells which are now officially gametes. Crossing over happens only during meiosis I. 

What is the end result of meiosis I?

At the end of meiosis I, two daughter cells with half the chromosome number of the parent cell (plus a copy or sister chromatid) are produced. Homologous chromosomes separate during meiosis I. 

What are the different phases of meiosis I? 

The phases of meiosis I in order are prophase I, metaphase I, anaphase I, and telophase I plus cytokinesis. 

What happens during anaphase I of meiosis I?

During anaphase I the spindle fibers, attached to the homologous chromosomes at the kinetochore, a region of the centromere, pull them towards opposite poles of the cell. Sister chromatids remain intact.  

What happens during meiosis I? 

During interphase, before meiosis I, the DNA is duplicated. During prophase I, crossing over, or the swapping of genes between homologous chromosomes occurs.During metaphase I, the homologous chromosomes line up side-by-side at the center of the cell. During anaphase I, homologous chromosomes are pulled towards opposite cell poles. During telophase I and cytokinesis, the cell membrane is pinched inward and two new daughter cells are formed. Daughter cells are haploid with a copy of each chromosome as well (in the form of sister chromatids).

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