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Meiosis I

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Biology

Do you ever split up your tasks to make them more manageable? That strategy isn’t just a great way to get work done; it’s also an efficient way to make sex cells. Meiosis, or the process of making sex cells (gametes), is split up into two parts: meiosis I and meiosis II. In the following, we will focus on learning about the details of meiosis I.

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.

Meiosis I: Definition & Steps with Diagrams

Meiosis I is the first stage of meiosis and produces two daughter cells with half the genetic information of the parent cell (duplicated). Each daughter cell will have one of the homologous chromosomes of the parent cell.

The steps of meiosis I are:
  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.

To get more information on these general stages, you can read our articles Mitosis and Meiosis or the Comparison between Mitosis and Meiosis.

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).

Homologous chromosomes contain the same genes, but one copy is derived maternally (from your mother), and the other is derived paternally (from your father). In other words, they contain different variations of the same genes.

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.

Crossing over is the process by which homologous chromosomes swap genes during meiosis.

Meiosis I Cell in prophase I of Meiosis I StudySmarterFigure 1: A cell during prophase I of meiosis I. Hailee Gibadlo, StudySmarter Originals.

  • 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).

Meiosis I Cell in metaphase I of Meiosis I StudySmarter

Figure 2: Cell during metaphase I of meiosis I. Hailee Gibadlo, StudySmarter Originals.

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.

Meiosis I Cell in anaphase I of Meiosis I StudySmarter

Figure 3: Cell during anaphase I of meiosis I. Hailee Gibadlo, StudySmarter Originals.

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.

Meiosis I Cell in telophase I of Meiosis I StudySmarter

Figure 4: Cell during telophase I of meiosis I. Hailee Gibadlo, StudySmarter Originals.

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!

Table 1: Differences between mitosis and meiosis I. Hailee Gibadlo, StudySmarter Originals.

Meiosis IMitosis
During prophase I, the homologous chromosomes form a tetrad and undergo crossing-over, a process in which they swap genetic information. During prophase, homologous chromosomes do not swap genetic material.
During metaphase I, the homologous chromosomes line up side-by-side at the metaphase plate.During metaphase, homologous chromosomes line up at the metaphase plate in a single line.
During anaphase I, the homologous chromosomes are pulled to opposite poles, meaning homologous chromosomes are separated. During anaphase, the sister chromatids, or identical chromatid copies, are split. Homologous chromosomes are not separated.
At the end of telophase I and cytokinesis, two haploid daughter cells with copies remain. Genes have been recombined during crossing-over, so these cells are not identical to the parent cell. Meiosis is not complete, meiosis II will begin.At the end of telophase and cytokinesis, two diploid (2n) daughter cells identical to the parent cell remain. Mitosis is complete.

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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

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. 

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. 

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

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.  

  1. During interphase, before meiosis I, the DNA is duplicated. 
  2. During prophase I, crossing over, or the swapping of genes between homologous chromosomes occurs.
  3. During metaphase I, the homologous chromosomes line up side-by-side at the center of the cell. 
  4. During anaphase I, homologous chromosomes are pulled towards opposite cell poles
  5. 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). 

Final Meiosis I Quiz

Question

What is the four-chromatid structure that forms during crossing over in meiosis I called?

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Answer

A tetrad

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Question

What is crossing over? 

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Answer

Crossing over is the process by which homologous chromosomes swap genes during meiosis. 


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Question

At what stage of meiosis does crossing over occur? 

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Answer

During prophase I of meiosis I, before homologous chromosomes are separated.

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Question

What occurs during mitosis, or cell division?

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Answer

Sister chromatids are separated.

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Question

During meiosis I, the following are all true except....

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Answer

Homologous chromosomes are not separated.

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Question

A chiasma is best described as _________________________. 

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Answer

The point where the non-sister chromatids are crossing over and can be observed under a microscope.

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Question

What happens during metaphase I in meiosis I? 

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Answer

The homologous chromosomes line up side-by-side at the metaphase plate. 

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Question

True or False: During metaphase I of meiosis I sister chromatids are separated.

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Answer

False- During metaphase I, homologous chromosomes line up at the metaphase plate to be separated, sister chromatids remain intact. 

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Question

Why is crossing over during prophase I an important step in meiosis? 

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Answer

Crossing over, the process by which homologous chromosomes swap some genes creates entirely new chromosomes, meaning genetic variation among gametes is increased. Genetic variation within gametes translates to a more genetically diverse population of organisms.

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Question

Anaphase I of meiosis I includes the separation of what? 

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Answer

Homologous chromosomes. They are pulled via spindle fibers to opposite poles in this stage. 

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Question

True or False: At the end of meiosis I four haploid daughter cells are produced. 

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Answer

False- Four haploid daughter cells are produced by the end of meiosis II. 

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Question

During what stage of meiosis I does the cleavage furrow form in preparation for cytokinesis? 

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Answer

Telophase I

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Question

True or False: During the prophase stage of mitosis, homologous chromosomes undergo crossing over?

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Answer

False- During mitosis, no crossing over occurs. Cells produced at the end of mitosis are diploid and copies of the parent cell. 

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Question

What is the purpose of the synaptonemal complex during crossing over?

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Answer

The synaptonemal complex is a protein structure that helps hold the homologous chromosomes together during crossing over.

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